Technological options to mitigate the effect of heating

From the reasoning given above, it is possible to deduce that heating is a matter of balance, and that a treatment should be sought that will result in maximum desired effects and minimum undesired effects. In order to find this balance in a scientifically sound way, the following must be known:

• what reactions are important for quality,

• how fast these reactions take place,

• technological knowledge in terms of time and temperature distributions in the equipment used to heat the food.

In order to understand the first aspect, knowledge of food science as well as knowledge of the raw material that is heated is needed. The ability to translate quality aspects into reactions that affect this quality aspect is required. Since quality is a multidimensional concept, this is not a simple thing to do. For the second aspect, the kinetics of the reaction must be studied, and in addition knowledge on the food matrix that is heated is also needed; i.e. on the composition and structure of the food. For instance, it is common knowledge that the Maillard reaction takes place in both sterilized milk and in fried potatoes. The fact that potatoes contain a substantial amount of free asparagine has a large impact on the formation of acrylamide, whereas acrylamide is not formed in heated milk products because they do not contain free asparagine. In addition, water activity and temperatures used in preparing fried potatoes and sterilizing milk are very different, factors, which will also influence acrylamide formation. Another important aspect is structure: sometimes foods do contain reactants but these may be physically separated in which case the reaction cannot take place. These simple examples illustrate the importance of knowledge on the materials that are being heated. The third aspect concerns engineering knowledge. Industrial equipment is large and the effects of residence time distribution can have a big impact. Heating-up and cooling-down periods may add considerably to the effective heat intensity, and process optimization in this respect can be very rewarding.

In general terms, it can be stated that industrial cooking leads to a much better control of desired and undesired changes than home cooking. Even though better equipment becomes available nowadays in the kitchen (microwave, steam cookers) it is not the intention of the average consumer to cook in a tightly controlled, standardised way as is the case in industrial processing. Moreover, home equipment will usually not allow fine tuning of the processes taking place, whereas this is the case for industrial equipment.

The ultimate proof whether or not a product is acceptable lies with the consumer. If he/she does not like it then a product fails. So if, for instance, the concentration of acrylamide in French fries can be reduced considerably, but at the cost of other quality attributes, such as texture and taste, then the product will not be accepted. In general, a consumer will assume that a food is safe, and consequently, will not care too much about this aspect, at least not in his/her buying behaviour. It therefore remains the task of the food technologist to make sure that quality aspects in addition to food safety are optimized. In other words, we should minimize the risk and optimize the benefits. This highlights the need for development of risk-benefit approaches.

Genetic resource

Hertog et al. ⁽Reference Hertog, Putz and Tijskens²⁸⁾ tested 10 different genotypes and found that the concentration of reducing sugars in immature crops (50 days before crop maturity) varied between 0·21 % (variety Semena) and 0·9 % (clone H1272/80). Genetically determined higher pre-harvest concentrations were correlated with higher concentrations induced by cold storage. Haase and Weber⁽Reference Haase and Weber²⁹⁾ tested four varieties on five sites in Germany during three growing seasons. At harvest, the variety Sempra consistently had the lowest concentration of reducing sugars (0·49 g/kg fresh weight, n = 30) and also the lowest increase in concentration after 6 months storage at 8°C (0·34 g/kg, totalling 0·83 g/kg), whereas the variety Agria had a concentration of 1·51 g/kg after storage. Cold-sweetening and senescence-sweetening were also found to be variety-dependent by Kumar et al. ⁽Reference Kumar, Singh and Kimar³⁰⁾. Amrein et al. ⁽Reference Amrein, Bachmann, Noti, Biedermann, Barbosa, Biedermann-Brem, Grob, Keiser, Realini, Escher and Amado²⁶⁾ found that potential for acrylamide formation varied from the variety Nicola at 2020 μg/kg to the variety Panda at 80 μg/kg; an 80-fold difference in acrylamide formation reflecting the large difference in the concentration of reducing sugars for the two varieties. In the following year the differences were similar although the absolute concentrations were different (1518 μg/kg for Nicola and 209 μg/kg for Panda). Pereira et al. ⁽Reference Pereira, Coffin, Yada and Souza Machado³¹⁾ reported an average of 340 mg reducing sugars/kg (range 180–460) in the processing variety Trent, and 1640 mg/kg (range 820–2470) for the variety Onaway which is much less suitable for processing. Amrein et al. ⁽Reference Amrein, Schönbächler, Rohner, Lukac, Schneider, Keiser, Escher and Amadò²⁷⁾ found that varieties especially bred for low reducing sugar content for crisps production (such as Lady Claire and Panda) maintained low concentrations of reducing sugars in the two years tested and also during adverse growing conditions with hot spells and regrowth leading to French fries with dark tips, the so called ‘sugar ends’. The mechanisms of cold sweetening are being elucidated through molecular techniques⁽Reference Sowokinos³²⁾ based on quantitative traits to assist breeding with molecular markers that show breeders in the laboratory that the desired trait is present or not in a genotype, without having to grow the plant in the field.

Discussion and conclusions: potatoes

The abundant occurrence of asparagine compared with reducing sugars (about 2·5 times molar abundance) and consequently an absence of a correlation between varying concentrations of asparagine and those of acrylamide after frying on the one hand and the strong correlation of acrylamide with reducing sugar concentrations on the other gave good reason for focusing on reducing sugars when looking for means of reducing acrylamide in potato products (although it should be kept in mind that a total removal of one of the precursors will be efficient irrespective of the initial abundance of any of the reaction partners, see the section ‘Health benefits of consuming whole grains compared with refined grains’ on asparaginase). The concentration of reducing sugars has three main interacting components: genetics, the physiological age of the tuber when harvested and storage conditions (especially temperature). These are summarized in Fig. 2.

Fig. 2

Schematic representation of reducing sugar concentration in potato tubers showing that high initial (pre-tuber maturity) reducing sugar levels lead to far higher levels at cold storage. The thin broken line represents tubers attached to the plant under ambient temperatures and the thick broken line represents post-harvest tubers stored at 9°C. Solid lines represent harvested immature (thin line) and mature (thick line) tubers stored at 4°C leading to cold sweetening. Dotted lines represent reconditioning at 18°C. Variety, levels of nutrients, weather and soil moisture determine tuber sugar levels during tuber growth and at harvest.

When tubers are physiologically younger at harvest they have higher concentrations of reducing sugars than in mature tubers, which rise higher during storage at low temperatures. Tubers are physiologically younger when harvested prematurely, i.e. before maximum yields and highest dry matter content have been achieved. Factors reducing the risks of harvesting too early are the use of earlier maturing varieties, and planting soils that allow late harvesting when conditions (especially rainfall) worsen. Selecting sites where harvests can be delayed may be a strategy for farmers and for procurement from which to source raw material. Less nitrogen and increased potassium application increase tuber maturity and reduce reducing sugar concentrations. High temperatures, that stop tuber growth even under moist conditions especially if re-growth follows when temperatures drop again, lead to physiologically younger tubers, which can partly be avoided by continued irrigation assuring unhampered growth.

Post-harvest treatments to reduce reducing sugar concentrations include an initial skin curing temperature for a few weeks of about 18°C followed by a storage period at 8°C for a few months. When prolonged storage is desired, lower temperatures of 4–5°C may serve followed by a reconditioning period of 3 weeks at 18°C. Where allowed, sprouting should be controlled chemically by spraying the crop before harvest or by treatment a few months after store loading. Irreversible senescence-sweetening is to be avoided by timely processing of susceptible lots.

Genetic resource

Asparagine concentrations in cereal species and varieties within a species vary widely. Elmore et al. ⁽Reference Elmore, Koutsidis, Dodson, Mottram and Wedzicha⁴⁰⁾ in a standardized procedure compared cakes made of potato flour, whole wheat and wholemeal rye cooked at 180°C from 5 to 60 min. The concentrations of asparagine and reducing sugars declined linearly with an increase of acrylamide formed during baking. After about 20 min the rye cake had formed about 3100 μg of acrylamide per kg dry weight cake after which it declined to less than 1600 μg/kg. Wheat cakes peaked at about 1100 μg/kg and declined to about 700 μg/kg. The greater ability of rye to produce acrylamide was related to the higher asparagine concentration in this cereal (634 mg/kg) compared with wheat (172 mg/kg). Asparagine, as a percentage of total free amino acids was also higher in rye (26 %) than in wheat (16 %). Table 2 also shows why in cereals, where sugars are abundant, asparagine (8 % of the molar concentration of reducing sugars in rye and 3 in wheat) is the limiting factor in acrylamide formation, whereas in fresh potatoes the asparagine concentration on a molar basis is more than double that of total reducing sugars and sugars are the limiting factor for acrylamide formation.

Table 2

Chemical composition of rye flour, whole wheat flour and potato flake, μmol/kg⁽Reference Elmore, Koutsidis, Dodson, Mottram and Wedzicha⁴⁰⁾

A CIAA paper⁽⁴¹⁾ reported free asparagine concentrations in wheat bran of 113 mg/kg, in wheat flour of between 267 and 434 mg/kg, depending on source (this is more than reported by Elmore et al. ⁽Reference Elmore, Koutsidis, Dodson, Mottram and Wedzicha⁴⁰⁾, levels tend to vary depending on author), in rye flour between 319 and 791 mg/kg which is similar as reported by Elmore et al. ⁽Reference Elmore, Koutsidis, Dodson, Mottram and Wedzicha⁴⁰⁾ in oat bran of 71 mg/kg and in oat flour of 50 mg/kg. Apparently oat flour and oat bran (48 mg/kg) have a similar asparagine content and a similar capacity to make acrylamide when baked. The same report mentioned that in another survey asparagine concentrations in 28 wheat samples varied between 150 and 450 mg/kg whereas 6 maize samples were all similar at about 70 mg/kg. From these data it can be concluded that wheat, maize, and rye or oats have decreasing asparagine concentrations and are likely to form less acrylamide when baked. Other data⁽⁴¹⁾, however showed that cake made in a standard process with dough in which the dominant cereal varied had the highest acrylamide when wheat dominated (about 400 mg of acrylamide/kg cake dry matter), about similar intermediate levels when maize and oat dominated (150 mg/kg) and lowest when made of rice (less than 100 mg/kg). For similar products made from potato and cereal (e.g. potato or maize chips), the acrylamide concentrations in potato crisps bought in supermarkets occasionally were above 1000 μg/kg whereas the maize (corn) crisps contained 55–498 μg/kg.

The differences in asparagine content between species are considerable, but also between varieties within a species. Taeymans et al. ⁽Reference Taeymans, Wood, Ashby, Blank, Studer, Stadler, Gonde, Van, Lalljie, Lingnert, Lindblom, Matissek, Muller, Tallmadge, O'Brien, Thompson, Silvani and Whitmore³⁹⁾ report a fivefold range in 31 wheat varieties grown mainly in the UK in 2002. For example, asparagine concentrations varied between 74 (variety Vault) and 664 mg/kg (variety Abbot). The correlation between asparagine concentration in wheat flour and acrylamide concentration of a baked product was examined using four wheat varieties with respectively 267, 321, 390 and 434 mg/kg asparagine (the reducing sugars concentration were very similar for the four varieties⁽⁴¹⁾). The resulting baked products had acrylamide concentrations of 656, 667, 923 and 874 μg/kg. The regression coefficient (R ²) for the correlation was 0·79, suggesting that almost 80 % of the variation in acrylamide formation was explained by the variation in asparagine content of the flour. Extended trialling may confirm and further strengthen the correlation⁽⁴¹⁾.

Breeding for varieties low in asparagine has possibilities, but little is known about the interaction between variety and environment so the stability of the trait is not known until environment × genotype studies are carried out. Genetic modification by blocking asparagine accumulation by altering asparaginase expression has been mentioned by a research group in the UK, but has not yet been officially published (J. S. Elmore, personal communication).

Environment

Different sites lead to different concentrations of asparagine in the flour and acrylamide in the finished product. The CIAA Report⁽⁴¹⁾ reported that an undisclosed processor baking wheat flour from four undisclosed regions in the same industrial process registered acrylamide concentrations varying between 13 and 796 μg/kg. Taeymans et al. ⁽Reference Taeymans, Wood, Ashby, Blank, Studer, Stadler, Gonde, Van, Lalljie, Lingnert, Lindblom, Matissek, Muller, Tallmadge, O'Brien, Thompson, Silvani and Whitmore³⁹⁾ reported asparagine concentrations in flour, e.g. variety Abbot from five different sites in the UK in 2005 of between 329 and 664 mg/kg. Three samples of the variety Claire varied between 163 and 232 mg/kg and five Rialto samples between 178 and 286 mg/kg. Occasionally the reason for site specific asparagine concentrations becomes evident: Lea et al. ⁽Reference Lea, Sodek, Parry, Shewry and Halford⁴²⁾ detected that increased saline concentrations were associated with higher asparagine concentrations, especially in sensitive wheat varieties. Muttucumaru et al. ⁽Reference Muttucumaru, Halford, Elmore, Dodson, Parry, Shewry and Mottram⁴³⁾ found decreasing asparagine concentrations in wheat grown in substrate or soil in both in vitro and ambient conditions subjected to increased levels of sulfur. Field trials reported by Granvogl et al. ⁽Reference Granvogl, Wieser, Koehler, Von Tucher and Schieberle⁴⁴⁾ showed a strong influence of sulphur fertilization on the amounts of free amino acids in wheat and correlation with baking properties as well as with 3-aminopropionamide and acrylamide generation during baking. Comparisons between years were not found but variation between years is not likely to be less than between sites.

Crop management

Crop management practices may include soil preparation, irrigation, crop protection and fertilisation. The effect of application of nitrogen fertiliser to increase protein content is very large. Figure 3 shows the impact of a range of nitrogen regimes on the percentage of protein in wheat flour⁽⁴¹⁾ and the high correlation between protein concentration and asparagine concentration. For low protein content Growers, to optimise yield, will tend to increase nitrogen inputs due to the positive correlation between fertilisation and grain yield.

Fig. 3

Correlation of asparagine level (mg/kg) with protein content for wheat grown over a range of nitrogen fertiliser regimes⁽⁴¹⁾.

