Total phenols

Phenolics are a major phytochemical class, and include the broad term ‘polyphenols’, meaning a molecule with one or more phenolic groups. The Folin–Ciocalteu assay is commonly used as a non-specific measure of ‘total phenols’ or ‘total polyphenols’. However, the assay reagent also reacts with sugars, ascorbic acid, aromatic amines and other non-phenolic substances, many of which are present in tree nuts. Thus, total phenols can be regarded as only an approximation of polyphenol content in tree nuts.

Flavonoids

Flavonoids are a subclass of phenolics, characterised by a chalcone C₆C₃C₆ structure, and include the main six subclasses: flavonols, flavones, flavanones, flavan-3-ols, anthocyanidins and isoflavones (Fig. 2). Flavan-3-ols, flavonols and anthocyanins are the main flavonoids present in nuts, while flavanones and isoflavones are found in lesser amounts.

Fig. 2 Major flavonoid classes and other representative phytochemicals present in tree nuts.

Proanthocyanidins

PAC are flavan-3-ol oligomers linked through carbon–carbon bonds. Type A PAC consist of a C₄ to C₆ or C₈ interflavan bond and a C₂-ether bond to the flavanol extension. Type B PAC have single C₄ to C₆ or C₈ interflavan bonding (Fig. 2). Tree nut PAC are comprised mainly of (+)-catechin and ( − )-epicatechin, but also include afzelechin (almonds) and epigallocatechin (hazelnuts, pecans, pistachios)⁽Reference Gu, Kelm and Hammerstone^1–Reference Monagas, Garrido and Lebron-Aguilar³⁾. Among tree nuts, the A-type PAC have been found only in almonds. The majority of tree nut PAC are highly polymerised (> 10-mers)⁽Reference Gu, Kelm and Hammerstone¹⁾.

Stilbenes

Stilbenes are similar to chalcones, as their structure consists of two phenyl groups interconnected through an ethene bond. The ethene bond is mainly in the trans configuration, but may revert to the cis form on exposure to light or heat. Resveratrol and piceid are the only stilbenes thus far identified in tree nuts, and are found in pistachios (Fig. 2).

Phytosterols

Phytosterols are lipophilic plant-synthesised steroids. Sterols consist of three six-carbon rings, one five-carbon ring attached to an aliphatic chain, and a C₃ hydroxyl group on the A ring⁽Reference McClements, Decker, Damodaran, Parkin and Fennema⁴⁾. Sterol esters have a fatty acid esterified to the C₃ hydroxyl group⁽Reference McClements, Decker, Damodaran, Parkin and Fennema⁴⁾. Sterols and stanols are differentiated by an alkene bond at the C₁ position on the B ring. Tree nut phytosterols are mainly composed of β-sitosterol (Fig. 2).

Carotenoids

Carotenoids are polyisoprenoids with differing degrees of conjugated double bonds (Fig. 2). In contrast to other plant foods, tree nuts contain very low amounts of carotenoids with α- and β-carotene, β-cryptoxanthin, lutein and zeaxanthin found in μg/100 g concentrations when present at all. Pistachios present a modest exception, with a mean β-carotene and lutein content of 0·21 and 4·4 mg/100 g dry weight, respectively⁽Reference Kornsteiner, Wagner and Elmadfa⁵⁾.

Other phytochemical classes

Tree nuts also contain phytates and the lipid subclasses of chlorophylls and sphingolipids. Other phenolic constituents consist of lignans, alkylphenols and hydrolysable tannins (HT). Walnuts also contain the alkaloid melatonin and the naphthoquinone juglone (Fig. 2).

US Department of Agriculture phytochemical databases

The USDA Nutrient Data Laboratory (Beltsville, MD, USA) established databases for total phenols, flavonoids, isoflavones and PAC^(6–9). The Nutrient Data Laboratory maintains these databases and encourages submission of data directly to their laboratory. In general, the USDA databases report phytochemical mean, minimum and maximum contents, a confidence code that judges the reliability of the data, and the source of information. The USDA phytochemical databases are not frequently updated. For example, the USDA flavonoid database was released in 2003, and updated in 2007. Phytochemical content is reported as aglycone equivalents, which in some cases has been converted from glycoside equivalents. Thus, the phytochemical content of some tree nuts may not match values reported in the literature. This method facilitates combining data from studies using hydrolysis to remove glycosides from polyphenols. The post-harvest processing of tree nuts is not distinguished in these databases, but it should be noted that the flavonoid data are from commercial samples⁽Reference Harnly, Doherty and Beecher¹⁰⁾.

US Department of Agriculture phytochemical databases: total phenols

The USDA Oxygen Radical Absorbance Capacity (ORAC) database includes the total phenol value of each tree nut (Table 2) ⁽⁶⁾. The data are derived from only one study, with seven or eight samples for each tree nut. Pecans, pistachios and walnuts have the highest total phenol content, with 1556 to 2016 mg gallic acid equivalents (GAE)/100 g.

Table 2 Total phenols content of nine tree nuts reported in the US Department of Agriculture (USDA)⁽⁶⁾ and Phenol-Explorer⁽Reference Neveu, Perez-Jiménez and Vos¹⁶⁾ databases

GAE, gallic acid equivalents.

US Department of Agriculture phytochemical databases: flavonoids

The USDA flavonoid database reports the flavan-3-ol, flavanone, flavonol and anthocyanin content of each tree nut (Table 3) ⁽⁸⁾. Pecans, pistachios, almonds and hazelnuts have the highest reported flavonoid content, with 12 to 34 mg flavonoids/100 g. The data for the USDA database are derived from only three studies and the number of samples ranged from 1 to 16. Therefore, the data populating the USDA flavonoid database are limited. Ideally, these values would be derived from multiple laboratories to arrive at reliable values for flavonoid content.

Table 3 Flavonoid content of tree nuts indexed in the 2007 US Department of Agriculture flavonoid database⁽⁸⁾

All tree nuts except for macadamias and Brazil nuts have flavonoid values listed in the USDA flavonoid database. In contrast, Yang et al. ⁽Reference Yang, Liu and Halim¹¹⁾ reported 107 mg catechin equivalents/100 g in Brazil nuts, and 140 mg catechin equivalents/100 g in macadamias, using a non-specific analytical technique. Therefore, more rigorous investigations into the flavonoid content of Brazil nuts and macadamias are warranted.

Flavan-3-ols, flavonols and anthocyanins are the main flavonoids in nuts. Cashews and pine nuts do not contain anthocyanins. Flavonols are also reported for almonds and pistachios, mainly as isorhamnetin and kaempferol for almonds and quercetin for pistachios. However, no studies are included in the database that measure isorhamnetin or kaempferol content in tree nuts other than almonds. Thus, a significant amount of phytochemical content remains uncharacterised from most tree nuts.

Pecans have the highest total flavonoid content among nuts at 34 mg/100 g, consisting mostly of flavan-3-ols and anthocyanins. Hazelnuts and almonds also have an appreciable content of flavonoids with 18 and 15 mg/100 g, respectively. Flavanones are only reported in almonds, but no studies included in the database have measured eriodictyol in the other tree nuts. Most confidence codes are A and B for all flavonoids and tree nuts, although an unidentified species of walnut received a C value for flavonoid data confidence.

US Department of Agriculture phytochemical databases: isoflavones

Last updated in 2008, the USDA isoflavone database contains entries for most tree nuts, except for macadamias and pine nuts⁽⁷⁾. To our knowledge, there are no studies examining isoflavone content of macadamias, but Kuhnle et al. ⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾ reported 32 μg isoflavones/100 g pine nuts. The isoflavone database includes daidzein, genistein and glycitein content and their sums, reported as ‘total isoflavones’. Values for the isoflavones formononetin and coumestrol are also reported. Pistachios have the highest isoflavone content of nuts at 3·63 mg/100 g mainly as daidzein and genistein, more than 100-fold greater than levels of other nuts (Table 4). Only one or two samples have been analysed for the isoflavone data, and are derived from three references. The confidence codes for these data are mainly C and D values, but Brazil nuts received a B value. Therefore, more work is needed to quantify isoflavones in tree nuts, especially to analyse a wider number of samples for inclusion into the USDA isoflavone database.

