The marked precipitation of Ca phosphate found to occur at ~ pH 6·8 when milk is heated to high temperatures may account for the minimum in the heat coagulation time (HCT)–pH curve at ~ pH 6·8. Dialysis of milk against water for about 3 h converted a normal type A milk to one with type B heat stability characteristics by reducing stability in the region of the HCT maximum while increasing stability in the region of the minimum. Reduction of the concentration of urea, lactose, Na or chloride did not cause these changes and gross micelle structure appeared to be intact following short dialysis as indicated by turbidity and sedimentability. Dilution of milk with water increased stability at the minimum without significantly affecting stability at the maximum. Pre-heating at 80°C for 10 min precluded the effect of dilution but not of dialysis.

α-Lactalbumin and SDS in addition to β-lactoglobulin introduced pH sensitivity to the heat stability–pH curve of serum protein free casein micelles particularly by increasing stability in the pH range 6·4–6·7. Bovine serum albumin, ovalbumin and lysozyme caused marked destabilization of milk and casein micelle suspensions throughout the pH range 6·4–7·4. Tetramethyl ammonium bromide caused destabilization of milk at pH values > 7·0, but had no effect in the region of maximum stability while the non-ionic detergents Triton X-100 and Tween 80 had no effect on heat stability.

The release of 12% (w/v) TCA-soluble N-acetylneuraminic acid from casein at 5 temperatures between 110 and 150°C was determined and showed a Q10 °C about 3; coagulation occurred when about 20% of the κ-casein was hydrolysed. Renneting of milk or colloidal calcium phosphate-free milk rendered the caseinate very heat-labile at its normal pH when more than about 20% of the κ-casein had been hydrolysed. Heat stability at the pH of maximum stability was not significantly decreased until after very prolonged renneting, but stability at pH values alkaline to the minimum was very sensitive to such hydrolysis suggesting that κ-casein is the principal factor responsible for heat stability in that region. Systems which do not have a maximum or minimum in the heat coagulation time–pH profile, i.e. serum protein-free casein micelles in milk diffusate, or milk which had been dialysed against water, were destabilized by renneting throughout the pH range 6·4–7·4.

The heat stability of the milk supply to manufacturing creameries in south-west Scotland was examined over 15 months from Nov. 1975 to Jan. 1977. For all but 2 months of this period the heat stability of the milk was very highly significantly correlated with the naturally occurring level of urea. Between 72 and 90% of the variation of coagulation time (CT), measured at the natural milk pH, was accounted for by changes in milk urea alone. For a short period, in May and June, the CT of the milk at natural pH fell within the minimum of the CT–pH profile, but insufficient data were available to allow the occurrence of this phenomenon to be related to changes in milk composition.

Whole goat κ-casein was prepared by chromatography of whole casein on hydroxyapatite. Chromatography of whole κ-casein on DEAE-cellulose separated 5 fractions. All of them were sensitive to chymosin. Their amino acid and carbohydrate composition, phosphate content and molecular weight were determined. Galactose, N-acetylgalactosamine, N-acetyl and N-glycolyl neuraminic acids were identified in whole κ-casein. It appears that goat κ-casein, like cow, buffalo and ewe κ-caseins, is composed of several fractions having identical peptide chains and differing in their carbohydrate contents. The main fraction, devoid of carbohydrate, was treated with chymosin. The para-κ-casein and caseinomacropeptide were isolated. Their amino acid composition and phosphate content were determined.

The proteins of the milk-fat globule membrane in homogenized cream have been studied using polyacrylamide-gel electrophoresis and electron microscopy. The effects of pasteurization and subsequent storage of the homogenized cream on the strength and composition of the membrane have also been investigated. Caseins and undenatured whey proteins are adsorbed to the fat–serum interface during homogenization; the caseins are the more dominant group, but with no apparent preference for any individual protein. After homogenization, but before pasteurization, whey proteins are more easily removed by washing than are the casein components. After subsequent pasteurization, and on storage, whey proteins become more tightly bound and are no longer readily removed by washing. Electron micrographs showed that the interfacial membrane between fat droplets and the serum phase consists of a protein composite material containing casein micelles, casein micellar sub-units and molecular, or non-micellar protein.

When caprine butterfat was heated in the presence of water vapour and the absence of air, γ- and δ-lactones and methyl ketones were produced which were qualitatively the same as those from bovine butterfat. The saturated δ-lactone and methyl ketone potentials of the caprine butterfat were of the order of one third those of bovine butterfat when the goats and cows received similar rations. γ-Dodecanolactone, γ-dodec-cis-6-enolactone, δ-tetradec-cis-8-enolactone and δ-tetradec-trans-8-enolactone potentials of the caprine butterfat were in the same range as for bovine butterfat when the animals were on similar diets. The γ- and δ-lactone and methyl ketone potentials and the fatty acid composition of caprine butterfat varied with the diet of the goat. Supplementing the diet with polyunsaturated fat, whether or not protected against biohydrogenation in the rumen, lowered the saturated δ-lactone and methyl ketone potentials as well as the proportion of C10,12,14,16 fatty acids in the caprine butterfat. Inclusion of crushed oats in the diet increased the γ-dodecanolactone potential. The γ-dodecanolactone and they γ-dodec-cis-6-enolactone potentials were also enhanced by feeding seed oil supplements which were not protected against ruminal hydrogenation. To prevent the development of the sweet off-flavour associated with γ-dodecalactones in heated, ruminant-derived meat and butterfat with elevated levels of linoleic acid, it appears that oats should be excluded from the basal ration of the ruminant and the polyunsaturated oil supplement should have the highest possible degree of protection against biohydrogenation in the rumen.