No reports on the influence of storage duration and conditions on asparagine and reducing sugar concentrations in cereals were encountered. However, large variations would not be expected since cereals contain about 15 % water whereas potatoes contain about 77 % water. This would suggest that metabolic rates in cereals would be much lower than in potatoes during storage.

Discussion and conclusion: cereals

In cereals, acrylamide formation is linked to the concentration of asparagine in the flour used to make the finished product. The concentration of reducing sugars, depending on species, may be 10–50 times higher than that of asparagine. Rye has the highest content of asparagine, about fourfold that of wheat. Rice has the lowest content and maize and oats are intermediate. Bran of oats but especially of wheat has higher asparagine concentrations than the corresponding flour from these species. Asparagine concentrations in wheat depend on G × E × M interactions where, besides genotype, environment seems to be a more influencing factor than management. It is not clear from the material available what causes the difference in sites. Further research is needed to establish the relative contribution of weather patterns, availability of soil moisture and nutrients or interactions of these factors. Different sites and years will lead to such different environments. A possible approach to tackle this problem may be a ‘factor analysis’ in which possible relevant environmental data of known samples can be related to asparagine concentration.

Varietal differences have been demonstrated but within varieties environmental differences are great. Research might show the robustness of low asparagine content in certain varieties.

The only management practice to indirectly influence asparagine concentrations is nitrogen fertilisation. Research on direct relations between nitrogen application rates in field trials (rather than comparing sites and years where many more factors than nitrogen rate differ) and asparagine concentrations are needed to corroborate the reported findings. Moreover they should establish the trade-off between reduced yields and improved quality when lowering nitrogen application dose. Assessment of other managerial factors such as application of nutrients other than nitrogen, water and crop protection may also reveal an influence on asparagine concentrations.

Currently, until further research shows additional evidence, the best options are to look for genotypes low in asparagine, grown on sites that have shown reduced potential for asparagines accumulation and grown under a moderate nitrogen regime. These combinations are likely to result in low concentrations of asparagine in the raw material and less acrylamide formation during processing.

Environment

Arabica and robusta coffee plants grow under special climatic conditions. Arabica originates from the highlands of Ethiopia, and requires a cool, tropical climate with relatively high rainfall. Trees grow on acidic soil with good drainage properties under partial shade and require cycles of wet and dry periods to trigger flowering and ripening of the fruit. Robusta developed in the lowlands of the Congo River basin with high rainfall and short dry periods, at constantly high temperatures and humidity. Robusta trees are therefore bigger with larger leaves.

Cultivars of coffee are generally propagated by seed which germinate 4–5 weeks after sowing. Under optimal conditions, young plants from seedlings flower 12–15 months after planting, producing a first light crop 2·5 years after planting. Temperature and seasonal changes in humidity (water stress) have a critical influence on stages of dormancy, flowering and ripening of the cherries. Repeated cycles of pruning are necessary to maintain a uniform annual harvest. Since coffee plants are perennial, breeding cycles take 20–30 years⁽Reference Anzueto, Baumann, Graziosi, Piccin, van der Vossen, Illy and Viani⁴⁶⁾.

Discussion and conclusions: coffee

Since coffee plants are perennial and consequently have a long life cycle, and their cultivation requires very specific climatic conditions, efforts to mitigate a specific process contaminant such as acrylamide through agricultural practice would/will take considerable time in breeding studies. In addition, coffee is sourced worldwide, thus the implementation in the country of sourcing of new breeding practices once established experimentally will take another considerable amount of time. However, practices of post-harvest processing might be one possibility to exploit the feasibility with respect to specific reduction of precursor levels (asparagine).

Acrylamide contents in potato products

Since the start of the acrylamide discussion, heated potato products have been in the focus of public interest due to the high acrylamide contents detected in some of these products. Fried potatoes, e.g. French fries, potato wedges, hash browns (Rösti), oven-fried potatoes and crisps are the most relevant products in this topic, whereas boiled potatoes contain virtually no acrylamide.

As mentioned in previous sections, these high amounts are mainly caused by large contents of free asparagine as one of the main precursors in potato tubers. This kind of product therefore is frequently monitored by government food authorities and a large amount of published data are available [see for example, ⁽Reference Lineback, Wenzl, Ostermann, de la, Anklam and Taeymans⁵⁵⁾ or ⁽⁵⁶⁾ for Germany or ^{(57, 58)} for the USA]. Maximum concentrations of 2310 μg/kg for ready-to-eat French fries and 4215 μg/kg for crisps were published by German food authorities for 2005⁽⁵⁹⁾. Median values are 212 μg/kg and 363 μg/kg, respectively, indicating a broad range of measured values in this kind of products.

Heated potato products, e.g. French fries or chips, often are consumed as a part of daily diets, especially in the case of children or adolescent people. Therefore, these make a significant contribution to acrylamide intake⁽Reference Dybing, Farmer, Andersen, Fennell, Lalljie, Muller, Olin, Petersen, Schlatter, Scholz, Scimeca, Slimani, Tornqvist, Tuijtelaars and Verger²⁾. According to Wilson et al. ⁽Reference Wilson, Rimm, Thompson and Mucci⁷⁾ acrylamide intake from fried potatoes and potato crisps represents 35 % of total acrylamide intake in USA and up to 46 % in The Netherlands. Biedermann-Brem et al. ⁽Reference Biedermann-Brem, Noti, Grob, Imhof, Bazzocco and Pferfferle⁶⁰⁾ estimated an additional intake of acrylamide of about 30 μg per day caused by one single portion of Rösti (250 g) per month.

Due to the high amounts of acrylamide measured in potato products and their significant contribution to daily acrylamide intake, many minimization efforts have been concentrated on these kinds of products. However, it has to be taken into account that heated potato products are also perceived as high quality products with respect to sensory properties. Therefore, lowering of the high quality standards established during decades is to be avoided when acrylamide mitigation is aimed at. In particular, colour (browning) and crispness are such quality parameters. The strong correlation between acrylamide content and degree of browning, which was found soon after detection of acrylamide in food, has driven the first measures to avoid an over-browning of these products. These include frying recommendations in the case of homemade French fries⁽Reference Grob, Biedermann, Biedermann-Brem, Noti, Imhof, Amrein, Pfefferle and Bazzocco⁶¹⁾ or in removing over-coloured pieces in industrial processes. Nevertheless, process interventions which result in pale and soft products due to insufficient cooking and evaporation conditions arising from low temperatures or short frying times are perceived as low quality and, therefore, are not acceptable for the food industry. Such approaches will not be discussed below.

To obtain potato products with a desired and expected degree of browning and low acrylamide contents, the kinetic de-coupling of different paths of Maillard reactions remains the main challenge for this kind of product. Some of the published approaches in this very complex topic will be described and discussed below. It is not intended to discuss all known interactions regarding acrylamide formation during potato processing. Only approaches for an aimed reduction verified under lab-scale or industrial conditions will be considered.

A summary of mitigation measures including possible adverse effects of these measures on nutrition and health is given at the end of the chapter.

French fries and similar products

This section deals with the French fries type potato products characterized by a crispy outer texture and a soft mealy core. This means that these products retain their moist core where the temperature during processing does not exceed 100°C. Acrylamide formation is limited to the outer dehydrated layers⁽Reference Franke, Sell and Reimerdes⁶²⁾. During frying, large temperature and moisture gradients exist in these products⁽Reference Gökmen, Palazoglu and Senyuva⁶³⁾ resulting in significantly different local reaction conditions. Accordingly mitigation measures for acrylamide also have to consider local differences, especially the material exchange processes at the interface between product surface and surrounding frying oil, e.g. water evaporation and oil penetration, play an important role for chemical reactions in the outer product layers. In contrast, measured acrylamide concentrations are averaged over the whole product.

It has to be kept in mind that French fries often are industrially par-fried and sold in a frozen state especially for private households or in a refrigerated state for catering and restaurants. At this stage the acrylamide contents are very low or negligible. The majority of acrylamide is formed during final preparation in the household or restaurant, and, therefore, outside of the supervision of industry itself. Such a multi-step preparation process complicates the application of mitigation measures for this product category.

Raw material influence and pre-treatment before processing

With respect to potatoes, the very strong correlation of acrylamide formation and precursor contents (reducing sugars) has been confirmed by numerous authors and has been discussed in detail above. Normally, concentrations of the precursor free asparagine exceed the amounts of reducing sugars, so that a slight variability in asparagine contents only has negligible effects on acrylamide formation in potatoes. However, in the case of relative low contents of free asparagine in potatoes, this precursor will also control the final acrylamide contents as shown by Franke and Reimerdes⁽Reference Franke and Reimerdes⁶⁴⁾.

In addition to selection of appropriate farming and storage conditions, pre-treatments of potato sticks during industrial fabrication of par-fried French fries and similar products can be considered for mitigation measures⁽³⁾. In particular, blanching and pre-drying of the sticks before par-frying are of interest with respect to possible interventions. Obvious measures are the reduction of precursors at the product surface and control of Maillard reaction pathways to suppress acrylamide formation during browning. The first way includes reduction of precursors by leaching, e.g. blanching, and by enzyme treatment. The enzymatic reduction of free asparagine by asparaginase in French fries has been described⁽Reference Heldt-Hansen, Rittig, Budolfsen Lynglev, Stringer and Ostergaard^65–Reference Elder, Fulcher and Leung⁶⁷⁾. Heldt-Hansen⁽Reference Heldt-Hansen, Rittig, Budolfsen Lynglev, Stringer and Ostergaard⁶⁵⁾ found acrylamide reductions of 80 % compared with untreated samples in lab scale experiments. Regarding sources and legal status of asparaginase application for reduction of amino acid asparagine see the section ‘Health benefits of consuming whole grains compared with refined grains’. The elimination of free amino acids has also been proposed by Tricoit et al. ⁽Reference Tricoit, Salucki and Rousset⁶⁸⁾ using silica gel as an adsorbing agent.

A shift of Maillard reaction pathways away from acrylamide generation can be achieved by modification of processing conditions during heating. One possible measure is lowering pH-value which reduces acrylamide formation. Pedreschi et al. ⁽Reference Pedreschi, Kaack and Granby⁶⁹⁾ immersed potato strips in a 1 % citric acid solution and found an average reduction of 53 % in acrylamide formation. Jung et al. ⁽Reference Jung, Choi and Ju⁷⁰⁾ reported reductions of about 80 % after soaking for about 1 h in 1 % citric acid. Baardseth et al. ⁽Reference Baardseth, Blom, Enersen, Skrede, Slinde, Sundt and Thomassen⁷¹⁾ also claimed that acid treatment of uncooked French fries prior to heating reduced the acrylamide content of the fried product. However, such a pre-treatment leads to distinct changes in sensory properties and is limited to special applications, e.g. French fries consumed with vinegar. Another possibility is the modification of the ionic strength in the outer layers, e.g. by soaking in salt (NaCl) solutions⁽Reference Reimerdes and Franke^72–Reference Lindsay and Sungjoon⁷⁴⁾. Reductions of up to 40 % due to salting were found for French fries. Earlier investigations indicated lower fat uptake of French fries after soaking in salt solutions and similar sensory properties compared with French fries salted after frying⁽Reference Bunger, Moyano and Rioseco⁷⁵⁾. Another approach to shift reaction pathways is the addition of other amino acids⁽Reference Elder, Fulcher and Leung^76–Reference Low, Koutsidis, Parker, Elmore, Dodson and Mottram⁷⁹⁾ or thiol components⁽Reference Elder⁸⁰⁾.

An enhanced pre-drying of blanched French fries before frying to reduce the frying time while maintaining similar quality with respect to browning and crispness can contribute to lower acrylamide contents in the final products⁽Reference Franke, Sell and Reimerdes⁶²⁾. In lab-scale experiments reductions of up to 75 % by application of salt treatment in combination with pre-drying were found. Recently, Erdogdu et al. ⁽Reference Erdogdu, Palazoglu, Gökmen, Senyuva and Ekiz⁸¹⁾ stated that application of microwave pre-treatment reduced acrylamide concentrations in French fries by up to 60 % compared with unblanched products. It has to be kept in mind that these reductions are measured in lab-scale experiments and possible effects of these measures in the industrial scale have to be checked for every product including its special quality requirements and for the processing lines.

Processing

The total heat load of French fries during processing, i.e. temperature and duration of frying, seems to have the major impact on acrylamide formation⁽Reference Matthäus, Haase and Vosmann⁸²⁾. Therefore, lower frying temperatures combined to slight extension of the frying time to obtain the desired degree of browning and crispness have been proposed⁽Reference Grob^83, Reference Fiselier, Bazzocco, Gamma-Baumgartner and Grob⁸⁴⁾. Application of these specific and selective interactions have been utilized during the very early stage in discussions on reduction of acrylamide and, in the case of French fries, included in preparation instructions for the consumer on packaging. A two-step frying process with lower temperatures in the second step to avoid excessive acrylamide formation has been postulated by Barry et al. ⁽Reference Barry, Burnham, Maganlal Desai, Joseph, Kin-Hang Leung, Masson, Mohan Rao, Saunders, Stalder and Topor⁸⁵⁾

Potato crisps, including fabricated crisps

In French fries a major part of the moisture is retained in the soft core limiting the core temperature to approximately 100°C, whereas in potato crisp products moisture of the whole product is normally lowered to levels below 2·5 % resulting in core temperatures above 120°C. Therefore, acrylamide is formed through the whole product resulting in higher contents than in French fries despite shorter frying times. This very low moisture is necessary for the desired texture properties and storage stability of these types of potato products.

Generally, similar mitigation measures as discussed for French fries are applicable but modifications are necessary due to different product sizes and manufacturing processes. Especially in the case of fabricated crisps prepared from dried and powdered potato components, e.g. granules or flakes, and other ingredients, extended possibilities for pre-treatments are available for potato processors.

Raw material influence and pre-treatment before processing

The raw material influence of potato tubers has been discussed above. However, due to the thinner product sizes compared with French fries, pre-treatments on the surface, e.g. soaking or fermentation, are more effective.