Table 4 Isoflavone content of tree nuts listed in the US Department of Agriculture database⁽⁷⁾

NR, not reported.

US Department of Agriculture phytochemical databases: proanthocyanidins

The USDA PAC database was released in 2004⁽⁹⁾. All of the tree nuts are included in the database, and values are based solely on the work by Gu et al. ⁽Reference Gu, Kelm and Hammerstone¹⁾. In this study, Brazil nuts, pine nuts and macadamias had no detectable PAC. Between five and eight samples were analysed for each tree nut, and each nut was assigned a confidence code of ‘A’.

The USDA PAC database does not distinguish between A- and B-linked PAC or the polymer units (for example, catechin, epicatechin) and reports content as catechin equivalents. Therefore, the database is an approximation of PAC content. In part, this is due to lack of available standards for quantification and characterisation purposes. Also, since monomers are included, catechin and epicatechin content may overlap with the USDA flavonoid database flavan-3-ol content.

The majority of tree nut PAC are highly polymerised (> 10-mers). However, cashews only have monomer and dimer PAC. PAC are the predominant polyphenols in almonds, hazelnuts, groundnuts, pecans and pistachios. Hazelnuts and pecans have the highest PAC content with 501 and 494 mg/100 g, respectively (Table 5). No PAC have been reported in Brazil nuts, macadamias or pine nuts, but Venkatachalam & Sathe⁽Reference Venkatachalam and Sathe¹³⁾ reported 10 mg catechin equivalents/100 g using a non-specific method of analysis.

Table 5 Total proanthocyanidins (PAC) content of tree nuts reported in the US Department of Agriculture database⁽⁹⁾

Future revisions of the USDA PAC database may include information about subunits, linkages, and more information about their characterisation. Prodanov et al. ⁽Reference Prodanov, Garrido and Vacas¹⁴⁾ and Monagas et al. ⁽Reference Monagas, Garrido and Lebron-Aguilar¹⁵⁾ provided qualitative and some quantitative information about almond and hazelnut PAC, but more work is needed to characterise PAC in these and other tree nuts, including developing quantitative methods.

Phenol-Explorer

Phenol-Explorer (PE) is a web-based database of the polyphenol content of foods⁽Reference Neveu, Perez-Jiménez and Vos¹⁶⁾. PE was developed by Augustin Scalbert and colleagues at INRA Clermont-Ferrand, Unité de Nutrition Humaine. PE's web-based interface allows queries for foods or polyphenols. PE has more phenolic classes than the USDA databases, including ‘total phenols’, flavonoids, lignans, stilbenes, phenolic acids and other phenolics (Table 6).

Table 6 Polyphenols included in the Phenol-Explorer database⁽Reference Neveu, Perez-Jiménez and Vos¹⁶⁾

Phenol-Explorer total phenol database

The PE database includes the total phenol value of each tree nut (Table 2). As compared with the USDA total phenol database derived from Wu et al. ⁽Reference Kornsteiner, Wagner and Elmadfa⁵⁾, total phenol values presented in this database are derived from studies of Wu et al. ⁽Reference Kornsteiner, Wagner and Elmadfa⁵⁾ and Kornsteiner et al. ⁽Reference Wu, Beecher and Holden¹⁷⁾. Thus, the data retrieved in the PE database could be more consequential than those in the USDA database. Though not considered in detail in the present review, we note that chestnuts are included in the USDA total phenol database and are identified as having the highest total phenol content, followed by pecans, pistachios and walnuts.

Phenol-Explorer phytochemical database

PE is more versatile than the USDA phytochemical databases as it allows for conversion between units such as aglycone equivalents (the USDA database preferred method) or mg/100 g or mol/100 g equivalents for polyphenol glycosides. The web portal has an option for displaying polyphenol class totals. The polyphenol results are organised by method of quantification, such as chromatography, chromatography after hydrolysis, Folin assay (the common measure of total phenols), pH differential method for anthocyanin content, and normal-phase chromatography.

PE also has a variety of summary reports. The selected reports include alcoholic beverages, cereals, cocoa, fruits and fruit products, non-alcoholic beverages, oils, seasonings, seeds, soya and soya products, and vegetables. While tree nut oils are not included in the ‘oils’ report, tree nuts are included in the ‘seeds’ report.

We list the PE entries used for almonds, cashews, hazelnuts, pecans, pistachios and walnuts (Table 7). Notably, no flavonoid, PAC, or phenolic acid entries exist for Brazil nuts, macadamias or pine nuts. While Brazil nuts and macadamias have total phenol entries, pine nuts are not listed in PE. The flavonoid and PAC data included in PE for most tree nuts are largely similar to those in USDA databases. Accordingly, the data are mainly based on the references from Harnly et al. ⁽Reference Harnly, Doherty and Beecher¹⁰⁾ and Gu et al. ⁽Reference Gu, Kelm and Hammerstone¹⁾ for flavonoid and PAC content. While groundnut isoflavones are listed in PE, tree nut isoflavone content is not. Also, while lignans are included in PE, the studies by Thompson et al. ⁽Reference Thompson, Boucher and Liu¹⁸⁾, Smeds et al. ⁽Reference Smeds, Eklund and Sjoholm¹⁹⁾ and Kuhnle et al. ⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾ which quantified lignans in many of the tree nuts are not currently entered. Therefore, existing and future studies of tree nut phytochemicals should be submitted to PE for inclusion. The PAC content of tree nuts is also listed in PE, and appears less than the values for the USDA PAC database (Table 8). This is because PE does not include PAC monomers in foods from Gu et al. ⁽Reference Gu, Kelm and Hammerstone¹⁾.

Table 7 Phenol-Explorer concentration of flavonoids, phenolics, stilbenes and other phenols in nuts⁽Reference Neveu, Perez-Jiménez and Vos¹⁶⁾

fw, Fresh weight.

Table 8 Phenol-Explorer listed concentration of nut proanthocyanidins⁽Reference Neveu, Perez-Jiménez and Vos¹⁶⁾

fw, Fresh weight.

PE includes data for phenolic acids, stilbenes and naphthoquinones, which are not currently accounted for in the USDA phytochemical databases. Since PE has the ability to compile data for more polyphenol classes, is updated more frequently than USDA databases, and contains a greater number of food types, PE may become more widely used among polyphenol researchers than USDA databases. Nevertheless, existing quality data regarding tree nut lignans, flavonoids, alkylphenols and isoflavones should be submitted to PE for consideration of inclusion in the database.

The PE database has been recently employed to identify the richest dietary sources of polyphenols (as the sum of the contents of all individual polyphenols) and their intake among a large cohort of adults. Pérez-Jiménez et al. ⁽Reference Pérez-Jiménez, Neveu and Vos²⁰⁾ reported five tree nuts (chestnuts, hazelnuts, pecans, almonds and walnuts) among the richest 100 dietary sources of polyphenols at a rank of 11, 21, 22, 40 and 79 containing 1215, 495, 493, 187 and 28 mg/100 g, respectively. Based upon serving sizes of 19, 28, 15 and 10 g, chestnuts, hazelnuts, pecans and almonds were ranked as 15, 21, 33 and 62 for their total polyphenol content of 230, 138, 69 and 19 mg/serving. It is noteworthy that using PE, the values obtained using the Folin assay were found to systematically exceed the total polyphenol content values. Using the French cohort of the SUpplémentation en VItamines et Minéraux AntioXydants (SU.VI.MAX) trial, Pérez-Jiménez et al. ⁽Reference Pérez-Jiménez, Fezeu and Touvier²¹⁾ assessed the dietary polyphenol intake of 4942 men and women aged 45–60 years and found total polyphenol intakes at 1193 (sd 510) mg/d per person with 8 (sd 18) mg/d derived from seeds, with 37, 27 and 14 % of this amount from walnuts, hazelnuts and chestnuts, respectively. Further, of the 41 (sd 39) mg/d per person intake of hydroxybenzoic acids, 8 % was derived from walnuts; of the 12 (sd 22) mg/d per person ( − )-epigallocatechin intake, 99 % was derived from tea but 0·2 % was consumed as almonds. In contrast, calculating total dietary polyphenols as the sum of extractable polyphenols, condensed tannins and hydrolysable polyphenols determined in their laboratory, Saura-Calixto et al. ⁽Reference Saura-Calixto, Serrano and Goñi²²⁾ reported a total bioaccessible polyphenol intake of 2533 mg/d per person in Spain, with 107 mg/d per person derived from consumption of a mean of 5·9 g nuts/d.

