The keeping quality (KQ) of whole milks pasteurized at 72 °C, 78 °C or 83 °C for 20 s and stored at 12 or 7 °C was examined by assessment of the naturally contaminating thermoduric microflora and of the numbers of Gram-negative rods, a single strain of the latter being introduced deliberately by post pasteurization inoculation. A pasteurization temperature of 83 °C reduced the KQ at 7 °C by about 15% compared with pasteurization at 72 °C or 78 °C, with a 2 d loss in the absence, and aid loss in the presence, of the post pasteurization contamination. The KQ of milks stored at 12 °C was not significantly affected by the pasteurization temperature. Milks pasteurized at 78 °C tended to have a slightly better KQ than those pasteurized at 72 °C, especially at 7 °C. However, pasteurization at both 78 °C and 83 °C caused a reduction in cream rising and considerably increased the risk of cream plugging.

Glucose levels in 188 quarter fore milks from different cows were determined by an enzymic procedure. Mean glucose content was 0·22 mM (standard error ±0·009) and results ranged from 0·02–0·57 mM. Abnormal quarters had lower glucose levels (P < 0·01) than normal quarters but variability within each classification was large. Glucose content was negatively correlated with both somatic cell count (r = –0·49) and N-acetyl-²-D-glucosaminidase level (r = –0·61). Milk glucose was considered to be of limited value as a diagnostic test for mastitis.

A survey was undertaken to update and extend available information on the vitamin content of pasteurized milk as produced at processing dairies in mainland UK and to investigate regional, seasonal and breed effects. The concentration of total retinol in milk from non-Channel Island (NCI) breeds averaged 61·9 βg/100g in summer and 41·2 βg/100g in winter. Concentrations of β-carotene were 31·5 and 10·5 βg/100g in summer and winter respectively. Concentrations of retinol in milk from Channel Island (CI) breeds were similar, but concentrations of β-carotene were on average 3 times higher. The concentration of vitamin D3 in milk from NCI breeds was 0·033 βg/100g in summer and 0·026 βg/100g in winter. There was no marked seasonal variation in the mean concentration of total vitamin C (14·5 βg/ml). Values for the concentration of B vitamins (βg/ml) were: folic acid 0·060, vitamin B12 0·0042, riboflavin 1·78, nicotinic acid 0·71, pantothenic acid 3·60, biotin 0·020, thiamin 0·46 and vitamin B6 0·61. Seasonal variation in the concentration was most marked for folic acid (c.v. 17·4%) and to a lesser extent for vitamin B12 (c.v. 10·3%). The only breed differences in the B vitamin content were for riboflavin and folic acid, the mean values obtained for milk from CI breeds being respectively 20 and 10 % higher than those from NCI breeds.

The detailed composition, pH, and the properties of rennet clotting, coagulum development and syneresis at pH 6·4 were estimated in milks from 4 Friesian cows during the first 9 weeks of lactation. The concentrations of casein, fat, citrate, Ca, Mg, Pi and Na decreased significantly during early lactation whilst the pH and concentration of lactose increased. Levels of whey protein and K and the casein:fat ratio were unaltered. Considerable variation occurred in the relative proportions of α-, β- and κ-casein. These changes in composition were associated with decreasing coagulum strength and an increased rate of whey drainage although rennet clotting time (RCT) did not follow a significant trend. RCT was positively related to levels of Na and whey protein but negatively correlated with Pi, K and lactose. There were significant positive correlations between coagulum strength and casein, fat, non-protein nitrogen, Pi, Ca, Mg and citrate. The correlation with α-casein was stronger than that with β- or κ-casein. Syneresis time was positively related to fat, Ca and coagulum strength, but negatively related to the concentration of lactose.

The action of rennet on homogenized milk was studied using turbidimetric and light scattering techniques, and compared with results obtained previously for skim milk. The time required for the onset of coagulation was shorter for homogenized milk than for skim milk. The rate of coagulation of fully renneted particles increased with increasing temperature, and with increasing Ca2+ concentration, but was only slightly influenced by changes in ionic strength. The von Smoluchowski rate constant for the coagulation reaction was two orders of magnitude smaller for homogenized milk than for skim milk. Results suggest that coagulation of homogenized milk is controlled in general by the same factors as skim milk, and that the reaction may be inhibited owing to a reduction in the amount of casein available for mutual interaction, rather than to disruption of the micelles on homogenization.

Concentrations were measured of different vitamins in milk as delivered to the home and after storage in the domestic refrigerator, during summer and winter periods. Vitamin C concentrations varied greatly, ranging from 0·1 to 18·4 εg/ml. Most of the lower vitamin C values were associated with known exposure of the milk to daylight and there was a clear tendency for the concentrations to fall with increased time of refrigerated storage especially when associated with prior exposure of the milk to daylight. Riboflavin values were mostly in the range 1·35–1·92εg/ml, but occasional lower values were associated with low content of vitamin C and in some instances with prolonged exposure to daylight on the doorstep. Concentrations of the other vitamins (εg/m1) were: folic acid 0·041–0·074, thiamin 0·38–0·48, vitamin B12 0·0032–0·0044 and vitamin B6 0·56–0·71. All fell within the normal ranges and were not significantly influenced by exposure of the milk to daylight or subsequent storage in the refrigerator.