Elder et al. ⁽Reference Elder, Fulcher and Leung⁷⁶⁾ suggested an addition of other amino acids to potato crisps to prevent higher acrylamide contents in the fried crisps. Additional blanching steps or soaking of the slices before frying which have diluting effects on precursors, can also contribute to lower acrylamide contents in the final product⁽Reference Matthäus, Haase and Vosmann^82, Reference Pedreschi, Kaack and Granby⁸⁶⁾. However, for shorter blanching times, which is the reality in industrial scale, these effects have not been found⁽Reference Wicklund, Ostlie, Lothe, Knutsen, Brathen and Kita⁸⁷⁾. Blanching of potato crisps can affect their final texture compared with untreated crisps. Additionally, the enzymatic degradation of reducing sugars (e.g. by glucose oxidase) before frying has been proposed in a patent specification as a way to reduce acrylamide formation⁽Reference Howie, Yau Tak Lin, Zyzak and Schafermeyer⁸⁸⁾. But no scientific publications are available on effects of enzymatic lowering of reducing sugars on acrylamide concentration and on product quality (browning, taste).

Furthermore, fermentation of crisps surfaces by micro-organisms, e.g. yeasts, to reduce precursors for acrylamide formation have been proposed⁽Reference Awad and George^89, Reference Gröting, Lichtsinn, Meuser and Jank⁹⁰⁾. Awad⁽Reference Awad and George⁸⁹⁾ claims reductions in acrylamide formation of up to 81 % compared with untreated samples for fabricated potato crisps fermented by a mix of bacteria and yeasts. In the case of fabricated crisps, the exchange of potato components by other raw material, having much lower contents of free asparagine, e.g. wheat starch, has been recommended⁽³⁾.

Processing

The influence of frying oil temperature is more significant for potato crisps compared to French fries due to the higher final product temperatures. Application of lower oil temperatures requiring longer frying times for dehydration reduces final acrylamide concentrations in the product⁽Reference Haase, Matthaus and Vossmann⁹¹⁾. Granda et al. ⁽Reference Granda, Moreira and Tichy⁹²⁾ reported acrylamide reduction rates of up to 90 % with the use of low-temperature vacuum frying (125°C instead of 180°C) in lab-scale. It has, however, also to be considered that lower frying temperatures often increase oil uptake during frying⁽Reference Saguy and Pinthus^93, Reference Ufheil and Escher⁹⁴⁾.

In contrast, very high frying temperatures above 200°C can reduce final acrylamide concentrations due to intensified degradation reactions of acrylamide⁽Reference Taubert, Harlfinger, Henkes, Berkels and Schomig^95, Reference Gökmen and Senyuva⁹⁶⁾. Therefore, ‘flash frying’ at very high temperatures with very short times can also contribute to lower acrylamide contents in potato crisps⁽³⁾.

Summary and conclusions

Table 3 lists mitigation measures for potato products mentioned in the literature and possible side effects with respect to nutrition and health, whereas possible effects on product quality, texture, taste, colour, are not fully considered. It can be assumed that all manufacturers will adapt their process interventions for acrylamide reduction on their own quality standards to make sure that their products maintain consumer acceptance. Therefore, mitigation measures resulting in distinct quality modifications will not be implemented in industrial scale by the food processors.

Table 3

List of potato products* and mitigation measures, which have been tested in labscale or pilotscale experiments

* For each product category qualities including taste and texture may also alter significantly.

† Reduction percentages are related to untreated samples.

It should be mentioned that the individual measures or interventions cannot be regarded as completely superimposable under industrial conditions because all percentage reductions are related to untreated control samples. This means that a combination of different interventions will be lower than the sum of each individual intervention relative to an untreated control. In addition, the possibility of interactions should be considered. Therefore, a simple addition of percentage reductions is not meaningful and all combinations of mitigation interventions have to be tested and verified under industrial conditions both with respect to achievable acrylamide reduction and with respect to possible quality impacts. In some cases, process simulations may be useful to predict effects of combinations of interventions on resulting acrylamide reduction⁽Reference Reimerdes and Franke⁷²⁾.

Due to the high complexity of the Maillard reaction pathways during cooking in real food systems with their heterogeneous composition, the understanding of formation of desirable and undesirable substances and their contribution to quality, nutrition and health relevant effects in human bodies is very incomplete and not sufficient for a fundamental assessment of these food heating processes. In particular the discovery of potential health promoting substances formed during heating, such as pronyl-lysine⁽Reference Lindenmeier, Faist and Hofmann⁹⁷⁾ and others, illustrates the need for further fine tuning of the heating process in order to eventually arrive at processing conditions that favour the generation of health promoting, at the expense of undesired substances⁽Reference Pedreschi, Kaack and Granby⁸⁶⁾.

Acrylamide content in cereal products

JRC (DG Joint Research Centre), in collaboration with CIAA, has set up a database collecting European monitoring data of the acrylamide content of various foods. To date, the database comprises 7150 data sets (latest update June 2006⁽⁵⁷⁾). In this database, cereal products are roughly classified into different food types: fine bakery ware (covers all types of biscuits, except infant biscuits and biscuits for diabetics), ginger bread, crisp bread (all kinds; rye, wheat, sesame, etc.), infant food, diabetics' products, and breakfast cereals. Since the acrylamide concentration in bread is relatively low, soft bread is not specified as a cereal product class in this database. However, high intake of a low acrylamide product can also form an important contribution to total acrylamide intake. According to the USFDA-CFSAN⁽⁹⁸⁾ mean acrylamide intake from soft bread was 0·019 μg/kg bw per day; this was considerably lower than intake from cookies (0·036 μg/kg bw per day), breakfast cereal (0·043 μg/kg bw per day) or French fries (0·058 μg/kg bw per day)⁽⁹⁹⁾. For the overall Dutch population cereal products contribute about 30 % of the dietary exposure to acrylamide⁽Reference Boon, de, van, van, Brette and van Klaveren¹⁰⁰⁾. Therefore, mitigation of acrylamide concentrations in cereal products can play a vital role in the reduction of exposure to acrylamide. Table 4 summarises mitigation measures for cereal products with potential levels of reduction and side effects.

Table 4

List of cereal products* and mitigation measures, which have been tested in labscale or pilotscale experiments

* For each product category qualities including taste and texture may also alter significantly.

† Reduction percentages are related to untreated samples.

‡ CBP = Chorleywood bread process, a rapid mechanical dough development method widely used in the UK.

Recipe

Several alterations in the recipe can lower the amount of acrylamide. For example, addition of trace amounts of trace elements such as Ca, Mg, and Zn is considered to reduce acrylamide concentrations. Addition of cocoa powder and glycine at relatively low concentrations results in acrylamide reduction. However, not all possible measures have been suitably evaluated for their impact on organoleptic factors. Also a benefit-risk evaluation is necessary before implementing these measures.

In laboratory scale experiments for fine bakery ware, ginger bread and crisp bread, the replacement of ammonium bicarbonate with sodium bicarbonate can lead to a reduction in acrylamide of up to 70 %⁽Reference Graf, Amrein, Graf, Szalay, Escher and Amado¹⁰¹⁾ and >60 % when combined with addition of organic acid⁽Reference Amrein, Schonbachler, Escher and Amado¹⁰²⁾. This substitution is easy to incorporate in the process, however, the taste of the product may be affected, and the disadvantage of higher sodium concentrations has to be assessed in the context of public health strategies aimed at reducing sodium intake.

Similar to lowering the amount of sugar to reduce the amount of acrylamide being formed, the substitution of reducing sugars by non-reducing sugars leads to a reduction of approximately 60–70 %⁽Reference Graf, Amrein, Graf, Szalay, Escher and Amado^101–Reference Vass, Amrein, Schönbächler, Escher and Amado¹⁰³⁾, however, browning of the product is insufficient after this replacement, leading to altered product characteristics. This measure has been successfully evaluated in fine bakery ware, ginger bread and crisp bread. Addition of citric acid has been tested in fine bakery ware, however, the reduction by this measure was relatively small at only 30 %⁽Reference Graf, Amrein, Graf, Szalay, Escher and Amado¹⁰¹⁾, although product integrity remained intact.

Processing

Extended yeast fermentation reduces the amount of acrylamide in crisp bread. However, this may alter product properties considerably. A potential risk is the formation of 3-monochloropropane-1,2-diol (3-MCPD) that is brought about by the increase in glycerol in combination with an acidic pH⁽Reference Hamlet and Sadd¹⁰⁴⁾. Thus in this case, reducing acrylamide concentrations by extended fermentation processes will result in increased concentrations of another potentially hazardous compound. Since the chemical reaction that forms acrylamide from asparagine is a result of high temperature, lowering the thermal input may lead to a substantial decrease in acrylamide. However, shorter preparation time and lower temperature lead to a product that spoils easily. Further, product assets like colour, flavour, and texture may change dramatically.

Addition of asparaginase as a processing aid to the raw product is a very promising mitigating measure⁽Reference Heldt-Hansen, Rittig, Budolfsen Lynglev, Stringer and Ostergaard^65, Reference Amrein, Schonbachler, Escher and Amado^102, Reference Vass, Amrein, Schönbächler, Escher and Amado^103, Reference Heikoop¹⁰⁵⁾. The naturally occurring enzyme converts asparagine into aspartic acid. As a result the amount of acrylamide in bread crust is decreased by 36–75 % and more than 80 % in crackers, cakes, cookies and Dutch honey cake⁽¹⁰⁶⁾. Asparaginase is produced by Aspergillus niger ⁽¹⁰⁷⁾ or Aspergillus oryzae ⁽¹⁰⁸⁾ and can be applied to an abundant variety of products. Aspergillus strains have been used for decades to produce food and feed enzymes, further, they produce asparaginase naturally. Many food enzymes, including those from Aspergillus, have been evaluated and accepted by JECFA, FDA and European national authorities. Also the GRAS notice on asparaginase as produced by Aspergillus niger ⁽¹⁰⁶⁾ has received no objections from FDA and furthermore its safety is endorsed by the French Food Safety Authority, AFSSA. The use of asparaginase has several advantages; relatively little amounts are needed for a significant reduction in acrylamide level, the product characteristics and organoleptic properties remain intact, as for potatoes no recipe changes are needed for cereals, addition of asparaginase does not affect the nutritional value of the product, nor diminishes specific beneficial properties like antioxidant capacity, and addition of the enzyme is easily incorporated into most industrial baking processes.

End product characteristics

Consumers could be advised to lower their demands for colour, texture, and flavour. When they accept a product with a lighter colour, a shorter shelf life, and less crispy, baking time and temperature could be reduced and consequently acrylamide levels would be lowered.

Discussion and conclusion

Lowering baking temperature or shortening baking time can effectively reduce the acrylamide content of food items. However these measures also significantly affect product characteristics. Selecting raw materials low in asparagine or other types of grain also changes product quality and may lower nutritional value. Several activities by CIAA and industry are ongoing to develop effective measures for acrylamide reduction without affecting product integrity. Examples of this type of measures are the use of sucrose instead of reducing sugars, and the use of asparaginase as a processing aid. A summary of possible mitigation processes is shown in Table 5.

Table 5

Acrylamide content of cereal products, possible reduction, mitigation measure used and sustainability of product quality

* AA, Acrylamide. Acrylamide concentrations adapted from JRC database⁽⁵⁶⁾.

† Median: the acrylamide content of 50 % of the considered samples were reported below that value.

‡ Change composition of breakfast cereals; avoid use of asparagine-rich grains like wheat bran. Limitation: wheat is an important source of fibre and other beneficial nutrients.

Recipe (including blending)

Coffee is considered a single ingredient product, manufactured by roasting only the green beans, i.e. without the addition of other basic ingredients. However, blending of coffees (different varieties, differently processed, from different provenances) is used to generate coffee with consistent quality and certain mechanical factors such as formation of crema during the extraction of espresso coffees. Blending can be done before or after roasting⁽Reference Bee, Brando, Brumen, Carvalhaes, Koelling-Speer, Suggi Liverani, Teixeira, Teixeira, Thomaziello, Viani, Vitzthum, Illy and Viani⁴⁷⁾. More complex mixtures ensure the maintenance of quality when some ingredients change. Consequently, also any changes in the composition with respect to acrylamide precursor concentrations in one coffee variety will be diluted in the blend, unless these changes are realised in other or all of the coffee components in the mixture. Thus, alternatives to reduce acrylamide by changing the recipe or blend are very limited.

Most of the procedures applied in processing of potato products or cereals are not applicable to coffee, such as changes in pH, addition of ingredients that may compete with asparagine in the Maillard reaction (glycine, di- and tri-valent cations) or fermentation⁽³⁾. Although some of the patent applications seem to involve the addition of compounds that may compete with the formation of acrylamide in the Maillard reaction, these methods have not been exploited experimentally and no quantitative data on acrylamide reduction are available.

Processing – roasting

Roasting of coffee is a complex process, involving transfer of heat through the three-dimensional structure of the bean, transport of water vapour, transport of CO₂ and volatiles generated in roasting, and changes of volume, structure, material and chemical composition. Studies on acrylamide formation during roasting demonstrated that concentrations rapidly increase to high levels that peak after approximately 70 % of the roasting time, and then decrease again⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen^25, Reference Taeymans, Wood, Ashby, Blank, Studer, Stadler, Gonde, Van, Lalljie, Lingnert, Lindblom, Matissek, Muller, Tallmadge, O'Brien, Thompson, Silvani and Whitmore^39, Reference Senyuva and Gökmen¹⁰⁹⁾. At the point where roasting is finished, acrylamide concentrations are reduced to 20–30 % of the maximum peak levels. Dark roast coffees, therefore, have a slightly lower average acrylamide content than regular or lighter roast coffees.

Table 7 illustrates the variability of acrylamide in roast and ground coffees from the market. The data variability is largest in the EU monitoring database⁽⁵⁷⁾, since it compiles data from a wide range of coffees from the European market analysed in different laboratories, using different analytical methods. Studies performed within a more limited scope with specific analytical methods show much less data variability.

