Lactating goats were given a close arterial infusion of [1-14C]leucine and [4,5-3H]4-methyl-2-oxopentanoic acid into one half of the mammary gland at 2–3 weeks and 34–39 weeks after kidding. Rates of protein synthesis, degradation and net output were determined from measurements of arteriovenous difference and blood flow using a model of leucine metabolism previously developed for muscle (Oddy & Lindsay, 1986). Protein leucine output in milk (Y μmol/min) correlated well with the difference between synthesis and degradation (X μmol/min) derived from the model:

There was substantial synthesis and degradation of protein within the mammary gland. Although only an approximate value could be obtained for the partitioning of protein synthesis and degradation between tissue and milk proteins, there was evidence of appreciable turnover of both. There was no significant difference between mammary leucine and protein metabolism in early and late lactation other than that imparted by a greater mass of mammary tissue in early lactation, although there was a tendency for greater oxidation of leucine in late lactation.

A milking machine claw incorporating valves to prevent movement of milk between teats during milking, and its contribution to the prevention of udder infection under experimental and field conditions, is described. Under experimental conditions a suspension of Streptococcus agalactiae and Str. dysgalactiae was introduced into the milking cluster during milking; 11 of 40 quarters became infected using a conventional claw piece whereas none of 40 quarters milked with the multivalve claw developed intramammary infection. In a 12-month experiment in ten commercial herds, each split into two equal susceptibility groups, the multi-valved cluster reduced the incidence of new infection with coliforms and Str. uberis by 17% (P < 0·1). The incidence of all new infections and new clinical cases was 14 and 25% lower compared to conventional clusters but these reductions were not significant.

Milk samples from 250 Jersey and Friesian cows were collected and used to investigate the pattern of interrelationships between milk constituents and to test the effects of breed, sire, parity, stage of lactation, season of calving and milking time on them. Breed and sire effects were significant for all constituents. Overall Jersey cows produced significantly higher milk constituent concentrations except lactose, but the test-day yields in Friesians were significantly higher. Milk constituents had varying responses to parity, days in milk and season of calving. Correlation between different constituents and their regression on each other suggested antagonistic relationships. The pattern of association between constituent concentrations in Friesian cows was similar to that between test-day yields, indicating an equilibrium between volume and content of milk.

Two light scattering methods for monitoring the precipitation of casein from skim milk (9% w/w) with mineral acid are presented. Experiments were carried out at precipitation temperatures in the range 25–53 °C and precipitation pH values in the range 3·9–5·1. The initial stages of precipitation were explored using a light fibre optics system attached to a closed-loop acid/milk mixing system. The later stages of particle formation and growth were examined using a laser doppler anemometer attached to an open-pipe flow system. It was found that there was a lag initially, followed by a period of rapid particle growth. During a third phase a dynamic equilibrium between particle growth, break-up and shrinkage was established. A model for isoelectric casein precipitation based on these observations is proposed and discussed.

Casein micelles separated by ultracentrifugation of raw skim milk were dispersed at a casein concentration of 2·5% in simulated milk ultrafiltrate and dialysed against 10 mM-imidazole buffer (pH 7·0) at 5 °C. The amounts of colloidal Ca and inorganic P decreased from 77 to 11 mg and from 31 to 2 mg respectively in 100 ml during 72 h of dialysis. Micellar casein content was reduced to 43 and 11% after 48 and 72 h of dialysis respectively. In high-performance gel chromatography of casein micelles in the presence of 6 M-urea, fraction 1, consisting of the casein aggregates cross-linked by colloidal Ca phosphate (CCP) decreased during dialysis and the retention time of the peak of fraction 1 was prolonged, suggesting that the cross-linkage between CCP and casein molecules was disrupted. The dissociation rates of the individual casein constituents from the casein aggregates cross-linked by CCP during dialysis were in the order β-> αs1 - > αs2-casein. The higher the ester phosphate content, the slower was the dissociation rate of the individual casein constituent. It is suggested that the strength of interaction between CCP and casein molecules depends on the ester phosphate content.

At pH 7·0, the emulsifying capacity (Ec) of ion-exchange fractionated whey protein powders in corn oil/water systems increased with increase in protein concentration up to 0·l% (w/v), where it was maximal. At higher concentrations Ec decreased because of packing effects of the oil droplets. At low protein concentrations Ec could be determined from linear equations involving the relative concentration of the individual proteins in the powder. The surface coverage of β-lactoglobulin on the oil was ∼ 1 mg m–2. Oil droplet size and Ec were maximal at pH 4·0–6·0, corresponding to the isoionic points of the proteins.

The rennet coagulation times of infant milk formulae or fresh skim milk (milk) samples heated at temperatures in the range 70–140 °C for 1–10 min decreased on acidification, usually to pH < 6·0. Heated milk samples acidified to pH 5·5 and reneutralized to pH 6·6 retained good rennet coagulability. Acidification of such milk samples before heating also reduced the adverse effect of severe heat treatment (95 °C for 1 min) on rennet coagulation. Addition of low concentrations of CaCl2 to heated milks offset the adverse effects of heating. Acidification of heated milks increased the [Ca2+], and reneutralization of acidified milk only partly restored the [Ca2+], i.e. acidified/reneutralized milk had a higher [Ca2+] than normal milk, suggesting this as the mechanism via which acidification/neutralization improves the rennet coagulability of heated milk. Approximately 50% of the whey protein can be incorporated into rennet gels in heated milks while retaining good coagulability and curd tension; this may be a useful technique for increasing cheese yield.