Changes in milk lipid composition were measured during the course of mastitis infections induced in cows by infusion of either Escherichia coli endotoxin or Streptococcus agalactiae into 2 quarters of the udder; untreated quarters were used as controls. Experiments were divided into 3 distinct phases: a pre infusion period during which several samples were collected before infusion; a post infusion period following infusion and corresponding to the occurrence of elevated cell counts in milk from infused quarters; a recovery period followed after a short pause and represented the return of the cell count in milk from infused quarters to the level in the corresponding controls.

Milk total fatty acid composition was unaffected by the infusion. Free fatty acid (FFA) composition did, however, undergo some alteration. There was a significant increase in long-chain saturated acids in milk from infused quarters relative to the corresponding controls during the post infusion period. FFA concentration (mequiv./100 g fat) also increased significantly during this period although the net increase was only slight. Phospholipid and cholesterol concentrations were significantly higher post infusion in milk of infused quarters.

The results suggest that changes in concentration and composition of those milk constituents synthesized and secreted by the mammary epithelium occur after secretion in the alveolar lumen and milk ducts.

The temperature at which milk fat was crystallized was found to influence its firmness values, crystalline conformation and the percentage solid fat at 5 °C. Extremely rapid cooling of milk fat using liquid N2 (LNT) gave a product which, although it contained a relatively high solid fat content, was less firm and possessed a wider plastic range than milk fat crystallized at 15, 5 or –20 °C. Polymorphic transitions occurred in the LNT milk fat when it was subsequently held at 15 °C but not at 5 °C. Apparently the particular polymorphic modification has an important role in rheological behaviour.

The viable counts, pH and production of lactic acid were determined for Lactobacillus lactis, L. helveticus and Streptococcus thermophilus after 0, 5 and 24 h anaerobic incubation at 44 °C in skim milk containing 0, 3, 5 or 7μg cadmium (Cd)/l. In skim milk containing 7μg Cd/l, a noticeable decrease in lactic acid production and inhibition of the decrease in pH was observed for all three strains after 24 h, although there was only a slight inhibition of growth. With 3 and 5μg Cd/l, there was only a slight inhibition in the decrease in pH. In addition, with 3 μg Cd/l some evidence of stimulation of lactic acid production was found. The inhibitory effect of low Cd concentrations on lactic acid bacteria may give rise to problems in the dairy industry.

Calcium and phosphate concentrations in milk ultrafiltrate prepared at 37 °C over the pH range 4.0–8.0 have been measured and used to calculate the apparent solubility products for various calcium phosphates in milk. Micellar calcium phosphate had a constant Ca/Pi = 1·58·10·03 at all pH values and a near-invariant ion activity product for CaHP04. Apparent solubility products have also been obtained for brushite, hydroxyapatite and amorphous tricalcium phosphate in milk salt solution over a similar pH range. No evidence was found for a hydroxyapatite or amorphous tricalcium phosphate phase in milk.

Bacterial post pasteurization contamination with psychrotrophic Gram-negative rods (GNR) was measured in commercial milks before and after transfer to retail containers. The tanks of pasteurized milk feeding the filling units contaminated milk less often (39% of samples) but usually at a higher level than the filling units (92% of samples). The number of GNR present had a considerable influence on the shelf life of milk, and the range found in commercially pasteurized milk was reflected in a wide range of shelf lives.

The use of a commercial laser Doppler electrophoresis apparatus to measure electrophoretic mobilities of milk constituents is demonstrated, and calculations of apparent ζ-potentials from these mobilities are given. Systems studied were: native and renneted casein micelles suspended in milk ultrafiltrate and different buffers containing Ca2+, particles from homogenized milk before and after renneting, fat globules, and homogenized fat/caseinate systems. The results confirmed previous estimates of the ζ-potentials of these materials, but also extended to cover the effect of changing the concentrations of Ca2+ and other salts on the electrokinetic properties of the particles. Increasing concentration of Ca2+ caused considerable decrease in the ζ-potentials of native and renneted casein micelles, and temperature increase caused the apparent ζ-potential to increase in both cases. Particles from homogenized milk showed ζ-potentials very similar to those of native casein micelles, and renneting of the homogenized milk gave behaviour similar to renneted casein micelles. The ζ-potential of the homogenized materials could be distinguished clearly from those of native fat globules. Likewise, milk fat which had been homogenized using whey protein showed distinct differences in its surface properties from particles based on caseinate or casein micelles.

The heat coagulation time (HCT) of concentrated whey-protein-free (WPF) milk measured at 120 and 130 °C was reduced to by addition of 5–20 mM-2-mercaptoethanol (ME). However, although the amount of soluble casein formed on heating was doubled by addition of ME, the shape of the HCT–pH profile was affected only slightly. The proportion of κ-casein in the soluble casein from heated concentrated WPF milk containing ME was very high, though it was somewhat lower than that of the soluble casein from heated concentrated WPF milk containing no ME. No solubilization of colloidal Ca phosphate was observed in either unheated or heated concentrated WPF milk on addition of ME. These facts suggest that ME probably promotes the formation of soluble casein with release of κ-casein from micelles on heating, thus destabilizing the casein micelles.