Table 7

Variability in acrylamide concentration (μg/kg) in roast and ground coffee

Throughout the European market, differences in consumer preferences exist regarding the roasting degree of coffees. In Germany, light to medium roast coffees are preferred, whereas in southern/Mediterranean Europe, consumers favour darker roast coffees. The degree of roast is inversely linked to acrylamide concentration in the roasted coffee, ranging from ~350 μg/kg in very light roast down to ~200 μg/kg in very dark roast⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen²⁵⁾. In addition, radical scavenging activity (as measured by Trolox activity) similarly decreases with higher roasting grade⁽Reference Summa, De la Calle, Brohee, Stadler and Anklam¹¹⁰⁾. In Spain, sugar is added during the roasting to achieve a stronger aroma (torrefacto coffees⁽Reference Clark¹¹¹⁾). No information is available on the impact of sugar coating on acrylamide formation. However, since sugars are not the limiting factor for acrylamide formation, the impact of the torrefacto process is expected to be low.

Robusta beans contain slightly higher amounts of acrylamide than arabica beans when roasted under similar conditions, a fact that is thought to be related to the slightly higher content of free asparagine in the green robusta beans⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen^25, Reference Murkovic and Derler⁵³⁾. In addition, this variation can also result from different processing practices that are traditionally applied to arabica and robusta green beans (harvest, washing, drying, see above) that also have an impact on the free amino acid profile in general and the free asparagine content in particular before roasting.

The amount of carbohydrates does not seem to be correlated to the amount of acrylamide formed after roasting. The limiting factor in the formation of acrylamide in coffee appears to be the concentration of free asparagine rather than the concentrations of reducing sugars in green coffee beans⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen²⁵⁾. The correlation is, however, weak, because the formation, which initially may be well correlated to the asparagine precursor concentrations, is later reduced by the destruction of acrylamide in the roasting process. No acrylamide is found in green coffee beans. Only a small fraction [less than 0·2 % on a molar basis, calculated from ⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen²⁵⁾] of asparagine is converted to acrylamide. Virtually all free amino acids are, however, consumed during roasting and participate in the generation of aroma and flavour.

Process parameters

The following parameters were investigated for their impact on the formation of acrylamide during roasting:

• ● method of heat transfer (convection or conductivity)

• ● roasting speed (1·5–16 min)

• ● degree of roasting (very light to very dark)

• ● cooling (air, water)

• ● moisture content of green coffee (7–14 %)

Due to the formation kinetics of acrylamide, degree and speed of roasting have an effect on acrylamide formation if analyzed over the whole process range including under- and over-roasting⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen²⁵⁾. At the same time these parameters are key to the organoleptic properties. For this reason there is only a narrow window for variation due to significant impact on the product characteristics, and acrylamide does not significantly change within that acceptable window. The method of heat transfer, cooling and initial moisture content did not affect the acrylamide concentration of the roasted product⁽⁴¹⁾.

Mitigation (patents)

Several patents have been filed that claim a reduction of acrylamide in several processed foods including coffee. One of the patents involves the enzymatic hydrolysis of asparagine using asparaginase⁽Reference Dria, Zyzak, Gutwein, Villagran, Young, Bunke, Lin, Howie and Schafermeyer¹¹²⁾. While trials using asparaginase have been successfully applied to the processing of fabricated potato crisps and cereals, no data are available on coffee processing. A reduction was furthermore claimed by Elder et al. ⁽Reference Elder, Fulcher and Leung¹¹³⁾, but no method was disclosed. A patent filed by Purtle et al. ⁽Reference Purtle, Eyal and Vitner¹¹⁴⁾ involves the addition of a reducing sugar and a reagent containing an amino acid to the raw beans. Sahagian and van Eijck⁽Reference Sahagian and van Eijck¹¹⁵⁾ claim to reduce acrylamide while retaining the organoleptic properties of the cooked food by the treatment with glycine prior to heating. The practical applicability of these methods and their impact on the final coffee product, however, remains to be established.

Storage of roasted coffee

Acrylamide concentrations in roasted coffee were reported to decrease with storage time (Table 8). Delatour et al. ⁽Reference Delatour, Perisset, Goldmann, Riediker and Stadler¹¹⁶⁾ reported a decrease from 203 to 147 μg/kg in roasted coffee over a 7 months period. Similar results were reported by Andrzejewski et al. ⁽Reference Andrzejewski, Roach, Gay and Musser¹¹⁷⁾ Levels of acrylamide decreased from 305 to 210 μg/kg and from 285 to 200 μg/kg in roasted and ground coffee and roasted coffee beans, respectively, after 3 months of storage⁽Reference Hoenicke and Gatermann¹¹⁸⁾. No decrease was however, observed in soluble coffee after several months of storage⁽Reference Delatour, Perisset, Goldmann, Riediker and Stadler^116, Reference Hoenicke and Gatermann¹¹⁸⁾.

Table 8

Loss of acrylamide during storage of coffee

Recently, the loss of acrylamide during storage of roast and ground coffee was investigated in more detail over time⁽Reference Lantz, Ternite, Wilkens, Hoenicke, Guenther and van der Stegen²⁵⁾. The loss was greatest during the first 2–3 months of storage, and depended on the storage temperature. Higher storage temperature (+37°C) led to a more rapid and more efficient decrease, while low temperatures ( − 18 to +4°C) had only a minor effect. The reported decrease from 350 down to 200 μg/kg acrylamide at room temperature after 3 months of storage is consistent with results previously shown by others (see Table 8). The mechanism behind this loss is currently unknown. One study indicates that acrylamide is (by an as yet unknown mechanism) bound to the coffee matrix during storage, since spiking of roast and ground coffee with radioactively labeled acrylamide showed an increased retention of radioactivity in the coffee matrix after brewing with increased storage time⁽Reference Böhm, Baum and Eisenbrand¹¹⁹⁾. It has to be stressed that reaction mechanisms that result in the reductions during storage are not an option to date to reduce acrylamide concentration in coffee, since it is directly linked to quality and organoleptic properties and, consequently, consumer acceptance. In addition, the storage loss of acrylamide is already to a certain extent occurring in the market, since coffees are already stored for some time before they reach the consumer. However, for the purpose of this paper, and since it is the only quantitative measure currently available, a partial decrease of 20 % is used in the modelling approach in order to assess the impact of acrylamide mitigation in coffee on human exposure and MOE.

Formation of other process related compounds

Important changes in chemical composition occur upon roasting of coffee, including caramelisation and degradation of carbohydrates, denaturation of proteins and reaction of free amino acids with carbohydrates in the Maillard reaction, leading to the formation of volatile and non-volatile aroma compounds, melanoidins, aldehydes, acids, etc. These reactions are discussed in detail elsewhere⁽Reference Clarke and Vitzthum^52, Reference Illy and Viani^120–Reference Charles-Bernard, Roberts and Kraehenbuehl¹²³⁾. Some process-related compounds, which are known for decades to occur in roasted coffee are discussed below. These compounds may be of importance in a risk-risk analysis of measures to reduce acrylamide. Quantitative data with respect to acrylamide reduction measures are unavailable to date, and hence will not be discussed within the scope of this publication.

3-Monochloropropane-l,2-diol (3-MCPD) was originally known to occur in acid hydrolysed vegetable proteins and soy sauce, but traces were also found in toasted bread and roasted coffee. Like acrylamide, 3-MCPD formation appeared correlated with the colour of roasted coffee beans, while arabica coffees contained lower 3-MCPD concentrations than robusta coffees⁽Reference Dolezal, Chaloupska, Divinova, Svejkovska and Velisek¹²⁴⁾. Furan (and various derivatives) is long known to be present in roasted coffee⁽Reference Maga¹²⁵⁾ and was more recently found to be formed in various heat-treated foods. Concentrations in coffee brew amounted to between 10 and 20 μg/kg. Since furan is highly volatile, its concentration decreases in coffee brew over time⁽Reference Goldmann, Perisset, Scanlan and Stadler¹²⁶⁾. Several groups have reported its occurrence in coffee⁽Reference Ho, Yoo and Tefera^127, Reference Hasnip, Crews and Castle¹²⁸⁾ and databases were generated by the EU (updated 2006) and FDA (updated 2006). Information on the conditions and kinetics of furan formation are scarce, but sugars seem to be important precursors [⁽Reference Mark, Pollien, Lindinger, Blank and Mark¹²⁹⁾ and references therein]. The oxidation of polyunsaturated fatty acids at elevated temperatures and the decomposition of ascorbic acid derivatives were also discussed to play a role⁽Reference Becalski and Seaman¹³⁰⁾. 5-Hydroxymethylfurfural (HMF) is known to be formed by dehydration of carbohydrates under acidic conditions in various foods, including roasting of coffee [⁽Reference Murkovic and Pichler¹³¹⁾ and references therein]. HMF is also considered an indicator of quality deterioration, as a result of excess heating or storage, and might indicate adulteration of honey with invert sugar syrup. HMF content is expected to increase with increased roasting.

Discussion and conclusions

No information is available on the reduction, nor the impact of reduction, of individual free amino acids and of asparagine, on organoleptic properties or the resulting acrylamide concentrations after roasting. Since free amino acids and proteins are strongly involved in formation of aroma and flavour, the quantity and type of amino acids directly affect the intensity and quality of aroma. Since the limiting factor for acrylamide formation seems to be the free asparagine content, the factors influencing the concentrations of the reducing sugar precursors fructose and glucose are of less importance.

Many studies have been performed on compositional changes during coffee roasting. The reactions taking place are complex, and involve many biologically/chemically active compounds. Up to now, one option to reduce acrylamide in coffee, that could potentially be made use of to a limited extent is its loss during storage. The practicability of this measure is, however, questionable. For the purpose of this work, as a working number a potential to reduce acrylamide by storage loss of 20 % was used. Grob et al. ⁽Reference Grob⁸³⁾ recently acknowledged that for coffee, no significant further mitigation appears in sight. The variations in acrylamide concentration in coffee are related to the roasting degree, which closely resembles the market range and consumer preferences in Europe. The formation of other process-related compounds has been studied for some time, the formation of which partially depends on precursors that are also involved in acrylamide formation (sugars). The quantitative relationship between these compounds and acrylamide formation during roasting has not been extensively exploited to date.

Carbohydrates

Sucrose is the major low molecular weight carbohydrate in green coffee, making up 7 % of the dry mass. Sucrose content is slightly higher in arabica coffee. Sucrose is rapidly degraded upon roasting, resulting in the formation of low molecular weight fragments and aliphatic acids (formic, acetic, glycolic and lactic acid). Sucrose is also consumed in caramelisation (dehydration) reactions forming important heterocyclic compounds (including aroma compounds) that can further polymerise to form melanoidin-type compounds. Furthermore, sucrose can react with amino acids or proteins in the Maillard reaction to form low or high molecular weight melanoidins that are responsible for the generation of the brown colour, taste, and volatile aroma compounds⁽Reference Arya and Rao¹²¹⁾. The monosaccharide content is relatively low (1 % in total of glucose, fructose, raffinose, stachyose, galactose, arabinose, rhamnose, mannose⁽Reference Knopp, Bytof and Selmar⁵⁴⁾). Glucose and fructose, which are also degradations products of sucrose, are rapidly degraded upon roasting⁽Reference Arya and Rao¹²¹⁾.

The high molecular weight fraction of green coffee carbohydrates mainly consists of arabinoglactan polymer, mannan and/or galactomannan and cellulose. Total polysaccharides make up around 50 % in green coffee beans and are degraded by up to 40 % depending on roast degree. Polysaccharides contribute to organoleptic properties such as mouth feel, viscosity and foam stability⁽Reference Fischer, Reimann, Trovato and Redgwell^132–Reference D'Agostina, Boschin, Bacchini and Arnoldi¹³⁴⁾. The insoluble hemicellulose (23–24 % of DM) and cellulose (13 % of DM) are largely unaffected by roasting, whereas structural changes (such as reduced degree of polymerisation) occur in arabinogalactans, galactomannans and mannans, leading to improved extractability⁽Reference Arya and Rao¹²¹⁾.

Acids

The profile of acids in coffee changes during roasting, while acidity increases. Main acidity is generated at the beginning of roasting, indicated by a fall in pH. Acids are responsible for 11 % of green bean weight, and 6 % of roasted bean weight. Acidity is also recognized as an important attribute of sensory quality (fine acidity of arabica)⁽Reference De Wilde, De Meulenaer, Mestdagh, Govaert, Vandeburie, Ooghe, Fraselle, Demeulemeester, Van, Calus, Degroodt and Verhe^38, Reference Ginz, Balzer, Bradbury and Maier¹³⁵⁾. Important acids in green coffee are chlorogenic, quinic, malic and citric acid, which also vary between different coffee varieties and origin. Citric, malic and chlorogenic acid decrease during roasting, while quinic acid increases due to the degradation of chlorogenic acid. Other aliphatic acids such as formic, acetic, glycolic and lactic acid are formed as products of sucrose degradation⁽Reference Ginz, Balzer, Bradbury and Maier¹³⁵⁾. Acidity of coffee also increases upon storage of coffee brew with time and temperature⁽Reference Verardo, Cecconi, Geatti and Giumanini¹³⁶⁾.

Oil

Oils make up around 15 % of dry mass in arabica beans, and 10 % in robusta beans. The loss of actual lipid matter upon roasting is low. Important components of the oil fraction are the coffee diterpenes cafestol and kahweol, free and esterified sterols, tocopherol and wax. Amounts of the diterpenes vary between arabica and robusta, and the concentrations of free diterpenes are low compared with the diterpenes bound in lipids. They are sensitive to heat, acid and light. The concentrations of esters also decrease with roasting temperature. Upon roasting, new diterpenes are formed (dehydrocafestol and kahweol), while cafestol and kahweol decrease (up to 80 % depending on roasting temperature). Some derivates are more stable during roasting (o-methyl-cafestol)⁽Reference Clarke and Vitzthum⁵²⁾. Cafestol and kahweol are thought to be responsible for an increase in serum cholesterol levels⁽Reference Higdon and Frei^137–Reference Bonita, Mandarano, Shuta and Vinson¹³⁹⁾. At the same time, they induce the activity of glutathione-S-transferase activity. This is considered a chemoprotective effect, shown to provide protection against aflatoxin B1 induced genotoxicity⁽Reference Bonita, Mandarano, Shuta and Vinson^140, Reference Cavin, Holzhaeuser, Scharf, Constable, Huber and Schilter¹⁴¹⁾.