A heat stable extracellular proteinase from the psychrotroph Pseudomonas fluorescens AH-70 was purified to electrophoretic homogeneity by affinity chromatography on a gramicidin S–Sepharose-4B column. Bacitracin linked to Sepharose-4B was unable to retain any proteolytic activity, whereas the same antibiotic bound to AH-Sepharose-4B retained ~ 25% of the total activity. The purification procedure on the gramicidin S–Sepharose-4B column was easy to perform, fast and reproducible; it resulted in a 207-fold increase in the specific activity and a yield of 41% of the original activity. The purified enzyme was a monomer with a mol. wt of 33000. The enzyme hydrolysed whole casein and its fractions whereas no activity was observed against bovine serum albumin. The enzyme was a metalloproteinase. It was heat stable, having D-values at 121, 135 and 150 °C of 3·8, 1·9 and 0·6 min respectively.

The effect of several parameters on the decarboxylation of α-acetolactic acid (ALA) to diacetyl and acetoin was studied in model experiments. At temperatures > 50 °C, relatively more diacety1 than acetoin was produced. Regardless of temperature, HC1 increased the production of acetoin (58 mol% conversion at max.) and prevented the production of diacetyl. Steam distillation of ALA caused ~ 55 mol%/h conversion to acetoin and 7 mol%/h to diacetyl. Production of acetoin was proportional to the ALA concentration. Based on these results two methods for the quantitative measurement of ALA were developed. Only one of 17 strains of Streptococcus diacetylactis (Lactococcus lactis subsp. lactis biovar. diacetilactis, new nomenclature) and three of three strains of Leuconostoc spp. produced ALA. The implications of these findings are discussed.

Synergic effects of proteolytic enzymes from two different microbial sources on the ripening of Swedish hard cheese were studied. When extracellular proteolytic enzymes from Bacillus subtilis (Neutrase) and/or heat treated cells of Lactobacillus helveticus (now L. delbrueckii subsp. helveticus) were added to the cheese milk, cheese ripening was accelerated; Neutrase effectively hydrolysed casein to give a softer body. Addition of heat treated lactobacilli did not accelerate hydrolysis of casein, but accelerated the breakdown of peptides which increased the amount of amino acid N in the cheese and also enhanced the intensity of cheese flavour. A bitter taste which developed in cheeses with added Neutrase could be eliminated by the simultaneous addition of heat treated lactobacilli.

When 10% citric, lactic or acetic acid was added to sheep's milk cheese whey during its heating for Myzithra cheese preparation, protein retention in the cheese was increased. Acidification of whey to pH 5·2 with lactic acid before heating followed by readjustment to pH 5·8 with NaOH was considered to be the best treatment for practical use, increasing whey protein retention in the cheese and yield without loss of flavour, aroma or texture.

A method of extracting cheese juice is described. Juice from a single one-month-old Cheddar cheese was used to investigate the relationship between the composition of the cheese juice and the occurrence of crystalline aggregates in the cheese. Two classes of crystalline aggregates were identified by light microscopy and also by electron microscopy in combination with X-ray microanalysis: the larger class contained both Ca and P whereas the smaller class contained Ca but not P. Concentrations of ions in the cheese and cheese juice were determined and corrected for the excluded volume of co-solutes and other phases present. When this was done, comparison of the concentrations of Na, K, Cl and lactate in the cheese and the cheese juice indicated that they were virtually entirely in solution, whereas appreciable amounts of the total Ca, P and P1 in the cheese were not extracted in the cheese juice. The amounts of the non-extractable salts were consistent with the volume fractions of the crystalline aggregates found by microscopy; calculations of the ion equilibria in the cheese juice showed that it was supersaturated with respect to various Ca phosphate salts and to tricalcium citrate. It is concluded that the physicochemical origin of the crystalline aggregates was the nucleation and growth of crystals from a supersaturated solution, though the sites of nucleation may have been formed by the microbial activity in the cheese. Of the free amino acids found in the juice, none was at a sufficiently high concentration to form a saturated solution, indicating that in this relatively young cheese the crystalline aggregates were not formed from amino acids.

Forty-five strains of staphylococci and 6 strains of micrococci isolated from pasteurized ewes' milk cheeses (Manchego and Burgos varieties) were identified to species level. Three strains of staphylococci were identified as Staphylococcus aureus. Amongst the 42 coagulase-negative staphylococci, 8 were novobiocin-sensitive and 34 novobiocin-resistant. The novobiocin-sensitive species found were Staph. auricularis (5 strains), Staph. caseolyticus (2 strains) and Staph. epidermidis (1 strain). Novobiocin-resistant strains were identified as Staph. gallinarum (5 strains), Staph. xylosus (4), Staph. saprophyticus (9), Staph. cohnii (6), Staph. sciuri (1) and Staph. lentus (3). The other 6 isolates could not be identified. Only 3 strains of micrococci were identified (Micrococcus kristinae, M. varians and M. sedentarius). The possible origin of the isolates is discussed.