US Department of Agriculture National Nutrient Database for Standard Reference for sterols

Phytosterols of tree nuts are reported in the USDA National Nutrient Database for Standard Reference, release 23 (SR-23)⁽²³⁾. The β-sitosterol campesterol and stigmasterol contents of most tree nuts are reported, excluding Brazil nuts and cashews (Table 9). Unlike the USDA phytochemical database, the tree nut phytosterol data in SR-23 are not derived from literature reports, but rather from data supplied by tree nut commodity boards and the Hammons Products Company in the case of black walnuts.

Table 9 Phytosterol content of tree nuts, according to US Department of Agriculture 2009 National Nutrient Database for Standard Reference, release 23⁽²³⁾

NR, not reported.

* Other, δ-5-avenasterol, sitostanol, campestanol and other minor phytosterols.

The SR-23 phytosterol data for tree nuts range from 72 mg/100 g in English walnuts to 214 mg/100 g in pistachios. Tree nuts are mainly composed of β-sitosterol, but are also composed of a number of minor sterols, sterol esters and stanols. Thus, the phytosterol values for most tree nuts in SR-23 are somewhat less than literature values due to the contribution of other minor sterols.

The SR-23 data could be immediately improved by submitting data for Brazil nuts and cashews that contain 154 to 190 mg sterols/100 g. The USDA Nutrient Data Laboratory encourages submission of data to support SR-23, so the phytosterol data could potentially be updated. Since the almond data for total sterol content include minor sterols, it is plausible that these data could be included for other tree nuts and would more accurately reflect tree nut sterol content.

US Department of Agriculture National Nutrient Database for Standard Reference for carotenoids

The USDA SR-23 database also includes vitamin A equivalents, β-carotene, α-carotene, β-cryptoxanthin, lycopene, lutein and zeaxanthin content of tree nuts. As previously reported, raw pistachios have the highest carotenoid content among tree nuts, which is reported mainly as 1205 μg lutein and zeaxanthin/100 g pistachios, resulting in 415 IU vitamin A/100 g. Raw and dry roasted pistachios also have 13 and 21 retinoic acid equivalents/100 g. Similar to the data on phytosterols, some SR-23 tree nut carotenoid entries are derived from data submitted by tree nut commodity boards, but also include data generated by the USDA Nutrient Data Laboratory and a study by Perry et al. ⁽Reference Perry, Rasmussen and Johnson²⁴⁾. As most tree nuts are not significant dietary sources of carotenoids, an effort to improve carotenoid data may not be worthwhile.

European Food Information Resource database

EuroFIR is funded through the European Union to develop a comprehensive food databank to be used as a single authoritative source for European food composition data⁽²⁵⁾. Additionally, EuroFIR has published reports about food and health, including a statement about nuts and risk of CVD. EuroFIR is developing a database for bioactive foods components. This database is unique in that it will compile both content and bioactivity data. The EuroFIR database, eBASIS (BioActive Substances in Food Information System), is not yet publicly available, but is currently available to member institutions.

Comparison of tree nut phytochemical database values relative to other foods

A report by Pérez-Jiménez et al. ⁽Reference Pérez-Jiménez, Neveu and Vos²⁰⁾ used the PE database to compare the richest sources of dietary polyphenols. The seeds category, which includes tree nuts, was ranked third behind spices and fruits for containing the most number of items in their list of the 100 foods with the highest polyphenol content. Walnuts, hazelnuts and chestnuts were identified as main contributors to polyphenol content within the seeds class.

To compare tree nut phytochemical content with other whole plant foods, we retrieved database values for fruits and vegetables from PE. Apples, highbush blueberries, purple grapes, green beans, spinach, tomatoes and yellow onions were selected because they are commonly consumed and available for consumption in the USA. Polyphenol values were retrieved from PE and carotenoid and phytosterol values were from the USDA SR-23 database.

The reference fruits and vegetables had phytosterol values of 4 to 15 mg/100 g, while tree nuts had 72 to 214 mg/100 g. Spinach, tomatoes and green beans carotenoid and xanthophyll content provide 690 to 9377 IU vitamin A, which is significantly greater than for tree nuts. Pecans, walnuts, pistachios and hazelnuts had more than double total phenols and PAC than the comparison foods (Fig. 3). Almond, Brazil nut and cashew total phenols were comparable with those of green beans, spinach, blueberries and apples. The PAC content of almonds was similar to those of grapes and blueberries. Polyphenol content by HPLC varied, with blueberries, grapes, spinach and apples having 56 to 310 mg/100 g, while tree nuts had 1·1 to 54 mg/100 g. The stilbene content of purple grapes was comparable with that of pistachios, with 0·19 and 0·11 mg/100 g, respectively. The database value for pistachio content is somewhat higher than the literature average of 0·8 mg stilbene/100 g (Table 10). Walnuts and walnut liquor were the only foods with naphthoquinone content in PE. Alkylphenol contents of cashews and pistachios are not reported in PE, or for the reference vegetables. However, bran breakfast cereals and whole grain flour bread have 286 and 25 mg/100 g, respectively. Several reports in the literature have assigned 144 and 44 mg alkylphenols/100 g for cashews and pistachios, respectively (Table 11). Similarly, while tomatoes have 0·02 mg lignan/100 g, tree nuts values which range from 0·02 to 0·97 mg lignin/100 g are not reported in PE (Table 11). Since all of the polyphenol classes have not been comprehensively surveyed in fruits, vegetables and tree nuts, it is difficult to draw reliable conclusions about the polyphenol abundance from HPLC analysis alone. More work is necessary to characterise, quantify and report fruit, vegetable and tree nut polyphenol values to phytochemical databases.

Fig. 3 Polyphenol content of reference foods and tree nuts in the Phenol-Explorer database. (), Total phenols; (□), polyphenols by HPLC; (■), proanthocyanidins.

Table 10 Phytochemical content of tree nuts reported in the literature*

ND, not determined in the literature; < LOD, below limit of detection.

* Values were determined by compiling data from all available literature reports. Means were based on the reported values for distinct samples; for example, if a study analysed two varieties of tree nuts, the content of both varieties would be included in the determination of the mean.

† mg gallic acid equivalents/100 g, a non-specific measure.

‡ Proanthocyanidin values include monomers that are also included in the flavonoid group.

§ mg catechin equivalents/100 g, a non-specific measure.

Table 11 Phytochemical content of tree nuts reported in the literature*

ND, not determined in the literature.

* Values were determined by compiling data from all available literature reports or database values. Means were based on the reported values for distinct samples; for example, if a study analysed two varieties of tree nuts, the content of both varieties would be included in the determination of the mean.

Total phenols

The total phenol values among the nine tree nuts reviewed here vary widely (Table 10), with walnuts and pecans being the top two, followed by pistachios with its value being 50 % lower. Almonds, Brazil nuts, cashews, macadamias and pine nuts all contain similar total phenol content between 200–300 mg GAE/100 g.

As compared with the data presented in the two databases (USDA and PE), total phenol values of almonds, Brazil nuts, cashews, macadamias, pecans and walnuts are not markedly different while mean literature values of hazel nuts and pistachios are 50 % lower than database values and pine nuts are 2-fold larger. There are numerous factors that contribute to the inconsistency in the reported values, including the specific methodology for extraction of the phenols, nut cultivars and nut processing. This issue is discussed below.

