The requirement for accuracy in the counting of somatic cells in milk has been increased by the use of this measurement as a basis of payment for milk. This paper reports on several factors that can substantially affect efficiency of staining and cell detection in various types of fluorescent cell counters. These mordant factors include sample age, chemical preservative, other sample preservation treatments and reagent properties. Experiments were conducted in several laboratories with Fossomatic 215 and 360 cell counting instruments to measure the effect of these mordant factors on both cell fluorescence and cell count. Counting efficiency was as low as 20% for some instruments counting some particular samples, depending upon the combination of adverse mordant factors. The problem of poor counting efficiency was also not detected by calibration samples that had more favourable mordant treatments applied. The adverse mordant factors were shown to reduce the intensity of fluorescence of cells for all instruments. We suggest that a significant reduction in counting efficiency due to these adverse mordant factors is most likely to occur when the particular instrument already has an intrinsic inability to separate cell fluorescence properly from background noise. This paper also discusses quality control samples, the impact of various factors on discriminator curves and the use of appropriate measures to maintain good calibration adjustment.

Physical characteristics of the bovine teat canal were examined for their influence on susceptibility to intramammary infection. All quarters of 18 cows were inoculated with 2 × 105 cfu Streptococcus agalactiae (Trial 1) and 20 cows with 105 cfu Str. Uberis (Trial 2) 3–4 mm into the teat canal every 3 d for 12 d. Incidence of quarter infection was similar for both pathogens, 30/72 (42%) in Trial 1 and 32/80 (40%) in Trial 2. Logistic regression analysis showed that probability of infection by Str. Agalactiae increased significantly with an increase in quarter peak flow rate (P < 0·05) whereas probability of infection increased for Str. Uberis with a decrease in teat canal length (P < 0·05). A significantly higher (P < 0·001) incidence of infection by Str. Uberis was observed in quarters that contained a low wet weight (< 1·8 mg) of removable keratin compared with those that contained > 1·8 mg keratin, but there was no correlation between weight of keratin and length of the teat canal. Infections by Str. Uberis took significantly less (P < 0·05) time to show a rise in somatic cell count above 7·5 × 105 cells/ml than Str. agalactiae. The results provide evidence that these pathogens use different mechanisms to pass through the teat canal.

A dynamic mathematical model of blood flow regulation in the mammary glands of a lactating cow was constructed from a principle of local vasodilator release in response to changes in intracellular adenylate charge. An equation was derived to predict uptake of the milk precursors acetate, glucose, amino acids and fatty acids, as affected by mammary blood flow rate. Metabolism of the precursors to milk components and CO2 was simulated with a set of empirically derived equations. Relative rates of ATP production and utilization regulated both the number of perfused capillaries and the conductance of arteriolar segments in the mammary glands. The model simulated local control phenomena of functional and reactive hyperaemia, and simulation of autoregulation under changing arterial pressure suggested a predominance of precapillary sphincter regulation. It was predicted that an increase in blood flow without the mammary capacity to utilize blood metabolites efficiently would be detrimental to milk synthesis. Conversely, increased blood flow through changes in mammary activity resulted in predictions of higher milk production. It was proposed that the equation for uptake,

be used in analysis of mammary arteriovenous differences.

Two experiments were conducted to test the hypothesis that responses of milk production to the abomasal infusion of casein or hydrolysates of casein can be influenced by the proportion of peptide-bound amino acids in the infusate. Eight lactating cows were used in two Latin square experiments with period lengths of 10 d. In Expt 1, the four cows were at a late stage of lactation and the four treatments were a basal diet of silage and a barley–soya supplement alone or with infusions into the abomasum of 180 g/d of sodium caseinate, an enzymic hydrolysate of casein or an acid hydrolysate of casein; all treatments supplied equivalent amounts of all the amino acids, this being achieved by addition of free amino acids as required. As infused, the three treatments contained 89 (caseinate), 40 (enzymic hydrolysate) and 15 (acid hydrolysate) % of their amino acids as peptides. The increase of milk production in response to infusion was small and there were no significant differences between the infusion treatments. In Expt 2, the cows received a basal diet of grass silage and a supplement containing feather meal as the main source of protein. Treatments were the basal diet alone and with three infusion treatments as in Expt 1, except that the amount infused was 230 g/d. The response to infusion was greater in this experiment, there were significant differences between infusions and the yield of milk protein with the infusion treatments was linearly related (P = 0·001) to the proportion of peptide-bound amino acids in the infusate. Moreover, there was a close inverse relation (P = 0·007) between the proportion of peptide-bound amino acids in the infusate and the concentration of total peptidebound amino acids in blood plasma.