Sterol compounds in coffee are not affected by roasting. Roasting reduces the concentrations of α-, β- and γ-tocopherols by 80–100 %. Coffee wax makes up 0·3 to 0·3 % of total coffee lipids, and is found in the outer layer of coffee beans. Waxes are partly decomposed by roasting. A component of the wax, 5-hydroxytryptamide, is thought to be the agent responsible for gastric irritation, and therefore coffee low in wax is termed ‘low irritating’⁽Reference Arnold, Ludwig, Kuhn and Moschwitzer^48, Reference Fehlau and Netter¹⁴²⁾.

Amino acids and protein

Free amino acid content in green beans is only 0·3–0·6 %⁽Reference Arnold, Ludwig, Kuhn and Moschwitzer⁴⁸⁾. They are largely transformed by roasting. With carbohydrates, they participate in reactions generating flavour and aroma compounds (Maillard reaction) and in the formation of melanoidins.

The protein content of coffee beans is reduced from 12 to 10 % upon roasting, corresponding to a loss of 21 %, taking into account the weight loss of the beans. Most of the protein (85 %) is represented by globulins (11S storage protein). The loss of individual amino acids during roasting depends on their reactivity (e.g. ε-aminogroups easily react to form melanoidins in the Maillard reaction). In addition, proteins are less soluble after roasting (extractability into the brew), which was attributed to protein denaturation, chemical reaction, degradation and conversion to aroma compounds⁽Reference Bekedam, Schols, van Boekel and Smit¹⁴³⁾.

Antioxidants and inducers of metabolic enzymes

Antioxidants are natural constituents in coffee and include phenolic compounds, chlorogenic acid, caffeic acid and caffeine. While many of the antioxidants naturally present in coffee are degraded (phenolic compounds, caffeic acid, ferulic acid, coumaric acid), others are formed by roasting in the Maillard reaction (melanoidins). Somoza et al. ⁽Reference Somoza, Lindenmeier, Wenzel, Frank, Erbersdobler and Hofmann¹⁴⁴⁾ identified high phase 1 enzyme inducing activity in freshly brewed coffee, as determined by dose dependent induction of metabolic enzyme inducing activity cytochrome c reductase (CCR) and phase II glutathione-S-transferase (GST) enzyme activities in a human intestinal cell culture system⁽Reference Somoza, Lindenmeier, Wenzel, Frank, Erbersdobler and Hofmann¹⁴⁴⁾. The authors identified N-methylpyridinium as a key inducing compound in the low molecular weight ( < 1 kDa) hydrophilic fraction of a coffee brew, the concentration of which amounted to 5·39 mg/l of coffee beverage. However, the antioxidant activity of N-methylpyridinium in vitro as determined by linoleic acid autoxidation was low. In contrast, 5-chlorogenic acid, a natural coffee compound with strong antioxidant activity with respect to linoleic acid autoxidation, did not show significant effects on phase I and II enzyme induction in the intestinal cell culture model. In rats, feeding either lyophilised coffee brew or N-methylpyridinium similarly increased blood antioxidant capacity (measured as trolox equivalents and blood tocopherol). Increased phase II enzyme activities in the liver (GST and UDP-glucuronosyltransferase; UDP-GT), however, were statistically non-significant. Pyrazines are important flavour compounds generated in heated foods including coffee. A recent study investigated the relation between formation of acrylamide, colour, pyrazines and antioxidants in asparagine/glucose model systems⁽Reference Ehling and Shibamoto¹⁴⁵⁾. The formation of pyrazines increased with time and temperature, but in contrast to the formation kinetics of acrylamide, they did not degrade at later stages but rather accumulated. Higher asparagine concentrations seemed to favour pyrazine formation. Acrylamide formation was strongly correlated to colour formation but not to antioxidant activity as determined by hexanal oxidation. A more recent study investigated the formation of radical scavenging activity in coffee roasted to different degrees⁽Reference Summa, De la Calle, Brohee, Stadler and Anklam¹¹⁰⁾. The authors demonstrate that degradation of acrylamide at the end of the roast process is strongly correlated to a decrease in radical scavenging activity and is furthermore correlated to the formation of colour.

Odorants

Hundreds of volatile compounds have been identified in roasted coffee, the profile of which is dependent on a number of factors, including species, provenance, and degree of roast, storage and brew. Odorants are mainly formed by the Maillard reaction, degradation of phenolic acids and carotenes, etc.⁽Reference Clarke and Vitzthum⁵²⁾

Caffeine

Caffeine is an important physiologically active compound in coffee and its effects have been and are still addressed and discussed in countless studies. Caffeine is well known for its stimulative effect on the central nervous system, elevation of blood pressure, increase in metabolic rate and diuresis⁽Reference Higdon and Frei¹³⁷⁾. Coffee beans contain between 0·8 and 2·8 % caffeine, which also contributes to the bitterness of coffee. Since concentrations of caffeine are unaffected by roasting, and decaffeination does not seem to have an effect on acrylamide formation, caffeine will not be addressed in this report. However, some biological and health effects of caffeine and coffee in general will be briefly discussed later in this document (see the section ‘Potential health benefits of coffee in the diet’).

Potato consumption

Potatoes are tubers – the edible, underground, swollen parts at the distal end of horizontally growing, underground stems or stolons. Worldwide, they are an important non-cereal energy crop. Fresh potato tubers have a much higher moisture content (approximate range 75–85 %) than cereals (14–20 %). Consequently, they are perishable but can be stored up to 8 or 9 months under controlled environmental conditions. Potatoes are sold fresh and as processed products which are dehydrated, canned or frozen and have a relatively long shelf-life. Cook/chill products, for example dishes containing mashed, boiled, baked or par-fried potatoes must be consumed within a few days of manufacture.

The history of the crop is well-documented⁽Reference Salaman¹⁴⁶⁾ and its significance as a major food source is evidenced by its revered position over 1000 years ago in the Inca civilisation of South America and by the devastation of the Irish Potato Famine. In recent years a notable feature has been that the quantities produced, processed and consumed in less developed countries have increased very rapidly, in some cases replacing traditional non-cereal energy root and tuber crops such as sweet potatoes, cassava and yams. Elsewhere on the other hand, the situation has been static or in decline. Currently according to FAO⁽¹⁴⁷⁾, almost 300 million tonnes of potatoes are produced annually. Over a third of this amount is in developing countries, which have shown an increase of about 11 % over the past 40 years [see ⁽Reference Andrzejewski, Roach, Gay and Musser^117, Reference Scott¹⁴⁸⁾ for a comprehensive analysis]. China is the largest producer (75 million tonnes in 2004) followed by Russia (37 million tonnes); India (25 million tonnes); USA (20 million tonnes). The EU25 produces about 60 million tonnes or about 20 % of world output of which over half are used for human consumption⁽¹⁴⁹⁾. About 10 % are used as seed to plant the next season's crop (the crop is vegetatively propagated from tubers); some specific, high dry matter content types are used for starch production, for example in The Netherlands⁽Reference Wustman¹⁵⁰⁾ and as a feedstock for alcohol production including vodka and bio ethanol as a renewable energy source. Surplus tubers, including potato processing waste are often fed to livestock or discarded.

McCance and Widdowson's the Composition of Foods⁽¹⁵¹⁾ gives detailed information about the composition of early/new/immature and old/maincrop/mature tubers cooked in different ways and with various additional ingredients. In general, potatoes are a good source of carbohydrate, but have a relatively low energy density. They are virtually fat free and have high contents of vitamins C and B₆, magnesium and iron and they are perceived by consumers as an important component of a healthy, balanced diet⁽¹⁵²⁾. Recently, the health benefits of including potatoes in the diet have been promoted vigorously by potato processors, packers, retailers and industry support organisations intending to maintaining or increasing market share in the face of competition from other carbohydrate sources and the fashion for low-carbohydrate diets⁽¹⁵²⁾. Because of their high carbohydrate content the majority of potatoes and potato products when eaten hot have a high glycaemic index (GI) and glycaemic load (GL). This capacity to elevate blood glucose concentrations has implications for development of type 2 diabetes, cardiovascular disease (CVD) and obesity. As a result there has been considerable interest in research on factors influencing GI of potatoes such as maturity, variety and cooking method⁽Reference Soh and Brand-Miller^153–Reference Henry, Lightowler, Strik and Storey¹⁵⁵⁾. Recent work by Henry et al. ⁽Reference Henry, Lightowler, Strik and Storey¹⁵⁵⁾, showed variation between seven varieties, with Marfona being the lowest with a GI of 56( ± 13) and Maris Peer the highest at 94( ± 16). Waxy textured varieties tend to produce medium GI values and floury potatoes high GI values. To achieve a low value, a variety would need to have a GI of ≤ 55 and if such varieties can be developed, then they have potential for use in diets where GI can be controlled. Buyken and Kroke⁽Reference Buyken and Kroke¹⁵⁶⁾ advocate the adoption of conventionally boiled, ‘immature’ tubers as part of low-GI diets (although early harvested tubers have higher initial reducing sugar contents which would make them less suitable for processing). In addition, consuming other foods together with potatoes as part of a meal can reduce the overall GI substantially⁽Reference Henry, Lightowler, Kendall and Storey¹⁵⁷⁾. In the home, potato consumers prepare fresh tubers for cooking or use partly or fully processed potato products that undergo heating and/or further cooking to finish off the dish. Some products are served and eaten immediately without any treatment such as crisps, pulped potato and potato dishes in ready-to-eat take-away meals. To some extent, the food value of potatoes and potato products may change because of differences in seasonal conditions, management in the field and in the store and the variety grown although effects on composition and nutritive value are likely to be relatively small. Dry matter content of potatoes changes throughout growth as do the ratios of sugars to starch and also the amount of fibre in the skin. Typically, early harvested, ‘new’ potatoes have very thin skins, low dry matter percentages of about 16 % and high concentrations of reducing sugars compared with tubers harvested at maturity or after storage. In some cases, towards the end of the storage period, sugars will increase as the tubers begin to sprout and the tuber will lose turgidity as it loses moisture. However, it is the way that the fresh potato is prepared and cooked and eaten that will tend to have a greater effect on its food value. Eating cooked tubers with intact skins, such as large, baked ‘jacket’ and small, boiled salad potatoes with ‘set-skins’ increases fibre intake and is considered to be a healthy option. On the other hand, the higher fat content of fried products compared with boiled or steamed potatoes, the type of cooking oil used (particularly with regard to the amount of trans-fats) and quantities of salt in the seasoning and any other additives, particularly in flavoured crisps, potato snacks and French fries, may be considered to compromise the healthy image of the potato. Moreover, inclusion of flavouring components from animal products may exclude these products from certain consumer groups.

Inherently, potatoes are a safe food but they do contain glycoalkaloids, which contribute to flavour, but can be poisonous⁽Reference Friedman and McDonald¹⁵⁸⁾. They play a role in the plants' natural defence against pests such as slugs, wireworms and cutworms. Glycoalkaloid concentrations increase when tubers are exposed to the light, form chlorophyll and turn green so that it is imperative to keep tubers well-covered with soil in the field, or in the dark in storage or to ensure pre-packed potatoes in store display cabinets are sold before the overhead lights cause greening. Green potatoes are usually rejected by the consumer (and often much earlier in the supply chain) or the green tissue may be cut off the tuber and discarded before cooking the remainder. Therefore, the risk to health from glycoalkaloids is small. Possible links between the consumption of tubers infected with tuber blight and the incidence of babies born with Spina bifida and anencephaly to mothers who had consumed blighted tubers, have been mentioned in the literature⁽Reference Burton^159, Reference Ulman, Taneli, Oksel and Hakerlerler¹⁶⁰⁾. However, it is difficult to envisage circumstances in which blighted tubers would be eaten by, or fed to, pregnant women. Pesticide residues are also an issue, partly because the potato crop is by far the biggest user of agrochemicals. Of course, crop protection chemicals must be used according to strict regulations to ensure that maximum residue limits (MRLs) are not exceeded, but concerns of many consumers about pesticide residues in food have contributed to the expansion in production and consumption of potatoes and other foodstuffs grown according to ‘organic standards’.

In 2003/04, the average rate of consumption of potatoes in the EU was 76·9 kg/head per year, ranging from 43·7 kg/head per year in Italy to 156·1 kg/head per year in Latvia⁽¹⁴⁹⁾. These figures are based on raw equivalents which are the weight of freshly prepared, cooked tubers eaten plus the weight of the raw equivalent of processed potato products, using the following ratios to convert weights of fresh, raw potatoes to processed products: canned 1:1; crisped 4:1; dehydrated 6·2:1; frozen 1·9:1⁽¹⁶¹⁾. In 2005 in the UK, excluding Northern Ireland, average consumption was 94·7 kg/head per year but from 1988 to 2005, was fairly constant at between 100 and 110 kg/head per year. Over this period, the trend has been for a decrease in the ratio of fresh to processed consumption. In 1988, the ratio was about 3:1, but by 2005 had reached 1:1 indicating that half of consumers' potato intake was in the processed form. This included products manufactured from about 3 million tonnes of UK-grown potatoes and imported products, mainly frozen French fries from The Netherlands and France equivalent to almost 1 million tonnes of raw potatoes. Over the same period, about a half of consumers' intake was through food service outlets, where ready cooked potatoes are served as a snack or part of a meal outside of the home⁽¹⁶²⁾ and these trends are expected to develop globally. In the USA, potato consumption is lower than in many European countries at about 56·3 kg/head per year forecasted for 2006, but almost two-thirds are processed in one form or another. As in Europe, frozen French fries account for the biggest proportion of consumption (42 %), followed by potato chips or crisps (13 %), and dehydrated products (11 %), whilst the consumption of canned potatoes is negligible⁽¹⁶³⁾.