Flavonoids

Literature values for flavonoid content of tree nuts range from 0·03 mg/100 g in pine nuts to 2700 mg/100 g in pecans (Table 10). The content of flavonoids has been reported for each tree nut, but comprehensive and systematic investigations characterising flavonoids from tree nuts are lacking. For example, Brazil nuts are reported to have 108 mg flavonoids/100 g by Yang et al. ⁽Reference Yang, Liu and Halim^11, Reference Yang²⁶⁾ using a non-specific measure of flavonoids. In contrast, Harnly et al. ⁽Reference Harnly, Doherty and Beecher¹⁰⁾ did not find flavonoids in Brazil nuts⁽Reference Harnly, Doherty and Beecher¹⁰⁾. Brazil nuts contain the isoflavones daidzein and biochanin A, albeit at low levels⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾. John & Shahidi⁽Reference John and Shahidi²⁷⁾ quantified catechin, ellagic acid and phenolic acids in Brazil nuts, but about 10-fold less than the report by Yang et al. ⁽Reference Yang, Liu and Halim¹¹⁾. Similarly, Yang et al. found 137·9 mg catechin equivalents/100 g in macadamias⁽Reference Yang, Liu and Halim¹¹⁾. Thus, a more careful and comprehensive analysis of macadamia and Brazil nut flavonoids and phenolics is warranted.

Flavonoids: pecans

Pecans have an apparently high value for flavonoid content because Malik et al. ⁽Reference Malik, Perez and Lombardini²⁸⁾ reported 1800 to 4700 mg catechin/100 g pecans. This is significantly larger than the report by Harnly et al. ⁽Reference Harnly, Doherty and Beecher¹⁰⁾ of only 7·2 mg catechin/100 g pecans. The reason(s) for this extreme difference is not clear, as both techniques employed direct extraction and used similar methods for quantification. Potentially important, Malik et al. ⁽Reference Malik, Perez and Lombardini²⁸⁾ used defatted kernels for extraction, whereas Harnly et al. ⁽Reference Harnly, Doherty and Beecher¹⁰⁾ did not.

Flavonoids: almonds

For almonds, a number of reports were published after the USDA flavonoid database release. These include studies from Monagas et al. ⁽Reference Monagas, Garrido and Lebron-Aguilar³⁾, Hughey et al. ⁽Reference Hughey, Wilcox and Minardi²⁹⁾, Garrido et al. ⁽Reference Garrido, Monagas and Gomez-Cordoves³⁰⁾, Mandalari et al. ⁽Reference Mandalari, Tomaino and Arcoraci³¹⁾, Bolling et al. ⁽Reference Bolling, Dolnikowski and Blumberg^32, Reference Bolling, Dolnikowski and Blumberg³³⁾ and Teets et al. ⁽Reference Teets, Minardi and Sundararaman³⁴⁾. However, earlier reports by Chen et al. ⁽Reference Chen, Milbury and Lapsley³⁵⁾ and Frison et al. ⁽Reference Frison and Sporns^36, Reference Frison-Norrie and Sporns³⁷⁾ were not included in the last revision of the USDA flavonoid database. Combining these reports yields an average of 25·01 mg flavonoids/100 g almonds.

Flavonoids: cashews

A systematic investigation into cashew flavonoid content is lacking, but Kuhnle et al. ⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾ added to the values of formononetin and biochanin A reported in the USDA isoflavone database and PE.

Flavonoids: hazelnuts

Polyphenols have been further characterised in hazelnut skins. Monagas et al. ⁽Reference Monagas, Garrido and Lebron-Aguilar¹⁵⁾ reported the catechin content and Del Rio et al. ⁽Reference Del Rio, Calani and Dall'Asta³⁸⁾ reported monomeric and oligomeric flavan-3-ols, flavonols, dihydrochalcones and phenolic acids in hazelnut skins. Although these studies did not report polyphenol content in terms of whole hazelnuts, monomeric and oligomeric flavan-3-ols were found to be the predominant polyphenol subclass, accounting for 95 % of total polyphenols⁽Reference Del Rio, Calani and Dall'Asta³⁸⁾. In other studies, Yurttas et al. ⁽Reference Yurttas, Schafer and Warthesen³⁹⁾ reported quercetin from hazelnut extracts, whereas Harnly et al. ⁽Reference Harnly, Doherty and Beecher¹⁰⁾ did not detect quercetin following acid hydrolysis of extracts.

Flavonoids: pistachios

In their analysis of pistachios, Ballistreri et al. ⁽Reference Ballistreri, Arena and Fallico⁴⁰⁾ and Seeram et al. ⁽Reference Seeram, Henning and Niu^41, Reference Seeram, Zhang and Henning⁴²⁾ found eriodictyol and anthocyanin. Gentile et al. ⁽Reference Gentile, Tesoriere and Butera⁴³⁾, Seeram et al. ⁽Reference Seeram, Zhang and Henning⁴²⁾ and Kuhnle et al. ⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾ also reported new isoflavone data for pistachios that could be included in future USDA database updates.

Flavonoids: walnuts

Catechin was detected in extracts of walnuts by Gómez-Caravaca et al. ⁽Reference Gomez-Caravaca, Verardo and Segura-Carretero⁴⁴⁾. Quercetin was found in black walnuts but not quantified by Anderson et al. ⁽Reference Anderson, Teuber and Gobeille⁴⁵⁾. Kuhnle et al. ⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾ added to the isoflavone data available for walnuts.

The widespread presence of flavonoids in tree nuts but the absence of data on the flavonoid content of macadamias, Brazil nuts and pine nuts suggests that thoughtful analyses of these nuts are warranted. Importantly, since truly comprehensive analyses of flavonoids in most tree nuts are lacking and further work to optimise extraction methods is required, further studies are required to confirm and extend the accuracy, breadth and reliability of the available quantitative data.

Phenolic acids and aldehydes

PE includes values for simple phenolics in tree nuts. Pecans and walnuts have high simple phenolic content at 2558 and 39·1 mg/100 g, respectively. The phenolic acids of pecans are primarily gallic acid, while those of walnuts are syringic acid. A high content of ellagic acid (28·5 mg/100 g) was found in Persian walnuts by Li et al. ⁽Reference Li, Tsao and Yang⁵¹⁾. Gómez-Caravaca et al. ⁽Reference Gomez-Caravaca, Verardo and Segura-Carretero⁴⁴⁾, Shahidi et al. ⁽Reference Shahidi, Alasalvar and Liyana-Pathirana⁵²⁾ and Alasalvar et al. ⁽Reference Alasalvar, Karamac and Amarowicz⁵³⁾ also reported quantitative data for phenols in walnuts and hazelnuts, but these data have not been entered into PE. Wijeratne et al. ⁽Reference Wijeratne, Amarowicz and Shahidi^54, Reference Wijeratne, Abou-Zaid and Shahidi⁵⁵⁾ reported simple phenolics in almond skin, but the quantitative data are not reported on a nut basis. More recently, Mandalari et al. ⁽Reference Mandalari, Tomaino and Arcoraci³¹⁾ have reported on a variety of phenolic acids in almond skin; these data are not included in the current version of PE.

No quantitative data for simple phenols are reported for cashews or pine nuts. Thus, a systematic evaluation of phenols in these tree nuts would be useful to establish the simple phenols present in these nuts.

Alkaloids

One report by Reiter et al. ⁽Reference Reiter, Manchester and Tan⁵⁶⁾ quantified melatonin in walnuts. There are no other qualitative reports of phyto-alkaloids in tree nuts.

Phytates

Tree nuts contain a significant amount of phytates, from 100 to 2500 mg/100 g (Table 11). Almonds have the highest reported phytate content, with pine nuts and Brazil nuts having the lowest content.

Chlorophylls

Pistachios have the highest chlorophyll content at 11 000 μg/100 g. In contrast, pine nuts (Aleppo pine nuts; Pinus halepensis Mill.) harvested in Tunisia contain 3 μg/100 g chlorophyll. No other published information is available on the chlorophyll content of other tree nuts.

Lignans

Lignans are included in the PE and eBASIS databases. Tree nuts contain 67–973 μg lignans/100 g, with the highest values reported for cashews (Table 11). To date, there have been no reports of lignans in macadamia nuts. Smeds et al. ⁽Reference Smeds, Eklund and Sjoholm¹⁹⁾, Thompson et al. ⁽Reference Thompson, Boucher and Liu¹⁸⁾ and Kuhnle et al. ⁽Reference Kuhnle, Dell'Aquila and Aspinall¹²⁾ each reported tree nut lignans. Future studies should also investigate the lignan content of macadamias.

Alkylphenols

Cashews and pistachios have significant alkylphenol contents at 144 and 44 mg/100 g, respectively. The alkylphenols in cashews and other tree nuts are principally cardanols, anacardic acid and cardols⁽Reference Lercker, Strocchi and Conte^57–Reference Trevisan, Pfundstein and Haubner⁵⁹⁾.