Five British Saanen goats were milk sampled during the first 39 weeks of lactation to determine changes in casein composition. Caseins were separated by anion- and cation-exchange FPLC to determine the relative amounts of the individual caseins. Acid, alkaline and SDS-PAGE were used to determine possible genetic polymorphisms and observe any lactational changes. Total casein nitrogen was determined using a micro-Kjeldahl method and this allowed the concentrations of individual caseins to be calculated. The milk of one animal, which had the deduced genotype αs1-CnAB, showed higher concentrations of both total and αs1-casein. The remainder of the group were either heterozygous αs1-CnBE or, more probably, homozygous αs1-CnE and produced milk of a generally lower protein concentration. Both FPLC and PAGE results showed that the relative amounts and concentrations of αs2-casein decreased with stage of lactation, consistent with its susceptibility to proteolysis. The relative amounts of the breakdown products of plasmin attack on β-casein, γ-caseins, were highly negatively correlated with milk yield (r = –0·942, P < 0·001) in the declining phase of lactation, reflecting the gradual involution of the gland at this time. The relative amount of κ-casein increased by ∼ 50% after peak lactation and its concentration almost doubled near the end of lactation. These compositional changes may alter the processing qualities of goats' milk in relation to cheese production.

The flux patterns for sweet whey and buttermilk were strongly influenced by pH. Increasing the pH of buttermilk from 6·6 to 8·0 tended to reduce initial flux values and reduce deposit formation on the membrane as indicated by lower values for the fouling coefficient. Flux was mostly controlled by concentration polarization. Reducing the pH below 6·6 increased the flux but caused more deposit on the membrane as indicated by high fouling coefficient values. Almost the opposite was found for sweet whey. The initial flux increased as pH increased, followed by considerable flux decline, which was linked to greater fouling of the membrane. Reducing the pH reduced the initial flux considerably but also reduced further fouling. These changes are thought to be brought about by the combined effects of pH change on the proteins and minerals, and in particular on calcium.

The cleavage of bovine κ-casein at the Phe105–Met106* bond by chymosin or pepsin is the first stage in casein micelle coagulation and casein digestion. The nature of the interaction of the peptide His98–Pro–His–Pro–His–Leu–Ser–Phe105–Met–Ala–Ile–Pro-Pro-Lys111 with chymosin and porcine pepsin was investigated using molecular modelling and energy minimization techniques. This study verified and extended a proposed model that electrostatic binding (involving His98, His100, His102 and Lys111 or Lys112) at either end of the active site cleft of chymosin is important for the positioning of residues 103–108 in the cleft. The peptide conformation remained unchanged in going from solution to binding into the active site cleft, with the exception that optimum binding of substrate to chymosin required the isomerization of the His98–Pro99 peptide bond from the trans to the cis conformation. The study also identified an acidic region in porcine pepsin that is in a position to form strong electrostatic interactions with the histidines at the N-terminus of the peptide.

The effects of pH and added NaCl or CaCl2 on gel development in heated whey protein concentrate (WPC) solutions were studied by oscillatory rheometry using a Bohlin rheometer, and by qualitative visual and tactile observations. The storage modulus (G′) exhibited maxima at pH values of about 4 and 7, while the phase angle (δ) exhibited minima at the same pH values. At pH 5·5, which lies within the range of the isoelectric points of the whey proteins, G′ was a minimum while δ was a maximum. At pH 7 and 8 G′ increased to a maximum and then decreased as added ionic strength was increased. At pH 4 G′ decreased steadily but slowly with increasing added ionic strength. The effects of CaCl2 were essentially the same as those of NaCl, but occurred at much lower added ionic strength. All the results are discussed in physicochemical terms. The main conclusions were, firstly, that changes in pH and salt concentration both affected gel development by altering the balance between attractive and repulsive protein-protein interactions, and secondly, that the effects of pH, salt concentration and salt type were interactive. Interaction both partly explains the wide variation in gel character and rheological properties among commercial WPC and potentially provides means of tailoring the gelling behaviour of a WPC so that it functions in a desired way in a given food system.

In order to predict the oozing susceptibility of heat-induced milk protein gels such as processed cheeses during storage, ultracentrifugation forces are required to accelerate the expulsion of water from the gel structure. Two predictive methods have been tested. Direct ultracentrifugation of processed cheese was used to study effects of centrifugal compression on the water-holding ability. The water release with optimal parameters (time, temperature and ultracentrifugation force) from a freshly manufactured processed cheese was correlated with visual assessment of oozing after 6 months storage. A second method was based on the susceptibility of gelled proteins to resist the dissociating action of solutions of chemical agents such as SDS, urea, EDTA and 2-mercaptoethanol. Most of the protein sedimented by ultracentrifugation in the presence of SDS represented that still in complexed form. This correlated with optimal water binding by the processed cheese. Response-surface methods, used to optimize the ultracentrifugation variables and the composition of the dissociation solutions, showed that the dissociation test was best with a dispersion of processed cheese in SDS solution (10 g/l) at a ratio of 1:6 (w/v) at 20°C, ultracentrifuged at 86000 g and 20°C for 25 min. Analysis of processed cheese cooked at 115°C and then dissociated in different solutions showed various protein–protein interactions in the gel network, whereas hydrophobic interactions were the most important stabilizers of the protein matrix of cheese cooked at higher temperature.