Potato tuber quality for the fresh market and for processing

The qualities of potato tubers relate almost entirely to their fitness for purpose, either for potatoes that are sold fresh or used for processing. Hardly any relate directly to the food value but more to the aesthetic properties of the raw material and processed products. Packers and processors have very specific qualities that their suppliers must meet, otherwise the price will be discounted or the consignment rejected. Some of the qualities are similar for both fresh and processing outlets; some are different and some are very specific to a particular process. For fresh tubers – the majority of which are washed before sale either pre-packed or loose – the main criteria relate solely to their appearance. They must be free from, or have very low levels of mechanical and pest damage, disease (particularly the blemishing diseases such as common scab, black and silver scurf, skin spot), sprouting, greening, secondary growth. They must also be of a particular size (e.g. small for salad potatoes; large for baking potatoes) and skin and flesh colour may also be important as is freedom from after-cooking blackening in boiled or steamed potatoes. Organoleptic attributes such as flavour, aroma and texture seem to be less important. However, with the growing interest in gastronomy, this is beginning to change and providing opportunities for the development of niche markets.

Potatoes for processing must also be free from damage, greening and secondary growth but the tuber blemish diseases are much less of a problem because the tubers are peeled prior to processing and the consumer does not see the potato in the raw state. Tuber dry matter content is extremely important because it affects the out-turn of the final product from the raw material and also its quality. At the factory, it is usually determined on the basis of specific gravity of a sample of fresh tubers rather than by oven-drying a sample to constant weight, because results are achieved within minutes rather than hours. For canning whole, new, immature potatoes must have low dry matter contents or they may disintegrate during the canning and cooking process. For fried products, i.e. French fries (often referred to as chips in the UK) and crisps (usually referred to as potato chips in North America), a high tuber dry matter percentage is required: it affects the texture/mouth-feel of the final product; the amount of water that must be driven off during frying and the amount of cooking oil that is absorbed. The latter influences the cost of the process because it determines the frequency with which the cooking oil must be replenished and also the fat content of the fried product, as previously discussed in this report. The amount of fat absorbed may also be influenced by the shape of the French fry (e.g. straight-cut or crinkle-cut), which affects the surface area per unit weight of raw material fried. Internal bruising or black spot (formation of necrotic areas resulting from reactions involving tyrosinase following internal damage) is also a problem close to the vascular ring, particularly for high dry matter processing varieties, which can bruise easily during harvesting and handling because it discolours the product. However, use of less susceptible varieties, careful harvest and handling techniques into and out of store and avoiding cold temperatures during these procedures can alleviate the problem. Green potatoes must also be avoided, especially in crisp manufacture as an unsightly green/purple discolouration with a bitter taste may be produced near the edge of the crisp.

As already discussed, the reducing sugar content is most important because of its involvement in the Maillard reaction that can lead to French fries and crisps that are unacceptably dark in colour. On reception at the processing factory, sugar concentrations of samples are measured and also small-scale test frying is done and the colour of the product is assessed relative to specific colour charts⁽¹⁶⁴⁾. Factors affecting sugar concentrations have already been discussed at some length in this report. Steps can be taken to ensure that reducing sugars in the raw material are at acceptable concentrations to begin with, but the cooking procedures can be adjusted to minimise excessive colouring with samples that have reducing sugar concentrations above the optimum (see the section ‘Summary and conclusions’, Table 3). Reducing sugar content may not be as critical for processors manufacturing products such as roast potatoes, wedges and croquettes which are flash-fried for a short time, than for products that are fried for a longer period such as crisps. Cooking of flash-fried products is finished by baking in the oven either at the consumer's home or the food service outlet, so that the degree of browning achieved is determined after the initial processing stage. With ‘hand-cooked’ brands of crisps, a finished product that is darker than the industry-standard, pale-straw colour seems to be acceptable and so reducing sugar concentration of the raw potatoes used for making them is probably less important than it is for conventionally cooked crisps. This may also be the case for fresh potatoes used to make crisps that are flavoured with dark-coloured ingredients and will mask the colour of the crisps as they come out of the frier. On the other hand, whilst reducing sugar concentrations of fresh tubers are not usually important in the manufacture of dehydrated potato mash or flakes, they may be when the reconstituted product is deep-fried to produce potato-based snacks.

Whilst the Maillard reaction and non-enzymatic browning is of primary concern, enzymatic browning is much less so because steam-peeling prevents it and the time between peeling and cooking is usually too short for the reaction to occur. For freshly peeled and cut chips, where there is a delay in frying, whitening preparations based on sodium metabisulphite as an anti-oxidant could be used.

The potato is a particularly good subject for genetic modification. Several GM varieties have been produced, particularly to provide pesticide-free control of certain pests and diseases. Some have also been modified to increase the protein content with the aim of improving the diets in some developing countries. Production of pharmaceuticals by GM potatoes is also an objective. Whilst this approach could offer potential solutions to a number of problems, there is intense debate and concern about the safety and acceptability of GM foods. Lack of consumer acceptance in Europe and Japan has led to the refusal of the major food service companies to include GM potatoes in their products anywhere in the world. Consequently, none are currently grown or processed commercially. However, China may be more willing to embrace this technology and provide a potential market for GM potato products⁽Reference Curtis, McCluskey and Wahl¹⁶⁵⁾.

Genetic modification may provide a potential solution to the problem of acrylamide in fried potato products. Recently in the USA, the variety Russet Ranger (an alternative to Russet Burbank for French-fry production) has been modified to overcome its susceptibility to bruising and cold-induced sweetening. Concentrations of acrylamide in French fries made from modified Russet Ranger following cold-storage were approximately a third (300–400 μg/kg) of those made from control, unmodified Russet Burbank and Russet Ranger tubers (1000–1200 μg/kg)⁽Reference Rommens, Ye, Richael and Swords¹⁶⁶⁾. As the modification was an all-native DNA transformation method that silenced specific genes and so did not involve insertion of foreign DNA and improved important quality traits, the authors suggest that such intragenic varieties may be more acceptable to consumers than transgenic food crops.

Potato varieties

Solanum tuberosum is the main species grown for consumption in Europe and North America but there are many species. Some of these are used for human consumption in South America and they are also an important resource for breeders to introduce pest and disease resistance and other qualities such as ability to grow at low temperatures, into cultivated types (the Commonwealth Potato Collection is one such resource⁽Reference Neilson¹⁶⁷⁾. There are many varieties of cultivated potatoes available in every country, but in practice a relatively small number dominate the area of production. This is because they are robust, yield reasonably well and are particularly suited to specific uses and markets. Indeed the buyers – the supermarkets and processors – are very specific about the varieties that they will accept and so contractual arrangements between growers and buyers are increasingly common. Some varieties are still in widespread use today that were introduced about a century ago. For example, in the UK, Jersey Royals and King Edwards are still premium varieties and there is currently growing interest in ‘Heritage’ potato varieties. For processing, the varieties Russet Burbank and Yukon Gold from North America are good examples of old varieties. Russet Burbank (1877) is still grown to produce high quality French fries for fast-food restaurants, but it is not a popular variety with growers because it is very susceptible to Late Blight, drought and damage and produces many mis-shapen tubers. However, many relatively newly introduced varieties have been adopted for processing because they have better characteristics than the more traditional ones. The build-up of reducing sugars during low temperature storage has already been mentioned and the practice of reconditioning at higher temperature to condense the reducing sugars is an important technique to consider. Not all varieties will recondition well and so breeders are keen to develop varieties that do not sweeten at low temperature. Once again it is varieties' fitness for processing, or their aesthetic or organoleptic properties if they are to be prepared from fresh, that far outweigh the nutritive aspects. In the UK, a good example of the latter is the recently introduced variety Mayan Gold, which is a new variety of the Phureja species⁽¹⁶⁸⁾. It has a very good flavour and texture and cooks more quickly than tuberosum varieties: it is a favourite of several ‘celebrity’ chefs. Other Phureja varieties seem to be in the pipeline. Vivaldi⁽¹⁶⁹⁾ is another new UK variety, said to have about 25 % lower carbohydrate than other varieties and therefore may be useful as part of a calorie-controlled diet with a lower GL compared with other varieties (see above). Potentially low GI varieties may also have lower reducing sugar contents and contribute either directly, or following further breeding, to less browning during frying or baking provided that the final products were acceptable to the consumer.

Breeding for varieties with longer dormancy is also an important objective as this affects the onset of sprouting and may delay the build up of reducing sugars and the process of senescence-sweetening and decrease the requirement for chemical sprout suppressants during long term storage.

Conclusion

Potatoes are a cheap, readily available, high carbohydrate food and perceived by consumers as being an important component of healthy, balanced meals and diets. The proportion eaten in the ready processed form (and as fried potato products in particular) and eaten outside the home through foodservice is increasing and at a faster rate in less developed countries than in North America and Europe. The concentrations of reducing sugars which are the key factor in the formation of acrylamide can be manipulated by crop agronomy in the field and store, influenced by breeding and managed by adjustments to processing methodology as described earlier in this report. Such approaches are well established and were developed long before the issue of acrylamide in potatoes appeared in media reports in spring 2002⁽¹⁷⁰⁾; the primary objective was to control the browning of fried products rather than the levels of acrylamide per se. None of these measures are likely to be detrimental to the nutritive value of the potato as a food, but of course, reduction of acrylamide levels is a desirable objective and must be explored.

Cereal grain consumption

The cereal grains consumed both by humans and animals are the edible seeds of the domesticated members of the Gramineae grass family⁽Reference Seal, Jones and Whitney¹⁷¹⁾. Worldwide grain production is dominated by wheat and rice, which collectively account for one-third and one-quarter of total grain production, these together with maize are the major cereals⁽Reference Slavin¹⁷²⁾. The minor grains are barley, sorghum, millet, oats and rye; non-cereal grains and seeds, which are structurally similar to the Gramineae with relatively similar nutritional characteristics include, amaranth, buckwheat, psyllium and quinoa⁽Reference Seal, Jones and Whitney¹⁷¹⁾. The relative amounts of grains consumed varies between countries, with quite marked differences between geographic areas best suited to the agronomic conditions required for their cultivation. For example, rice can be grown in water in tropical and subtropical climates, wheat production occurs in temperate zones and rye, which accounts for less than 2 % of world grain production, is an important crop in cooler climates such as those found in Scandinavia and Eastern European countries. Grains are important plant foods as they constitute dietary staples in most diets and are a major source of energy, protein, B vitamins and minerals for the world population⁽Reference McKevith¹⁷³⁾. Grains provide approximately two-thirds of the energy and protein intake in the world although, again, exact proportions vary between countries. In the UK, cereal foods provide approximately 30 % of energy, 25 % of protein and almost 50 % of available carbohydrate intakes, and are dominated by wheat-based products such as breads, breakfast cereals, biscuits and cakes. In contrast, in less affluent countries, the importance of cereals increases with many parts of rural Africa and Asia deriving more than 70 % of energy intake from cereal sources⁽Reference Southgate, Garrow, James and Ralph¹⁷⁴⁾. The per capita consumption of rye in countries with high consumption ranges from the nearly 40 kg/year in Belarus and Poland to 10–20 kg/year in the Baltic and Scandinavian countries, Ukraine, Austria, Czech Republic, Slovakia, and the Russian Federation⁽¹⁴⁷⁾. Rye is traditionally used for bread (fresh bread, crisp bread, and thin crisp) made with sourdough containing wholemeal rye flour, water, salt and starter culture⁽Reference Kujala¹⁷⁵⁾. Pumpernickel rye bread, which is more popular in the central Europe, is made of rye kernels, wheat flour and malt⁽Reference Poutanen¹⁷⁶⁾. In addition to bread, there are different types of flours, groats, and flakes for baking, and porridges and breakfast cereals, as well as rye-based Easter pudding and rye-dough covered baked dish containing fish and pork traditionally used in Finland. Modern rye products are rye pasta, rye hamburgers and rye-based snacks.

Cereal grains provide a wide range of nutrients other than energy to the diet, varying somewhat between the different grains. The endosperm, which constitutes approximately 80 % of the grain by weight is a rich source of starch but also contains soluble fibres, protein and considerable quantities of riboflavin and pantothenic acid. The bran, or the protective ‘outer shell’ of the grain provides predominantly insoluble fibre, B vitamins, trace minerals, phytochemicals (such as lignans, tocotrienols and phenolic compounds) and antinutrients (including phytic acid, tannins and enzyme inhibitors). Finally, the germ contains further B vitamins, vitamin E, trace minerals, phytochemicals, antioxidants and lipids⁽Reference Slavin^172, Reference Slavin, Jacobs, Marquart and Weimer¹⁷⁷⁾. Examples of how the relative amounts of the bran and germ constituents can differ from one species to another have been described by Seal⁽Reference Seal¹⁷⁸⁾. For example, brown rice has a very low bran content (30 mg/g) compared with maize (around 60 mg/g) and especially wheat ( ≤ 160 mg/g). With the exception of rice, whole grains are high in dietary fibre; wheat is a valuable source of insoluble dietary fibre whilst oats, barley and rye supply soluble fibre to the diet⁽Reference Seal, Jones and Whitney^171, Reference Slavin^172, Reference Smith, Kuznesof, Richardson and Seal¹⁷⁹⁾. Wholegrain foods contribute significantly to dietary fibre intake and consequently to the intake of many fibre-associated compounds such as lignans, phenolic acids, alkylresorcinols, phytosterols, and some vitamins and minerals. In the Finnish diet rye bread is the most important single food as a fibre source, providing on average 1/3–1/2 of the daily fibre intake of adults. The concentration of individual nutrients in grains is also very variable; for example fat concentrations range from 2·0 g/100 g in whole rye flour to 8·7 g/100 g in oatmeal. For further data on compositional differences between grain flours see Holland et al. ⁽Reference Holland, Unwin and Buss¹⁸⁰⁾