The chemical composition and rennet coagulation properties at pH 6·4 of milks from 2 commercial herds of Friesian cows were monitored during the change from winter diet to spring grazing. There was considerable variation both in composition and in coagulation properties during this period. There were significant trends for increasing concentrations of casein, Na and lactose, and decreasing concentrations of fat, citrate, K and Mg following the change. Many correlations occurred between concentrations of components, some of which may have physiological significance. Coagulum strength increased after the change to spring grazing and was related to the concentrations of casein, citrate and some of the minerals. Syneresis time did not follow any trend during this period, but was significantly related to concentrations of fat, Na and K. Rennet clotting time did not follow a significant trend immediately after the dietary change, but tended to increase as the summer progressed; it was significantly related to concentrations of Ca, Pi Na and K, as well as to the original pH of the milk. When the concentration of lactose in milk was adjusted, variations in lactose concentration did not affect the coagulation properties of milk.

The acetolactate synthase of Leuconostoc lactis NCW1 was studied. The Mn2+ content of cell free extracts was 3·2 µ/mg protein. The enzyme did not require Mn2+ for activity, had an optimum pH between 5 and 6 and was labile. Incubation at 21 °C or addition of thiamine pyrophosphate followed by storage at 4 °C stabilized the enzyme. It was allosteric with at least two binding sites for pyruvate and was inhibited by several products of glucose metabolism (6-phosphogluconate, 3-phosphoglycerate, 2-phosphoglycerate, PEP and ATP) at pH 5·4. Except for ATP, which became more inhibitory, the inhibition disappeared completely (6-phosphogluconate, 2-phosphoglycerate, and PEP) or partly (3-phosphoglycerate) at pH 4·7. The role of these compounds in the regulation of acetoin production from citrate by leuconostocs is discussed.

κ-casein from porcine milk was isolated and purified by affinity chromatography on thiopropyl Sepharose followed by hydroxyapatite chromatography. The amino acid composition of this protein is similar to that of bovine κ-casein. The sugar composition was established and sow κ-casein was found to be highly glycosylated. It contains 12 N-acetylneuraminic acid, 10 N-acetylgalactosamine, 2 N-acetylglucosamine and 22 galactose residues.

Sections (1–5 μm thick) of epoxy resin-embedded cheese curd, yogurt, whipped cream and cream cheese were etched by treatment with warm alcoholic NaOH. Thin specimens were produced which could be metal coated and examined by scanning electron microscopy using accelerating voltages of at least 30 kV without producing charging problems. The images observed were easy to interpret and could be used to produce quantitative results. Although native milk fat globules did not survive the etching process, homogenized milk fat was not adversely affected. Critical point drying of etched sections produced results superior to those obtained by air drying.

Milk with 1·5% fat was treated to increase its viscosity to that of whole milk on the assumption that the sensation produced in the mouth, its mouthfeel, would be improved. Selected treated 1·5% fat milks were assessed for similarity to whole milk in taste panel trials. Panelists were chosen because of their ability to distinguish between whole milk and that containing 1·5 % fat by taste in at least one of two tests. Only 34% of the persons screened were suitable. Throughout the trials, the proportion of panelists able to distinguish between the two milks served alone and in tea remained at about 50%, but rose to 80–90% for the milks in coffee. In general, addition of KOH to 1·5% fat milk to pH 7·4 and of 6 mM-citrate overcame about 75 and 50%, respectively, of the difference in mouthfeel between whole and 1·5% fat milk. Whole and 1·5% fat milks were easily distinguishable visually, but the appearance of the latter was improved by adjustment to pH 7·4.

Whole milks concentrated 1·5–4-fold and acidified and citrated milks concentrated 2·8-fold by ultrafiltration at 50 °C were analysed for chemical changes relevant to further processing, storage or nutrition. Fat and protein were entirely retained in the concentrate. The retention of water-soluble vitamins, Ca, Mg, phosphate and trace minerals depended on the proportion bound to the protein. Ascorbic acid was rapidly destroyed during concentration. Because of the differential retention of nitrogenous components, protein comprised a progressively higher proportion of the total N as the milk became more concentrated. No denaturation of whey protein or disruption of casein micelles was detected during concentration of whole milk, but some solubilization of the casein occurred after citration. Reduction of fat globule size occurred early in the concentration process, damage to the fat globule membrane was indicated and the milk became more susceptible to lipolysis. Apart from a tendency for preacidified or precitrated concentrates to gel, no change in the susceptibility of the milks to heat damage was detected.

The mRNA coding for prochymosin (prorennin) has been partly purified from calf abomasum. The in vitro translation products of the total polyadenylated RNA show a major band on gel electrophoresis which reacts with antibody raised against purified chymosin. The mol. wt of 43000 is higher than expected from the reported amino acid sequence and would correspond to prochymosin with an unprocessed signal sequence of ∼ 17 amino acids. The synthesis of chymosin mRNA is age-related, and ceases by 3 months even in milk-fed calves.