The effects of pH and added salts or chelating agents on the gel strength and dynamic rheological properties of acid–heat-induced gels made from reconstituted skim milk (200 g solids/l) were investigated. Gel strength increased as pH was lowered between 5·75 and 5·15 except in the range pH 5·45–5·25 where a local maximum in gel strength was obtained at pH 5·35. Gel characteristics were affected by addition of salts or chelating agents but each of their effects was different, depending on the final pH of the milk gel. The addition of CaCl2 or chelating agents (Na2HPO4, disodium citrate or the disodium salt of EDTA) which affected micellar calcium phosphate, non-sedimentable casein and Ca2+ activity in different ways all resulted in decreased gel strength at pH 5·5. The addition of CaCl2 or MgCl2 caused a decrease in tanδ (ratio of the viscous modulus G″ to the elastic modulus G′) whereas disodium citrate or the disodium salt of EDTA addition caused an increase and Na2HPO4 addition did not cause a change. The addition of NaCl (up to 50 mM), which causes an increase in ionic strength but has no effect on non-sedimentable casein and Ca2+ activity, decreased gel strength but did not change tanδ. The addition of a range of other salts (KCl, NH4Cl, NaSCN, NaNO3 or Na2SO4) also decreased gel strength at pH 5·5.

Acidic and basic fibroblast growth factors (FGF) were identified in bovine cheese whey after partial purification using a two step procedure. Cation-exchange chromatography produced a mitogen-rich extract which was loaded on to a heparin-sepharose column and eluted stepwise with 0·8, 1·2 and 2·0 M-NH4HC03. Mitogenic activity was found in all three fractions by cell growth assays using Balb/c-3T3 fibroblasts. Immunoblotting identified acidic FGF in the 1·2 M-eluate and basic FGF in the 2·0 M-eluate, but neither acidic nor basic FGF was detected in the 0·8 M-fraction. Quantitative radioreceptor assays indicated 5·8 ng of acidic FGF-like activity and 19·8 ng of basic FGF-like activity per 1 whey in the appropriate eluates. This study represents the first direct demonstration of FGF in milk.

Adherence of bacteria to various milk contact sites was examined by scanning electron microscopy and transmission electron microscopy. New gaskets, endcaps, vacuum breaker plugs and pipeline inserts were installed in different areas in lines carrying either raw or pasteurized milk, and a routine schedule of cleaning-in-place and sanitizing was followed. Removed cleaned and sanitized gaskets were processed for scanning or transmission electron microscopy. Adherent bacteria were observed on the sides of gaskets removed from both pasteurized and raw milk lines. Some areas of Buna-n gaskets were colonized with a confluent layer of bacterial cells surrounded by an extensive amorphous matrix, while other areas of Buna-n gaskets showed a diffuse adherence over large areas of the surface. Most of the bacteria attached to polytetrafluoroethylene (PTFE or Teflon™) gaskets were found in crevices created by insertion of the gasket into the pipeline. Examination of stainless steel endcaps, pipeline inserts, and PTFE vacuum breaker plugs did not reveal the presence of adherent bacteria. The results of this study indicate that biofilms developed on the sides of gaskets in spite of cleaning-in-place procedures. These biofilms may be a source of post-pasteurization contamination.

Unlike that of other lactic acid bacteria, the growth of Leuconostoc spp. in milk is poor and the resulting acidification cannot be used to distinguish different strains. An acidification test based on the use of high initial numbers (109 cfu/ml) has been developed and proved to be an efficient tool for discriminating between 110 Leuconostoc strains isolated from French raw milk cheeses. The pH values after 24 h ranged from 6.55 to 4.05 and distinguished four acidification groups. All 34% of the strains that acidified milk to at least pH 5.1, coagulating it and should be considered as Lac+. The differences in rates and degrees of acidification could not be related to the proteolytic activity which was, from all 27 representative strains tested, similar to that of Lactococcus lactis Prt– variants.

The influence of diet quality and stage of lactation on Cheddar cheese moisture and yield were segregated and the effects of other potential variables (cow breed, milk protein genotype, age and level of mastitis) were minimized. An experiment was conducted using milk of cows in mid lactation or late lactation fed on high or low quality diets. Increased moisture in cheese was largely due to the influence of cow diet. Milk from cows given high quality diets produced lower moisture cheese. Cheese yield was influenced by diet and stage of lactation. Cows in late lactation fed on high quality diets always gave milks yielding the most cheese. Fat and protein recovery in cheese was not influenced by diet or stage of lactation. The results suggest that low quality diets may induce changes in the complex interactions of milk proteins and other components that affect cheesemaking.