Processing of grains

The majority of grains undergo some processing before consumption to improve flavour, texture, appearance and shelf stability. Grains used for muesli and in some granary breads undergo mild heat treatments which will inactivate enzymes but which are not expected to generate acrylamide⁽Reference Slavin¹⁷²⁾. The range of processing techniques used include milling, heat extractions, cooking and parboiling⁽Reference Richardson¹⁸¹⁾. Ready-to-eat breakfast cereals are also produced using techniques such as flaking, extruding, puffing and shredding⁽Reference Fast, Fast and Caldwell¹⁸²⁾. Cereal flours are subjected to yeast or bacterial fermentation during the production of different breads. These processing techniques have a profound impact on the nutrient profile of the product. Refined grains, where the outer layers of the grain are removed during milling, have reduced concentrations of dietary fibre, vitamins, minerals, phytic acid, lignans, phytooestrogens and phenolic compounds⁽Reference Smith, Kuznesof, Richardson and Seal^179, Reference Slavin¹⁸³⁾. The changes are most significant for nutrients and components found in the bran and germ layers which are stripped from the grain in modern milling processes. The degree of reduction is dependent on the extraction rate, ranging from no reduction in wholemeal flours (100 % extraction, containing all fractions of the grain) to the highest reduction in white flours (approximately 70 % extraction with only the starchy endosperm remaining). Some minerals and vitamins are added to refined flours to compensate for the losses incurred during milling. Processing may significantly influence the concentration and bioavailability of the fibre-associated bioactive compounds in cereals since they are mostly concentrated in the bran-layers of the grain, and are only present at low concentrations in the flour endosperm⁽Reference Liukkonen, Katina, Wilhelmsson, Myllymaki, Lampi, Kariluoto, Piironen, Heinonen, Nurmi, Adlercreutz, Peltoketo, Pihlava, Hietaniemi and Poutanen¹⁸⁴⁾. Concentrations of folate and easily extractable phenolic compounds increase during germination and sourdough baking of rye flours, while those of tocopherols and tocotrienols are reduced and only negligible changes in the concentrations of sterols, lignans and alkylresorcinols are observed⁽Reference Liukkonen, Katina, Wilhelmsson, Myllymaki, Lampi, Kariluoto, Piironen, Heinonen, Nurmi, Adlercreutz, Peltoketo, Pihlava, Hietaniemi and Poutanen¹⁸⁴⁾. Phytate concentrations are also reduced during sourdough baking. The increase in concentration of folate seen during germination of rye grains compensates for the reduction observed during heat processing (extrusion, autoclaving, puffing, roasting), and the final product contains more folate than the native grains.

Health benefits of consuming whole grains compared with refined grains

There is a growing body of evidence, predominantly from epidemiological and cohort studies, demonstrating strong relationships between consumption of wholegrain foods and disease risk. For the major non-communicable diseases such as CVD, type 2 diabetes and some cancers these relationships are strongly inverse; i.e. with increasing whole grain consumption the disease risk is reduced. There are numerous reviews of the literature summarizing these observations. The strongest relationships have been found for CVD⁽Reference Seal^178, Reference Anderson, Hanna, Peng and Kryscio^185–Reference Flight and Clifton¹⁸⁸⁾ where the reduction in risk of CVD between the lowest whole grain consumers and the highest whole grain consumers is reportedly between 20 and 40 %. Many of the studies reported in these reviews are based on large cohorts from the US and Northern Europe. The large numbers of subjects in the individual studies allows for good statistical interrogation of the data and, in particular, for adjustment for lifestyle and other confounding factors. In the majority of studies, for example, those who consumed more wholegrain foods were more likely to, smoke less, eat more fruits and vegetables, eat less red meat and be more physically active⁽Reference Seal^178, Reference Anderson, Hanna, Peng and Kryscio¹⁸⁵⁾. Nevertheless, the relationships hold true when these factors are taken into account, and the effects remain stronger in many cases for whole grains compared with cereal fibre alone. The mechanisms by which whole grains exert this beneficial effect remain unclear, however, for oats, barley and rye the effect may be due in part to the soluble fibre present in these grains. Whole grain rye contains high amounts of soluble arabinoxylans, which can lower serum cholesterol concentration, a known risk factor of CVD. For example, an inverse association was found between the amount of dietary fibre in the diet, to a large extent derived from rye, and CVD in Finnish men⁽Reference Pietinen, Rimm, Korhonen, Hartman, Willett, Albanes and Virtamo¹⁸⁹⁾. The benefit of rye was demonstrated in a dietary intervention during which serum cholesterol, serum total and LDL cholesterol concentrations were reduced during the rye-bread-consumption period⁽Reference Leinonen, Liukkonen, Poutanen, Uusitupa and Mykkänen¹⁹⁰⁾ in hypercholesterolaemic men. The cholesterol-lowering effects of oats have been well documented and a health claim is authorised, both in America⁽¹⁹¹⁾ and in the UK⁽¹⁹²⁾, based on the presence of the soluble fibre, β-glucan. A similar claim for barley has also now been approved⁽¹⁹³⁾.

The relationship between whole grain consumption and cancer risk has also been investigated in a number of studies, with similar reductions in risk. The evidence supporting the assertion that consumption of wholegrain foods was associated with reduced risk of cancers was first reported in a review of 40 case–control studies published between 1984 and 1998 from the US and Europe⁽Reference Jacobs, Marquart, Slavin and Kushi¹⁹⁴⁾. For these studies, the pooled odds ratio averaged across all studies was 0·66 (95 % confidence interval, 0·60–0·72), and was in the range of 0·50–0·80 with only breast cancer (0·86) and prostate cancer (0·90) being higher. A similar systemic review of case–control studies conducted using a common protocol in northern Italy between 1983 and 1996 (some of which were included in the Jacobs' meta-analysis) also showed an inverse relationship between frequency of whole grain consumption and risk for cancer⁽Reference Chatenoud, Tavani, La Vecchia, Jacobs, Negri, Levi and Franceschi¹⁹⁵⁾. Although there is a common agreement that a fibre-rich diet containing grains helps to reduce the risk of some cancers, especially those of the lower bowel, the data derived from human and animal studies on the potential of specific cereal products in reducing the risk of cancer is inconclusive. Decreased intake of lignans has been proposed to explain the incidences of many types of cancers⁽Reference Adlercreutz¹⁹⁶⁾, but the observational studies following this suggestion have yielded conflicting results. Using an animal model, multiple intestinal neoplasia (Min) mice, Mutanen et al. ⁽Reference Mutanen, Pajari and Oikarinen¹⁹⁷⁾ showed that rye bran was beneficial against intestinal tumorigenesis. However, recently this group has shown that in the same animal model individual plant lignans do not protect against tumour production⁽Reference Pajari, Smeds, Oikarinen, Eklund, Sjoholm and Mutanen¹⁹⁸⁾, so the mechanisms for the possible protective effects remain unclear.

Epidemiological studies have also consistently shown significant inverse relationships between the risk for type 2 diabetes and increased consumption of whole grains. Pooled data from seven prospective cohort studies from the US showed the average relative risk of developing type 2 diabetes was 0·70 comparing the highest and lowest categories of whole grain intake⁽Reference Liu¹⁹⁹⁾. In northern Europe a similar protective effect was observed for Finnish men and women⁽Reference Montonen, Knekt, Jarvinen, Aromaa and Reunanen²⁰⁰⁾. These studies also demonstrate the inverse association between dietary fibre intake and reduced risk of type 2 diabetes and, with the exception of the Finnish study, they all show no benefit or even harmful effects of refined grain products when they are eaten in place of fats or wholegrain foods⁽Reference Liu¹⁹⁹⁾. Studies in healthy subjects have shown that postprandial plasma insulin responses are significantly lower after the consumption of a whole grain rye bread compared with a white wheat bread, but no significant differences in plasma glucose concentrations were found⁽Reference Leinonen, Liukkonen, Poutanen, Uusitupa and Mykkänen^190, Reference Juntunen, Niskanen, Liukkonen, Poutanen, Holst and Mykkanen^201, Reference Juntunen, Laaksonen, Autio, Niskanen, Holst, Savolainen, Liukkonen, Poutanen and Mykkanen²⁰²⁾. It is possible that wholegrain rye slows the digestion and absorption of dietary carbohydrates since it has a high content of soluble fibres, especially arabinoxylans. However, the lower insulin response after rye bread consumption was not explained by the amount of fibre because a low-fibre rye bread was shown to reduce the insulin responses similarly to a high-fibre rye bread⁽Reference Juntunen, Laaksonen, Poutanen, Niskanen and Mykkanen²⁰³⁾. Structural properties and differences between rye and wheat breads, rather than the fibre content, may explain the observed findings. A diet containing high-fibre rye bread for 8 weeks improved insulin secretion (measured by an intravenous glucose tolerance test) in healthy individuals and in subjects suffering of metabolic syndrome⁽Reference Juntunen, Laaksonen, Autio, Niskanen, Holst, Savolainen, Liukkonen, Poutanen and Mykkanen^202, Reference Laaksonen, Toppinen, Juntunen, Autio, Liukkonen, Poutanen, Niskanen and Mykkanen²⁰⁴⁾.

The epidemiological evidence that whole grain consumption is of benefit to health is a powerful indicator of a relationship, however such relationships do not demonstrate causality. To date evidence from controlled dietary intervention studies with large numbers of subjects showing clear benefit of increased consumption of wholegrain foods on markers of disease risk are very few, and attempt to describe mechanisms of action, therefore, are speculative⁽Reference Seal, Brownlee and Jones²⁰⁵⁾. However, many plausible explanations have been proposed including the increased intake of antioxidant substances, bio-active components such as plant lignans, phytooestrogens, soluble and insoluble fibre, phytates and phenolics. Intakes of many of these components have been linked to improved immune function, antioxidant status, endothelial function and blood pressure, tumour suppression, and inflammation. A small number of studies published recently have shown benefit of incorporating whole grains into the diet, although these have used ‘at risk’ subjects. For example, Behall et al. ⁽Reference Behall, Scholfield and Hallfrisch²⁰⁶⁾ showed that wholegrain foods whether high in soluble or insoluble fibre reduced blood pressure in mildly hypercholesterolemic men and women. Similarly, Qi et al. ⁽Reference Behall, Scholfield and Hallfrisch^207–Reference Lopez-Garcia, van Dam, Qi and Hu²⁰⁹⁾ showed that intakes of whole grain were associated with decreased concentrations of systemic inflammation in diabetic women. Wholegrain food consumption has also been linked to weight management and obesity control, but most of the evidence is from cohort studies with self-reported weight.

Promoting whole grain consumption

The evidence for improved health with increased whole grain consumption has spawned the development of health claims for use on foods, and to the incorporation of wholegrain messages in healthy eating campaigns. The American Food and Drug Administration (FDA) were the first to approve a health claim based on wholegrain foods in 1999 although this was modified in 2003⁽²¹⁰⁾. Health claims in Europe did not arrive until 2002 in the UK⁽²¹¹⁾ and 2003 in Sweden⁽²¹²⁾. In the UK and the US, wholegrain foods were defined as foods containing >51 % whole grain by weight and >50 % in Sweden. The most commonly used definition of whole grain was produced and adopted in 1999 by the American Association of Cereal Chemists as follows: ‘Whole grains shall consist of the intact, ground, cracked or flaked caryopsis, whose principal anatomical components – the starchy endosperm, germ and bran – are present in the same relative proportions as they exist in the intact caryopsis’⁽²¹³⁾. Slight modifications of this definition are proposed by the Food and Drink Administration in the US⁽²¹⁴⁾ and have been adopted in Australia and New Zealand⁽²¹⁵⁾. There is no standardised definition agreed for the UK.

Specific recommendations for whole grain consumption tied to target quantities of consumption currently exist only in the US, where they are embedded in the US Dietary Guidelines for Americans⁽²¹⁶⁾. The guidelines give an explicit recommendation for adults to consume at least three ounce-equivalents of whole grain per day, and that at least half of the grains consumed should be whole grains. Currently there are no dietary recommendations for whole grains in the UK, although they now receive specific mention in the Food Standards Agency's Eat Well, Be Well programme⁽²¹⁷⁾. Similar advisory recommendations to eat more whole grains now exist in Switzerland and Australia.

Despite the scientific interest in promoting whole grain consumption, and the growing industry drive to promote consumption of wholegrain foods, especially breakfast cereals, consumption levels across Europe and the US remain very low. Data from the US collected in the late 1990s showed that adults consumed an average of only 1 serving (about 16 g) of whole grain per day with only 8 % of the population achieving the target of 3 servings (48 g)⁽Reference Cleveland, Moshfegh, Albertson and Goldman²¹⁸⁾. More recent analyses of data from the 1999–2002 National Health and Nutrition Examination Survey once again confirmed that the majority of Americans fell well short of the 2005 Dietary Guidelines for total and whole grain consumption with just 11 % from whole grains compared with the recommended 50 %. Just 4 % of the population met the target of consuming at least half of their grains as whole grains⁽²¹⁹⁾. The situation is no better in the UK with about one-third of British adults consuming no whole grain at all and less than 5 % meeting the US target⁽Reference Lang, Thane, Bolton-Smith and Jebb²²⁰⁾. For young people aged 4–18 in the UK whole grain intake is even lower⁽Reference Thane, Jones, Stephen, Seal and Jebb²²¹⁾. Comparisons between whole grain intake in the UK between 1986–7 and 2000–1 show a decline in intake in this 14-year period⁽Reference Thane, Jones, Stephen, Seal and Jebb²²²⁾. Typical consumers of wholegrain foods in the UK tend to be older, from a high socioeconomic group, less likely to smoke and more likely to exercise⁽Reference Lang and Jebb²²³⁾. In Finland rye bread is consumed most frequently in rural parts of the country among the lower educated population groups, more frequently among men than women, and more frequently among the old than young⁽Reference Prattala, Helasoja and Mykkanen²²⁴⁾. Finnish men and women consume on average 100 and 66 g of rye bread per day, respectively⁽Reference Männistö, Ovaaskainen and Valsta²²⁵⁾ providing approximately 55 and 36 g of whole grain per day from this one dietary source. Despite this one case of relatively high consumption much work is needed across most of Europe and North America if public health strategies are to achieve the intake levels for wholegrain foods advocated by the scientific community.