Naphthoquinones

Pecans and walnuts have 41 and 12 mg juglone/100 g, respectively. Naphthoquinones have not been reported in other tree nuts. Data from Colaric et al. ⁽Reference Colaric, Veberic and Solar⁶⁰⁾ for walnut juglone are included in PE. The pecan juglone content reported by Hedin et al. ⁽Reference Hedin, Langhans and Graves⁶¹⁾ (regarding tree leaves) and Borazjani et al. ⁽Reference Borazjani, Graves and Hedin⁶²⁾ used less reliable methods for their analyses and, thus, are unlikely to be included in the PE database. Thus, an updated survey of juglone in pecan kernels is necessary to include this nut in PE.

Hydrolysable tannins

HT have only been quantified in walnuts, with 27·56 mg/100 g. Glansreginins A and B are the only HT to be quantified, but more than fifty HT have been characterised in walnuts⁽Reference Anderson, Teuber and Gobeille^45, Reference Cerda, Tomas-Barberan and Espin^63–Reference Zhang, Liao and Moore⁶⁸⁾. Future efforts should focus on quantifying these HT in walnuts. The presence of ellagic acid in pecans and almonds and gallic acid in pecans and hazelnuts also suggests that more work is needed to characterise HT in these nuts. Thus, efforts should be made to determine structures of HT in walnuts and other tree nuts.

Sphingolipids

Sphingolipids, mainly squalene and cerebroside, have been reported in all tree nuts except macadamias. Reported sphingolipid content ranges from 0·4 mg/100 g in pistachios to 612 mg/100 g in walnuts. Brazil nuts also have high sphingolipid content at 593 mg/100 g. The majority of the sphingolipid data have been derived by Miraliakbari & Shahidi⁽Reference Miraliakbari and Shahidi^69–Reference Miraliakbari and Shahidi⁷¹⁾ utilising a non-specific method of quantification. Thus, more effort is needed to gain reliable data for the sphingolipid content of tree nuts.

Genetics (variety)

The pathways for the synthesis of phytochemicals are dictated principally by genetics. The data presented in the ‘Phytochemical databases with tree nuts’ and ‘Tree nut phytochemical values reported after database publication’ sections reveal a large difference in phytochemical profiles between tree nuts. This variation is a consequence of divergence of gene expression between nut types. In addition, there is a substantial diversity in phytochemical synthesis between genotypes. Comprehensive research investigating the effect of genotype on phytochemical content within each tree nut type has not been conducted. A few studies have reported that the genotype effect was significant on total phenol, flavonoid and ‘total antioxidant capacity’ (TAC) of almonds, hazelnuts, pecans, pistachios and walnuts.

Genetics (variety): almonds

There are three available almond studies examining the effect of genotype on phytochemical content in California almonds. Milbury et al. ⁽Reference Milbury, Chen and Dolnikowski⁷⁴⁾ observed that among seven almond genotypes, total phenol content in Padre almonds at 241 mg/100 g GAE was 100 % larger than in Fritz almonds (Table 13). TAC in almond skins of seven genotypes also varied with a 142 % difference between the highest and the lowest ferric-reducing antioxidant power (FRAP) value. Bolling et al. ⁽Reference Bolling, Dolnikowski and Blumberg³²⁾ and Milbury et al. ⁽Reference Milbury, Chen and Dolnikowski⁷⁴⁾ characterised twenty flavonoids and phenolic acids in seven and eight California almond genotypes, respectively. They found the sum of their concentrations varied 2·7-fold between almond genotypes. They also found that polyphenol profile between almond genotypes was able to be discriminated based on their heredity (Fig. 5). Particularly, the concentration of total phenols in Sonora almonds was 170 % larger than in Fritz almonds. Hughey et al. ⁽Reference Hughey, Wilcox and Minardi²⁹⁾ also reported that Carmel almonds had 47 % more flavonoid content than Nonpareil almonds. Similarly, a study of ten Portuguese almond genotypes showed a 4- and 18-fold difference in flavonoids and total phenols between genotypes, respectively⁽Reference Barreira, Ferreira and Oliveira⁷⁵⁾.

Table 13 Polyphenol content, total phenols and ferric-reducing antioxidant power (FRAP) values of seven almond genotypes over 3 years⁽Reference Milbury, Chen and Dolnikowski⁷⁴⁾

(Mean values and standard deviations)

GAE, gallic acid equivalents; TE, Trolox equivalents.

^a,b,c Mean values within a column with unlike superscript letters are significantly different (P ≤ 0·05; ANOVA and Tukey's honestly significant difference (HSD) test).

Fig. 5 Canonical discriminant analysis of almond genotypes based on polyphenol content and antioxidant activity⁽Reference Bolling, Dolnikowski and Blumberg³²⁾. M, Mission; F, Fritz; B, Butte; C, Carmel; N, Nonpareil; Mo, Monterey; S, Sonora; Can 1, first canonical variable; Can 2, second canonical variable. Data represent the first two canonical variables of almond samples by genotype with 80 % confidence ellipses.

The concentration of individual flavonoids in almonds depends upon the genotype. Bolling et al. ⁽Reference Bolling, Dolnikowski and Blumberg³²⁾ found that Sonora almonds are particularly high in catechin and isorhamnetin-3-O-glucoside compared with Nonpareil almonds. Thus, genotype affects the type and concentration of flavonoids synthesised in almonds. The genotype effect on other phytochemicals in almonds remains to be determined.

Genetics (variety): hazelnuts

Amaral et al. ⁽Reference Amaral, Casal and Citová⁷⁶⁾ observed that phytosterol contents differed between nineteen hazelnut genotypes harvested from Vila Real in 2001. The concentration of total sterols ranged from 134 to 263 mg/100 g oil, and the concentration of the main phytosterol, β-sitosterol ranged from 108 to 220 mg/100 g oil. Cristofori et al. ⁽Reference Cristofori, Ferramondo and Bertazza⁷⁷⁾ reported that the total phenol content of hazelnuts varied among seven genotypes. Jakopic et al. ⁽Reference Jakopic, Petkovsek and Likozar⁷⁸⁾ reported that the total phenol content of hazelnuts ranged from 70 to 478 mg GAE/kg kernels, with an average of 189·5 mg/kg among twenty hazelnut genotypes harvested in Maribor, Slovenia. Further, they reported that the concentrations of myricetin-3-O-rhamnoside ranged from 0·2 to 2·1 mg/kg, with an average of 1·0 mg/kg kernels; quercetin-3-O-rhamnoside ranged from 1·4 to 9·9 mg/kg, with an average of 3·6 mg/kg; quercetin-pentoside ranged from 0·05 to 0·15 mg/kg, with an average of 0·08 mg/kg; catechin ranged from 1·4 to 26·3 mg/kg, with an average of 7·3 mg/kg; gallic acid ranged from 0·09 to 0·52 mg/kg, with an average of 0·26 mg/kg; and protocatechuic acid ranged from 0·5 to 2·9 mg/kg, with an average of 0·9 mg/kg. Solar & Stampar⁽Reference Solar and Stampar⁷⁹⁾ also reported that epicatechin content ranged from 14·0 to 74·5 mg/kg kernels in sixteen hazelnut genotypes harvested in Slovenia. The impact of genotype on polyphenols in hazelnuts is consistent with that on almonds.

Genetics (variety): pecans

Vilarreal-Lozoya et al. ⁽Reference Villarreal-Lozoya, Lombardini and Cisneros-Zevallos⁸⁰⁾ and Lombaridini et al. ⁽Reference Lombardini, Villarreal-Lozoya and Cisneros-Zevallos⁸¹⁾ reported that total phenol content of five pecan genotypes (Desirable, Kanza, Nacono, Pawnee and Shawnee) grown in Texas (USA) varied almost 1-fold, with concentrations ranging between 62 and 113 mg/100 g defatted kernels. Vilarreal-Lozoya et al. ⁽Reference Villarreal-Lozoya, Lombardini and Cisneros-Zevallos⁸⁰⁾ also reported that TAC values in defatted kernel of five pecan genotypes (Desirable, Kanza, Nacono, Pawnee and Shawnee) grown in Texas were significantly different from one another. Hydrophilic ORAC values and 2,2-diphenyl-2-picrylhydrazyl (DPPH) values of the Kanza genotype, which were the highest, were 119 and 67 % larger than those of the lowest Desirable genotype. The relationship of genotype to individual flavonoid content has not been studied.