The quantity of γ- and δ-lactones and of methyl ketones produced when bovine butterfat was heated in the presence of water vapour and the absence of air, and hence the flavour quality of the butterfat, varied widely with the diet of the lactating cows. There was a marked increase in the γ-dodecanolactone potential when the cows were transferred from pasture to a mixture of chopped lucerne hay plus crushed oats but not when they were transferred to lucerne hay only. Feeding crushed sunflower seeds, whose oil was protected against biohydrogenation in the rumen, but not feeding sunflower meal from which the oil had been extracted, raised the γ-dodec-cis-6-enolactone potential, lowered the saturated δ-lactone potential and the methyl ketone potential and simultaneously raised the proportion of linoleic acid in the butterfat at the expense of fatty acids up to and including C16.

Streptococcus lactis C10, grown in tryptone–yeast extract–lactose broth containing [14C] bicarbonate, incorporated radioactivity into the protein and nucleic acid fractions of the cell as well as into compounds which were excreted by the organism into the growth medium. Aspartic acid was the first compound to be labelled and was the only amino acid labelled in the cell protein. All 4 bases were labelled in the cell RNA. Aspartic, succinic and lactic acids were the radioactive compounds excreted into the growth medium.

Resting cells of Streptococcus lactis C10 were able to synthesize aspartic acid de novo but could not actively transport aspartic acid into the cell. Intracellular aspartate was excreted from the cell in the presence of glucose but did not exchange with any extracellular amino acids. The results indicated that Str. lactis C10 obtains the aspartic acid it requires for growth by bicarbonate fixation instead of by the utilization of extracellular aspartic acid.

All 47 non-nisin producing strains of Streptococcus lactis and 12/13 strains of Str. lactis subsp. diacetylactis examined produced bacteriocins, for which the term lactostrepcins is suggested. Seven strains of Str. cremoris examined produced no bacteriocins active against 3 lactic streptococci strains used as indicators. The strains examined were divided into 3 groups: I, those producing lactostrepcins active against only one streptomycin resistant mutant of Str. lactis 60 indicator strain; II, those producing lactostrepcins active against all 3 indicator strains; III, those not producing lactostrepcins active against the indicator strains employed. The lactostrepcins were sensitive to various proteolytic enzymes and to phospholipase D, but retained full or partial activity after dialysis. Most of the bacteriocins studied were fully active only within the pH range 4·2–5·0 and were reversibly inactivated at pH 7·0 or 8·0. Results suggested occurrence of 4 different lactostrepcins. The lactostrepcins produced by all group I strains were the same, but there were differences among the lactostrepcins produced by group II strains. Lactostrepcins killed some β-haemolytic streptococci and some strains of Lactobacillus helveticus. One of the lactostrepcins was also active against certain Leuconostoc strains, but not against other Leuconostoc strains, nor against L. helveticus or other Gram-positive bacteria.

Thirty-one strains of mesophilic and thermophilic lactic acid bacteria and their respective phages were tested for their minimum, optimum and maximum multiplication temperatures. Culture growth was strongly influenced by temperature during the first few hours of incubation, but less so after 24 h. Most of the phages showed the same pattern of development as their hosts, but one phage lysing a thermophilic lactobacillus and 3 phages lysing mesophilic streptococci proved temperature-sensitive, having a lower maximum temperature than that of their hosts. One phage was unusual in that its minimum development temperature was 7 °C above that of its host. Differences in temperature sensitivity were insufficient to reduce risk of phage infection by temperature control in industrial processes.

Strains of coryneform bacteria which produce methanethiol from L-methionine in laboratory culture have been isolated from Cheddar cheese and raw milk Many Gram-negative raw milk isolates including Pseudomonas fluorescens, Ps. putida, Ps. fragi, Ps. aureofaciens, Acinetobacter, Aeromonas and Serratia marcescens also produced methanethiol. Representative strains of coryneforms and pseudomonads produced methanethiol only from free L-methionine; glycyl-L-methionine and D-methionine were not degraded. The aerobic nature of the methanethiolproducing bacteria and the instability and alkaline pH-dependence of their thiol producing enzymes explained their failure to increase methanethiol concentrations in experimental cheese, even when large numbers (5 × 107/g milled curd) of pregrown cells were added. Other commonly occurring cheese bacteria such as starter streptococci, micrococci or lactobacilli either did not produce methanethiol or produced only traces. It is concluded that the methanethiol normally present in Cheddar cheese is unlikely to be the product of bacteria or their enzymes.