Conclusion

The current drive to increase consumption of wholegrain foods presents a problem in the context of acrylamide consumption since without mitigation of acrylamide content any increase in whole grain consumption could potentially result in increased intake of acrylamide. However, the relative disease risk increase associated with increased acrylamide consumption must be balanced against the relative disease risk reduction associated with increased whole grain consumption. For example, take a non-whole grain consumer who adopts the recommendation to consume 48 g of wholegrain per day⁽²¹⁶⁾. If this individual consumes this whole grain all in the form of ‘dark rye wholemeal crisp bread’ with an average acrylamide content of 1520 μg/kg (mean of 6 samples, range 1250–1670⁽⁵⁷⁾), assuming a wholegrain rye content of 84 % on a dry matter basis for the rye crisp bread⁽Reference Seal, Brownlee and Jones²⁰⁵⁾, this would equate to an increase in acrylamide consumption of approximately 87 μg/day. This assumes that this change in intake was an addition to the diet rather than a substitution for a non-whole grain baked alternative. In the case of substitution, the change in acrylamide intake could be very small or indeed result in a reduction in acrylamide intake compared with some foods. The additional ‘risk’ of this consumption is hard to calculate but is likely to be small based on the lifetime exposure calculations described in the section ‘Background’. This would be in contrast to the much greater population health benefits such as reductions in risk of CVD, type 2 diabetes and cancers associated with increased whole grain consumption predicted from epidemiological and cohort data described above which are in the order of 20–30 %. These calculations, and the assumptions therein, need careful consideration and further evidence and research is clearly needed before appropriate health messages can be developed.

Ingredients in coffee

Despite 20 years of reassuring research, many people still avoid coffee because they worry about its health effects. However, current research reveals that a moderate coffee intake of two to four cups per day is not only safe but may even offer some health benefits. Some of the health effects of moderate coffee intake have been already attributed to chemically characterised coffee ingredients. The substances which, during brewing, dissolve in water to form the beverage are classified as non-volatile taste components. These include caffeine, trigonelline, chlorogenic acid, phenolic acids, amino acids, carbohydrates and minerals. Volatile aroma components formed during roasting include, for example, organic acids, aldehydes, ketones, esters, amines, and mercaptans.

Effect of caffeine

Probably the most widely studied physiologically active substance in coffee is the alkaloid caffeine, also called guaranine or methyltheobromine. Caffeine is a naturally occurring substance found in the leaves, seeds or fruits of more than 60 plants, including coffee and cocoa beans, cola nuts and tea leaves. One cup of coffee, depending on its strength, may contain some 20–100 mg of caffeine. Caffeine acts as a mild stimulant of the central nervous system, helping to reduce feelings of drowsiness and fatigue.

The hypothesis that high doses of caffeine ingestion adversely affects human health has been investigated by Nawrot et al. ⁽Reference Nawrot, Jordan, Eastwood, Rotstein, Hugenholtz and Feeley²²⁶⁾ based on reviews of published human studies obtained through a comprehensive literature search. Based on these data, it was concluded that for the healthy adult population, moderate daily caffeine intake at a dose level up to 400 mg per day is not associated with adverse effects such as general toxicity, cardiovascular effects, effects on bone status and calcium balance, changes in adult behaviour, increased incidence of cancer and effects on male fertility. The data reviewed by Nawrot et al. ⁽Reference Nawrot, Jordan, Eastwood, Rotstein, Hugenholtz and Feeley²²⁶⁾ also show that only reproductive-aged women and children are ‘at risk’ subgroups who may require specific advice on moderating their caffeine intake. Based on available evidence, it is suggested that reproductive-aged women should consume not more than 300 mg caffeine per day, equivalent to 4·6 mg/kg bw per day for a 65 kg person. Children should not consume more than 2·5 mg/kg bw per day. With a moderate consumption of two to four cups of coffee, the resulting caffeine intake is unlikely to cause adverse effects in healthy adults.

Coffee and reduced disease risk

Latest research has not only confirmed that moderate coffee consumption does not cause harm (Table 9), it has also uncovered possible health benefits. Most of these benefits have been identified through statistical studies that track a large group of subjects over the course of years and match incidence of various diseases with individual habits, like drinking coffee, while controlling for other variables that may influence that relationship. According to recent large epidemiological and population-based cohort or cross-sectional studies, moderate coffee drinking may lower the risk of numerous diseases listed in Table 9.

Table 9

Diseases for which the consumption of coffee is associated with a significant risk reduction

According to the data published so far, strong evidence exists for a significant reduction of type 2 diabetes, which is predominantly influenced by lifestyle factors and nutrition. In a prospective follow-up study in 10 188 Finnish men and 11 197 women aged 35–74 years without a history of stroke, coronary heart disease (CHD) or diabetes, hazard ratio of type 2 diabetes in participants who drank 0–2, 3–6 and ≥ 7 cups of coffee per day were 1·00, 0·77 and 0·66 in men and 1·00, 0·71 and 0·52 in women, respectively⁽Reference Hu, Jousilahti, Peltonen, Bidel and Tuomilehto²²⁷⁾. The relationship between coffee consumption and risk of CVD, being one of the major late complications of type 2 diabetes, has been examined in a study by Bidel and colleagues⁽Reference Bidel, Hu, Qiao, Jousilahti, Antikainen and Tuomilehto²²⁸⁾. This study was designed to assess the association between coffee consumption and CVD mortality among 3837 Finnish patients with type 2 diabetes. During the average follow-up of 20·8 years, 1471 deaths were recorded, of which 909 were coded as CVD, 598 as CHD and 210 as stroke. The respective multivariate-adjusted hazard ratios in participants who drank 0–2, 3–4, 5–6 and ≥ 7 cups of coffee daily were 1·00, 0·77, 0·68 and 0·70 for total mortality (P < 0·001 for trend), 1·00, 0·79, 0·70 and 0·71 for CVD mortality (P = 0·006 for trend), 1·00, 0·78, 0·70 and 0·63 for CHD mortality (P = 0·01 for trend), and 1·00, 0·77, 0·64 and 0·90 for stroke mortality (P = 0·12 for trend). These results clearly show that, in type 2 diabetic patients, coffee drinking is associated with reduced total, CVD and CHD mortality⁽Reference Tuomilehto, Jousilahti, Rastenyte, Moltchanov, Tanskanen, Pietinen and Nissinen²²⁹⁾. Caffeine, however, is probably not the active coffee component affecting the risk of type 2 diabetes. This has been demonstrated in a recently published study by van Dam et al. ⁽Reference van Dam, Willett, Manson and Hu²³⁰⁾ In a cohort of 88 259 US women of the Nurses' Health Study II, associations of coffee consumption and the risk reduction of type 2 diabetes were similar for caffeinated and decaffeinated coffee as well as for filtered and instant coffee.

The risk of colon cancer has been reported to decrease by about 25 %⁽Reference Tavani and La Vecchia²³¹⁾, that of gallstones by 22 %⁽Reference Leitzmann, Stampfer, Willett, Spiegelman, Colditz and Giovannucci²³²⁾ and that of cirrhosis of the liver by 60 %⁽Reference Klatsky, Morton, Udaltsova and Friedman²³³⁾. A 29 % risk reduction of having current asthma symptoms have been observed for regular coffee drinkers suffering from asthmatic symptoms when compared with non-coffee drinking patients⁽Reference Schwartz and Weiss²³⁴⁾. Other benefits discussed include a reduction in the risk of Alzheimer's disease⁽Reference Lindsay, Laurin, Verreault, Hebert, Helliwell, Hill and McDowell²³⁵⁾ and Parkinson's disease⁽Reference Hernan, Takkouche, Caamano-Isorna and Gestal-Otero²³⁶⁾ and, at least among a large group of female nurses tracked over many years, fewer suicides⁽Reference Kawachi, Willett, Colditz, Stampfer and Speizer²³⁷⁾.

Coffee intake has also been shown to improve cognitive performance⁽Reference Klatsky, Morton, Udaltsova and Friedman²³³⁾ as well as endurance performance⁽Reference Graham²³⁸⁾.

The mechanisms, however, by which coffee consumption may reduce the risk of the above mentioned diseases are still unknown. In some cases, like for the onset of type 2 diabetes, data on different types of coffee⁽Reference Tuomilehto, Hu, Bidel, Lindstrom and Jousilahti^239, Reference Wu, Hankinson, Willett and Giovannucci²⁴⁰⁾ and on different coffee constituents such as caffeine⁽Reference Greer, Hudson, Ross and Graham^241–Reference Lane, Barkauskas, Surwit and Feinglos²⁴³⁾, chlorogenic acids⁽Reference Johnston, Clifford and Morgan²⁴⁴⁾, quinides⁽Reference Shearer, Farah, de Paulis, Bracy, Pencek, Graham and Wasserman²⁴⁵⁾, and magnesium⁽Reference de Valk²⁴⁶⁾ affecting glucose metabolism are available from animal studies and human trials. A recently published study by Atanasov et al. ⁽Reference Atanasov, Dzyakanchuk, Schweizer, Nashev, Maurer and Odermatt²⁴⁷⁾ demonstrated an inhibitory activity in vitro of coffee extracts on glucocorticoid induced expression of phosphoenolpyruvate carboxy kinase, a key enzyme of the gluconeogenic pathway, which may result in decreased blood glucose concentrations. Some studies have also demonstrated an increase in plasma antioxidant capacity after coffee consumption⁽Reference Somoza, Lindenmeier, Wenzel, Frank, Erbersdobler and Hofmann^144, Reference Natella, Nardini, Giannetti, Dattilo and Scaccini²⁴⁸⁾. The antioxidants which dominate quantitatively in coffee are chlorogenic acids⁽Reference Somoza, Lindenmeier, Wenzel, Frank, Erbersdobler and Hofmann¹⁴⁴⁾ and phenolic compounds, such as caffeic and ferulic acid⁽Reference Fujioka and Shibamoto²⁴⁹⁾, whereas also N-methylpyridinium ions have recently been demonstrated to increase plasma antioxidant activity after dietary intake⁽Reference Somoza, Lindenmeier, Wenzel, Frank, Erbersdobler and Hofmann¹⁴⁴⁾.

Conclusion

The results of controlled trials on health benefits of moderate coffee consumption are often conflicting, as they are based on substantially varying study designs using different coffee beverages administered in varying doses. In particular, the results on caffeine are difficult to interpret, as chronic caffeine intake leads to metabolic adaptation⁽Reference Robertson, Wade, Workman, Woosley and Oates²⁵⁰⁾ which makes it difficult to compare an outcome of short term controlled trials with that of long term epidemiological cohort or cross-sectional investigations. What can be stated about possible health effects of coffee is that there is emerging evidence that, in moderate doses of two to four cups per day, long-term coffee consumption offers some health benefits. The question of whether these health benefits can be improved by applying processing technologies by which the contents of health promoting compounds such as chlorogenic acid or N-methylpyridinium ions can be increased, or by which the contents of harmful compounds such as acrylamide can be lowered, and simultaneously has to be answered in future epidemiological and appropriately designed and controlled human trials.

Gaps of knowledge

• ● Seasonal variability and weather conditions are unpredictable factors, and a source of a certain unavoidable variability in potato tuber quality and sugar content.

• ● As for potatoes, seasonal variability is a factor of uncertainty in cereals, and soil composition is a factor not entirely under control of the farmer. It remains to be seen if low nitrogen fertilisation which interferes with crop yield would be acceptable to the producer.

• ● Very little information is available on possible interventions regarding the cultivation, harvest and storage of coffee. Post-harvest processing such as washing and drying may be the only point of intervention and a subject for further study.

Gaps of knowledge

• ● Although several interventions have been identified, they are often specific to particular products with certain processing characteristics, and a generalisation of measures over a whole food category is therefore impossible.

• ● Most measures to date still need to be evaluated with respect to the impact on product quality and industrial practicability.

• ● Heat processing of foods is usually strongly adapted to product characteristics such as appearance, taste, flavour and texture which are strongly influenced by consumer acceptability and preferences. Any compromise on one of these factors should be avoided, because any mitigation is useless if the consumer does not enjoy (and subsequently does not buy) the product anymore.

Gaps of knowledge

• ● Ongoing research and epidemiological studies will further shed light on the health benefits and risks of the overall human diet as well as particular components of the human diet, particularly with respect to worldwide growing public health concerns linked to obesity, physical inactivity, diabetes and CVD.

Gaps of knowledge

• ● Mitigation measures applied are mostly laboratory scale, and reductions are unlikely to be translated 1:1 into factory scale.

• ● Individual mitigation measures are applicable to certain products, but not necessarily to a whole category of foods.

• ● Similarly, a food category likely consists of different foods, to which individual mitigation strategies are applicable and also it is likely that a fraction of foods for which no mitigation measures are available.

• ● This causes uncertainty in the exposure assessment and consequently in the calculations of MOE.

Justification of classification of acrylamide and sodium intakes in relation to health risk

The classes of sodium intake were based on epidemiological studies demonstrating effects of sodium intake on blood pressure, a known risk factor for CVD.

In this calculation the total population was included. However, if the focus were on overweight persons the increase in sodium intake resulting from the mitigation measures applied might be of more concern because of the possible higher susceptibility of this group in the population to the effects of salt.

This example demonstrates that the risk-benefit model can calculate a simultaneous change in exposure levels of two compounds and that these types of models can be applied to judge the effect of mitigation. The model will be applicable for other mitigation measures as long as reliable quantitative data on concentration of compounds are available and if the relationship(s) is (are) known between the dose of a compound and possible health outcomes.

Gaps of knowledge

• ● A risk-benefit analysis has been attempted for one of the mitigation measures and its side effect is described in this article. The analyses might be applied to other risk-benefit cases, depending on availability of quantitative data.

• ● More exposure data is needed for vulnerable groups with respect to sodium intake, e.g. over weight consumers which are more susceptible to elevated blood pressure.

• ● Limited dose-response data for carcinogenic effects of acrylamide are available. Whether a linear extrapolation model can be used to predict cancer risk in humans at realistic exposure levels, is not clear, however. Further studies using appropriately designed experiments and animal models investigating metabolic fate and biological effects across the full range of acrylamide exposures are required.

• ● Quantification of uncertainties in data and modelling is needed.