Genetics (variety): pistachios

Tokusoglu et al. ⁽Reference Tokusoglu, Unal and Yemis⁴⁸⁾ reported that the trans-resveratrol content varied between five pistachio genotypes (Uzun, Kirmizi, Halebi, Ohadi and Sirrt) grown in Turkey. Uzun pistachios with 9 μg trans-resveratrol/100 g were 17·5-fold lower than the content of Ohadi pistachios.

Genetics (variety): walnuts

The genotype effect on walnut polyphenols has not been reported, but its effect is suggested by other data; for example, the tocopherol and fatty acid composition in nine walnut genotypes (Franquette, Lara, Marbot, Mayette, Mellanaise, Parisienne, Arco, Hartley and Rego) harvested in northeastern Portugal were different⁽Reference Amaral, Alves and Seabra^82, Reference Amaral, Cunha and Alves⁸³⁾. Further, walnut leaves collected from the same nine genotypes have varied contents of nine constituent polyphenols⁽Reference Amaral, Valentao and Andrade⁸⁴⁾.

Amaral et al. ⁽Reference Amaral, Casal and Pereira⁸⁵⁾ examined the phytosterol content of six walnut genotypes grown in northeastern Portugal (Table 14). The total sterol content of the Marbot genotype was 68 % greater than the Parisienne genotype. The difference was not ascribed to a geographic effect because walnut trees of all genotypes were planted in the same orchard.

Table 14 Genotype effect on sterol contents in walnuts⁽Reference Amaral, Casal and Pereira⁸⁵⁾

Almonds

Seasonal factors include climate, soil nutrition, infestation and related parameters. Bolling et al. ⁽Reference Hughey, Wilcox and Minardi²⁹⁾ examined total phenols, polyphenols and FRAP values in almonds across three harvest seasons and found that season had a small but statistically significant effect on total polyphenol content in almonds while there were not differences in total phenols and FRAP values (Table 15). These results suggest the overall phenol production, as well as TAC, may not fluctuate importantly between seasons. However, a more robust study with a larger sample size is necessary to confirm the stability of phenol production between seasons.

Table 15 Effect of harvest season on polyphenol content and antioxidant activity of Butte, Carmel and Nonpareil almonds⁽Reference Bolling, Dolnikowski and Blumberg³²⁾

(Mean values and standard deviations, n 9)

GAE, gallic acid equivalents; FRAP, ferric-reducing antioxidant power; TE, Trolox equivalents.

^a,b Mean values within a column with unlike superscript letters are significantly different (P ≤ 0·05; ANOVA and Tukey's honestly significant difference (HSD) test).

Pecans

De la Rosa et al. ⁽Reference de la Rosa, Alvarez-Parrilla and Shahidi⁸⁶⁾ characterised the content of ellagic, gallic, p-hydroxybenzoic and protocatechuic acids and TAC in pecans grown in three locations of the State of Chihuahua, Mexico. They found that growing location did not affect TAC measured by ORAC, DPPH, 2,2′-azino-bis(3-ethylbenzothiazolin-6-sulfonic acid (ABTS) and hydroxyl radical-scavenging assay. The content of the four phenolic acids also was not different, with average values of 5, 220, 55 and 21 mg/g, respectively.

Hedin et al. ⁽Reference Hedin, Langhans and Graves⁶¹⁾ reported that juglone contents in pecans were increased along with harvesting months (from June to September) within a season from 47·9 (June and July) to 116 mg/100 g (August and September).

Pistachios

The effect of geographic location on the phytochemical content of pistachios has been studied in Europe. Bellomo & Fallico⁽Reference Bellomo and Fallico⁸⁷⁾ measured anthocyanins, chlorophylls and xanthophylls in pistachios collected from Italy and Turkey in the 2001–2002 season (Table 16) and found that pistachios from Turkey generally had a much lower phytochemical content. However, Grippi et al. ⁽Reference Grippi, Crosta and Aiello⁴⁹⁾ found in a small study with twelve pistachio samples that Sicilian pistachios collected from orchards in Bronte and Agrigento, regions about 100 miles (160 km) apart, had comparable total resveratrol content at 0·7 mg/100 g kernels. The geographic effect on phytosterols in pistachios was also reported by the same group, where the distribution of nine phytosterols in pistachios harvested in Italy (Bronte and Agrigento), Turkey, Greece and Iran differed⁽Reference Arena, Campisi and Fallico⁸⁸⁾. β-Sitosterol, the main phytosterol found in pistachios, was found at 85·5 and 88·0 %, respectively, with Δ5-avenasterol at 9·2 and 5·7 %, respectively, in Italy and Iran.

Table 16 Geographic effect on anthocyanins, chlorophylls and xanthophylls in pistachios⁽Reference Bellomo and Fallico⁸⁷⁾

Walnuts

Hedin et al. ⁽Reference Hedin, Langhans and Graves⁶¹⁾ reported the juglone content in black walnuts was increased from 79 to 193 mg/100 g along with harvesting months (from June to September) within a season. Amaral et al. ⁽Reference Amaral, Valentao and Andrade⁸⁴⁾ found, in walnut leaves, that there was a seasonal effect on nine polyphenol compounds (3-caffeoylquinic, 3-p-coumaroylquinic and 4-p-coumaroylquinic acids, quercetin 3-galactoside, quercetin 3-arabinoside, quercetin 3-xyloside, quercetin 3-rhamnoside, a quercetin 3-pentoside derivative and a kaempferol 3-pentoside derivative), implicating a seasonal effect on walnut polyphenols.

Almonds

Almonds are currently subjected to mandatory pasteurisation or roasting treatments before consumption due to food safety concern. Bolling et al. ⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾ showed that three pasteurisation procedures did not change TAC of phytochemicals in almond skins, measured by FRAP and total phenol assays, or individual flavonoid concentrations (Table 17). However, because there were large variations in study outcomes within the pasteurisation treatments, a more robust study investigating the pasteurisation effect on almond phytochemicals is warranted. It has been recognised that roasting is potentially destructive to phytochemicals. Bolling et al. ⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾ found that roasting decreased total phenol content and FRAP values in almonds, but did not change concentrations of flavonoids. This discrepancy might be attributed to specific breakdown of heat-labile antioxidant constituents other than flavonoids.

Table 17 Effect of pasteurisation and roasting on almond skin flavonoids and phenolic acids, total phenols and ferric-reducing antioxidant power (FRAP) values⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾

(Mean values and standard deviations)

GAE, gallic acid equivalents; TE, Trolox equivalents; PPO, polypropylene oxide; S1, steam treatment 1; S2, steam treatment 2.

* Significance was assessed by a paired t test for roasted samples (n 12) (P ≤ 0·05).

During long-term storage, nutrients in nuts are subjected to changes of temperature and humidity. Bolling et al. ⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾ hypothesised that long-term storage could have a detrimental impact on the polyphenol content and TAC of almonds. In contrast, total polyphenol and total phenol content and FRAP values of raw almond skins were found to increase with storage duration, independent of storage temperature at 4 or 23°C (Fig. 6). Further, individual polyphenol compounds appeared to have different degrees of increase (Fig. 7). In particular, p-hydroxybenzoic acid increased about 400-fold after 15 months of storage.

Fig. 6 Storage of raw almonds in darkness at 4°C (-Δ-) and 23°C (-□-) for 15 months increased flavonoids and phenolic acids determined by liquid chromatography–MS (a), total phenols (b) and ferric-reducing antioxidant power (FRAP) (c) of almond skins⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾. Values are means (n 13), with standard errors represented by vertical bars, representing seven almond varieties. * Mean value was significantly different from that at 4°C (P ≤ 0·05).

Fig. 7 (a) Changes to flavonoid aglycone and phenolic acid content in almond skins upon storage at 23°C for 15 months⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾. (), p-Hydroxybenzoic acid; (), dihydroxykaempferol; (), kaempferol; (), eriodictyol; (), catechin; (), epicatechin; (), procatechuic acid; (), isorhamnetin; (), quercetin; (), naringenin. (b) Changes to flavonoid glycoside content in almond skins upon storage at 23°C for 15 months⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾. (), Quercetin-3-O-galactoside; (), isorhamnetin-3-O-glucoside; (), kaempferol-3-O-glucoside; (), isorhamnetin-3-O-rutinoside; (), naringenin-7-O-glucoside; (), kaempferol-3-O-rutinoside; (), kaempferol-3-O-galactoside; (), quercetin-3-O-rutinoside.

Cashews

The kernel of the cashew nut is removed from the shell by a process known as shelling, which can be achieved by various methods such as drying, steam roasting, oil-bath roasting, or cooking under high-pressure steam. Trox et al. ⁽Reference Trox, Vadivel and Vetter⁹⁰⁾ examined that the effect of oil-bath roasting, steam roasting, drying, open pan roasting and Flores hand-cracking on carotenoid and tocopherol contents in kernels. They reported that shelling, roasting and drying promoted decreases in β-carotene, lutein, and α- and γ-tocopherol as compared with raw kernels, with widely varied magnitude of reductions (Table 18). The open pan roasting in general exhibited the highest level of decrease in phytochemicals probably due to the direct action of fire; and the Flores hand-cracking method led to a lower level of reduction of phytochemicals. In order to preserve phytochemicals, the selection of the shelling method for cashew nuts is critical.

Table 18 Effect of shelling on contents (μg/100 g DM) of selected carotenoids and tocopherols in cashew nut kernels⁽Reference Trox, Vadivel and Vetter⁹⁰⁾

Hazelnuts

Schmitzer et al. ⁽Reference Schmitzer, Slatnar and Veberic⁹¹⁾ reported that roasting had a negative effect on individual phenolics but not on the total phenolic content and TAC (determined by DPPH) of hazelnuts. They found that the roasting decreased protocatechulic acid, phloretin-2-O-glucoside, catechin and epicatechin contents in whole hazelnuts of six genotypes harvested in Maribor, Slovenia. The percentage reduction in the four phenolic compounds could be as high as 84 %. On the other hand, gallic acid content was increased by the roasting, with the magnitude of the increase being from 2 to 818 %. The increase in gallic acid after roasting might be ascribed to degradation of HT.

Pecans

Food irradiation is a food safety procedure to eliminate pathogenic microbial contaminations and involves exposure of food to different sources of ionising energy. Villarreal-Lozoya et al. ⁽Reference Villarreal-Lozoya, Lombardini and Cisneros-Zevallos⁹²⁾ studied the effect of irradiation on phytochemical contents in Kanza and Desirable pecans collected in the autumn of 2004 in Texas. Pecan kernels were treated with 0, 1·5 and 3 kGy irradiation, followed by storage at 40°C and 55 to 60 % relative humidity for up to 134 d. They found that this accelerated storage condition promoted decreases in total phenols (about 20 % reduction as compared with the initial values), condensed tannins (about 30 % reduction), and TAC measured by ORAC (no significant changes) and DPPH (about 15 % reduction for the Desirable genotype), while irradiation itself before storage did not have any major effect on the studied measures. The storage effect on phenol compounds in pecans is largely consistent with those found in almonds.

Pistachios

After harvesting, pistachios are typically sun dried in Italy. Ballistreri et al. ⁽Reference Ballistreri, Arena and Fallico⁴⁰⁾ found that sun drying for 3 d decreased total phenol content and the majority of determined flavonoids and stilbenes in Bronte pistachios collected in Italy in 2007 (Table 19). The magnitudes of reduction ranged from 30 to 83 %.

Table 19 Content of phenolics (mg/100 g DM) in edible pistachio kernels of Bronte Pistacia vera ⁽Reference Ballistreri, Arena and Fallico⁴⁰⁾

GAE, gallic acid equivalents.

Gentile et al. ⁽Reference Gentile, Tesoriere and Butera⁴³⁾ reported that roasting at 160°C for 40 min deceased total phenols, TAC, PAC and trans-resveratrol content in Bronte pistachios collected in Sicily in 2005 (Table 20). Interestingly, isoflavones in pistachios were not degraded by the roasting treatment. These results are consistent with those observed in almonds that roasting decreased FRAP and total phenol values and did not alter flavonoid concentrations⁽Reference Bolling, Blumberg and Oliver Chen⁸⁹⁾.

Table 20 The effect of roasting on total phenols, total antioxidant capacity (TAC) and flavonoids in edible kernels of pistachios⁽Reference Gentile, Tesoriere and Butera⁴³⁾

GAE, gallic acid equivalents; TEAC, Trolox equivalent antioxidant capacity; TE, Trolox equivalents.

Pistachios in China are often bleached before retailing. Seeram et al. ⁽Reference Seeram, Zhang and Henning⁴²⁾ examined the effect of bleaching treatment with different concentrations of H₂O₂ for 1 min, followed by 20 min of roasting at 60°C (140°F) on anthocyanins and TAC of in-shell pistachios. They found that bleaching significantly decreased cyanidin-glucoside and -galactoside concentrations and TAC measured by the Trolox equivalent antioxidant capacity (TEAC) assay in pistachio skins in a H₂O₂ concentration-dependent manner (Table 21). They also found that roasting alone significantly deceased cyanidin-glucoside, cyanidin-galactoside and TAC values compared with raw pistachio skins.

Table 21 Effect of bleaching and roasting on anthocyanins and total antioxidant capacity values in pistachio skins⁽Reference Seeram, Zhang and Henning⁴²⁾

TEAC, Trolox equivalent antioxidant capacity.

Bellomo et al. ⁽Reference Bellomo, Fallico and Muratore⁵⁰⁾ examined the storage effect on chlorophyll-a and -b and lutein in pistachios at 10, 25 and 37°C for up to 14 months. The pistachios were collected from Bronte, Italy in 2003. In general, these phytochemicals decreased over time, dependent on storage temperatures. On average, chlorophyll-a and -b and lutein were decreased by 50, 30 and 44 % at 14 months, respectively.

Walnuts

Sze-Tao et al. ⁽Reference Sze-Tao, Schrimpf and Teuber⁹³⁾ observed that 5 min of roasting at 204°C caused a small but statistically significant decrease in tannins (catechin polymers) in walnuts (585 v. 520 mg catechin equivalents/100 g dry weight). This result is similar to the effect of roasting pistachios at 160°C for 40 min where the content of PAC decreased by 87 %.

Almonds

Since organic solvents employed to extract polyphenols do not closely reflect the physiological bioaccessibility of polyphenols in the gastrointestinal tract, Chen & Blumberg⁽Reference Chen and Blumberg⁹⁴⁾ compared the amount of total phenols extracted from almond skins using acidified methanol solvent (M; acetic acid–methanol–water, 3·7:50:46·3 by vol.) and a gastrointestinal mimic juice solvent (GI, containing pepsin and pancreatic juice). They found that the total phenol content of almond skins was different by 4.8-fold (M v. GI solvent: 91.2 v. 14.9 μmol GAE/g or 15.5 v. 2.5 mg/g; P ≤ 0.05). Further, the LC-MS chromatograms of almond skin polyphenols extracted with M and GI at the same GAE concentration showed qualitatively different profiles of flavonoids, largely because of the absence in the GI solvent of catechin, epicatechin, kamperfol-3-O-glucoside, kaemperfol-3-O-galactoside, dihydroxy-kampferol, quercetin-3-O-glucoside, quercetin-3-O-galacoside, rutin, isorhamnetin, kaempferol, naringenin, quercetin and eriodictyol (Fig. 8). These results raise again the question regarding what extraction solvent(s) should be used for characterising nut phytochemicals.

Fig. 8 Typical liquid chromatography − MS/MS extracted ion chromatograms of almond skin polyphenolics from (a) acidified methanol extraction and (b) gastrointestinal juice mimic extraction⁽Reference Chen and Blumberg⁹⁴⁾. Peak numbers correspond to compounds as (1) catechin, (2) epicatechin, (3) quercetin-3-O-galactoside, (4) naringein-7-O-glucoside, (5) rutin, (6) quercetin-3-O-glucoside, (7) dihydroxylkaempferol, (8) kaempfer-3-O-galactoside, (9) kaempferol-3-O-glucoside, (10) kaempferol-3-O-rutinoside, (11) isorhamnetin-3-O-runtinoside, (12) eriodictyol, (13) quercetin, (14) naringenin and (15) isorhamnetin.

Hazelnuts

Alasalvar et al. ⁽Reference Alasalvar, Karamac and Amarowicz⁵³⁾ compared the extraction efficiency of 80 % ethanol and 80 % acetone on total phenols in hazelnuts. They found that 80 % acetone was more efficient to extract phenols and tannins than 80 % ethanol (Table 22). Ghirardello et al. ⁽Reference Ghirardello, Prosperini and Zeppa⁹⁵⁾ compared extraction of phenolic compounds using three different solvent mixtures (80 % (v/v) ethanol, methanol, and acetone with water) at different temperatures. Consistent with the results from Alasalvar et al. ⁽Reference Alasalvar, Karamac and Amarowicz⁵³⁾, they found that 80 % acetone at 50°C exhibited the best extracting capacity with the highest total phenol content and TAC.

Table 22 Total phenols, condensed tannins and total antioxidant capacity measured by Trolox equivalent antioxidant capacity (TEAC) in 1 g dry extracts of hazelnuts⁽Reference Alasalvar, Karamac and Amarowicz⁵³⁾

Lipid-lowering effects

Most randomised clinical trials of nuts conducted thus far have focused on the hypocholesterolaemic effects of whole nuts, and not their individual constituents. In a systematic review of twenty-three intervention studies, Mukuddem-Petersen et al. ⁽Reference Mukuddem-Petersen, Oosthuizen and Jerling¹¹⁹⁾ reported the consumption of 50–100 g of almonds, groundnuts, pecans or walnuts five or more times per week, as part of a heart-healthy diet moderate in fat (about 35 % of energy), significantly lowered total cholesterol (2–16 %) and LDL-cholesterol (2–19 %) in both normo- and hyperlipidaemic individuals compared with control diets without nuts or with a different fatty acid profile. Similarly, in their review, Griel & Kris-Etherton⁽Reference Griel and Kris-Etherton¹²⁰⁾ concluded that tree nuts, i.e. walnuts, almonds, macadamias, pecans, pistachios and hazelnuts, reduced LDL-cholesterol 3–19 % when compared with Western and lower-fat diets. Subsequent trials of hazelnuts⁽Reference Mercanligil, Arslan and Alasalvar^121, Reference Tey, Brown and Chisholm¹²²⁾, pistachios⁽Reference Sheridan, Cooper and Erario¹²³⁾, macadamia nuts⁽Reference Griel, Cao and Bagshaw¹²⁴⁾ and Brazil nuts⁽Reference Maranhao, Kraemer-Aguiar and Oliveira¹²⁵⁾ have also confirmed their hypocholesterolaemic effect.

In a pooled analysis of twenty-five nut intervention trials, Sabate et al. ⁽Reference Sabate, Oda and Ros¹²⁶⁾ determined a mean intake of 67 g/d lowered total cholesterol 5 %, LDL-cholesterol 7 %, LDL:HDL 8 %, total cholesterol:HDL 6 % and TAG 10 % (among those with initial levels ≥  150 mg/dl (1500 mg/l) only). Their analysis also revealed that the lipid-lowering effect of nuts was dose-related and greatest among subjects with high LDL and low BMI, regardless of the type of nut consumed.

Nuts are rich in chemically related phytosterols, a class of compounds that interfere with intestinal cholesterol absorption which, according to Segura et al. ⁽Reference Segura and Javierre¹²⁷⁾ might explain part of the cholesterol-lowering effect of nut intake beyond that attributable to fatty acid exchange. Given the evidence that polyphenols also lower cholesterol absorption⁽Reference Zern and Fernandez⁹⁸⁾, it is plausible that there are multiple mechanisms of action and/or potential synergistic effects between the individual nut constituents. However, no definitive studies have examined the extent of the contribution of polyphenols to the observed lipid-lowering effect.

Inflammation and endothelial function

A limited number of human studies have examined the effects of nuts on inflammation, endothelial function and vascular reactivity. In the Prevención con Dieta Mediterránea (PREDIMED) Study of 772 asymptomatic adults, aged 55–80 years, a Mediterranean diet including nuts (30 g/d) reduced levels of circulating IL-6 (P < 0·018), ICAM-1 (P < 0·003) and vascular cell adhesion molecule-1 (VCAM-1) from baseline after 3 months, but not CRP⁽Reference Estruch, Martínez-González and Corella¹²⁸⁾. Compared with a low-fat control diet, between-group differences in endothelial function were also observed, but no P values were reported. In this study, lowered fasting glucose (P = 0·039), insulin (P < 0·001), homeostatic model assessment (HOMA) index (P < 0·001), and systolic (P < 0·001) and diastolic blood pressure (P = 0·001) were also observed after 3 months with the nut diet when compared with the control diet.

A Mediterranean diet in which 32 % of the energy from MUFA fat was replaced with walnuts improved VCAM-1 status (P < 0·05), a circulating marker of endothelial activation, but had no effect on ICAM-1, CRP or homocysteine after 4 weeks when compared with a cholesterol-lowering Mediterranean diet⁽Reference Ros, Nunez and Perez-Heras¹²⁹⁾. Flow-mediated dilation, a measure of brachial artery vasomotor function and a biomarker of CHD risk, also improved significantly after both acute and chronic walnut feeding in twenty-four hypercholesterolaemic subjects from this cohort⁽Reference Ros, Nunez and Perez-Heras^129, Reference Cortes, Nunez and Cofan¹³⁰⁾. A similar substitution with pistachios also improved endothelial function (P < 0·002) and lowered levels of IL-6 (P < 0·001), as well as fasting glucose (P < 0·001), with no change in either CRP or TNF-α after 4 weeks in a cohort of thirty-two healthy young men living in a controlled environment⁽Reference Sari, Baltaci and Bagci¹³¹⁾.

Macadamia nuts, when incorporated into the diet at 15 % of energy (40–90 g/d), lowered plasma leucotriene B₄ levels from baseline after 4 weeks in seventeen hypercholesterolaemic men⁽Reference Garg, Blake and Wills¹³²⁾. A non-significant reduction in the plasma thromboxane B₂:prostacyclin I₂ ratio was also observed following macadamia nut consumption.

Among healthy adults, incorporating almonds into the diet at 10 and 20 % of energy (34 and 68 g/2000 kcal (8368 kJ), respectively) for 4 weeks lowered CRP when compared with a nut-free control diet, although no dose–response relationship was observed. E-selectin was also significantly lower, but only with the higher almond dose⁽Reference Rajaram, Connell and Sabate¹³³⁾.

A controlled feeding trial incorporating walnuts as a source of α-linolenic acid (ALA) demonstrated the anti-inflammatory effects of an ALA-rich diet in hypercholesterolaemic subjects⁽Reference Zhao, Etherton and Martin¹³⁴⁾. In this study, a diet containing about 37 g walnuts/d and 15 g walnut oil/d reduced CRP (P < 0·01) and ICAM-1 (P < 0·01) when compared with an average American diet, while also lowering VCAM-1 (P < 0·01) and E-selectin (P < 0·01) when compared with a diet high in linoleic acid. This walnut diet also lowered levels of IL-6, IL-6β and TNF-α production by peripheral blood mononuclear cells (P < 0·05)⁽Reference Zhao, Etherton and Martin¹³⁵⁾. In contrast, a controlled feeding trial of walnuts and cashews showed no effect on serum CRP⁽Reference Mukuddem-Petersen, Stonehouse Oosthuizen and Jerling¹³⁶⁾ in sixty-four subjects with the metabolic syndrome. While the anti-inflammatory effects shown by Zhao et al. ⁽Reference Zhao, Etherton and Martin^134, Reference Zhao, Etherton and Martin¹³⁵⁾ were most probably due to the walnut fatty acids, neither feeding trial controlled for the presence of dietary antioxidants or phytochemicals, which alone can make an impact on these biomarkers.