The caseinolytic activities at pH 6·8 of polymorphonuclear (PMN) and mononuclear leucocyte homogenates (equivalent to a level of 106 cells/ml milk) were less than the levels of natural milk proteinase activity found in milk from healthy cows. Bulk milks contained ∼ 4 times more milk proteinase activity than the composite milks from individual healthy cows. Isolated blood leucocytes, when added to raw milk of good bacteriological quality and stored at 5 °C, did not readily degenerate and had no detectable effect on the milk proteins even when these cells were completely disrupted by homogenization of the milk. Pasteurization of milk which contained leucocytes caused loss of cell vitality. Extracellular proteinases of psychrotrophic bacteria growing in milk were not detected until the early stationary phase of growth. The total viable count at which this occurred varied greatly. Proteinase production by a pure culture of Pseudomonas fluorescens was not detected in milk stored at 5 °C until a viable count of ∼ 109 colony forming units (c.f.u.)/ml was obtained, whilst normal bulk milks stored at 5 °C produced detectable levels of extracellular proteinase(s) when the psychrotrophic flora reached 107–108 c.f.u./ml. Casein proteolysis by PMN and mononuclear leucocyte homogenates resulted in similar polypeptide maps, but plasmin and bacterial proteinase isolated from a strain of Serratia marcescens resulted in polypeptide maps different from each other and from that produced by the leucocyte proteinase(s). The rate of proteolysis of caseins by the different proteinase sources appeared to be in the order αsl- > β- > > κ-casein for the leucocyte extracts, β- > αsl- > > > κ-casein for bovine plasmin and β- ≈ κ- > αsl-casein for S. marcescens proteinase.

The effect of storing Cheddar cheeses at elevated temperatures on proteolysis and flavour development was examined. Cheeses were stored under the following ripening conditions: (1) 8 °C for 32 weeks (control); (2) 15 °C for 8 weeks then 8 °C for 24 weeks; (3) 17·5 °C for 8 weeks then 8 °C for 24 weeks; (4) 20 °C for 8 weeks then 8 °C for 24 weeks; (5) 15 °C for 32 weeks; (6) 17·5 °C for 32 weeks; and (7) 20 °C for 32 weeks. The rates of build up of products of proteolysis and flavour increased in the order of the above treatments and substantial acceleration of ripening was achieved. However, after 32 weeks at 17·5 °C and 16 weeks at 20 °C, cheeses received significantly lower preference scores than the control cheeses (P < 0·05) mainly owing to the development of off flavours. The maximum temperature at which cheeses could be stored for 32 weeks without reduction in quality was 15 °C. Cheeses stored at this temperature for the entire period had levels of products of proteolysis at 12 weeks of age which were similar to those of controls at 32 weeks, and estimations of age at 20 weeks which were similar to those of controls at 32 weeks.

The stability of lactase from Streptococcus thermophilus at 55 °C increased 7-fold, 2-fold and 1·5-fold in the presence of lactose, galactose and glucose respectively; maltose had no effect. Total stability over an 8 h period was more than 10-fold better in milk and sweet whey than in lactose solution, owing to the stabilizing influence of the milk proteins and the milk salts. Ovalbumin and reduced glutathione provided some extra stability but were not as effective as the milk components. In the absence of lactose the enzyme was less stable in milk and was not protected at all by sweet whey constituents. None of the milk protein fractions was as effective in the absence of lactose as when it was present. Enhanced thermostability of the enzyme in milk and sweet whey is due to contributions by all major milk components, but binding of lactose to the enzyme is the major factor controlling the extent of stabilization by other components.

The composition of the diet fed to cows, the composition and coagulating properties of the milk produced by them and used for cheesemaking, the cheesemaking methods and the chemical composition, texture and grade of Cheshire cheese obtained from four farmhouse producers during the period December to June 1982–83 were investigated. The presence of pockets of loose fat in the cheese structure, which can lead to downgrading in the late winter and spring, was not related to the chemical composition of the milk and may arise from the handling of the milk before cheesemaking. Generally the composition of the forage and the milk varied with date, but not between the different farms, whereas the composition and properties of the cheese were dependent on the manufacturers but were fairly constant during the period of study.

Flavour, aroma and appearance of milks fermented with commercially available kefir grains were compared with similar characteristics of heat treated (90 ΰC/30 min) milks that had been fermented with aroma producing starters in addition to the kefir grains. The starters used were either a mixture of Streptococcus lactis subsp. diacetilactis and Leuconostoc cremoris (lactic culture) or of Str. thermophilus and Lactobacillus bulgaricus (yogurt culture).

Two methods of fermentation were investigated involving removal of the grains followed by (i) a second fermentation of the kefir with the aroma-producing organisms and (ii) combining the kefir with aroma-producing organisms for use as a starter to ferment a further sample of heat-treated milk. Products were assessed by a taste panel and marked for acceptability. Addition of lactic or yogurt cultures resulted in products with increased mean scores for aroma and more acceptable flavours.

A bacteriophage, øC2(W), which attacked Streptococcus lactis strains C2, ML3 and 712, is described. It had a prolate head and a non-contractile tail and produced large haloes around plaques. Infection of eight paired-strain cultures, each containing Str. lactis C2, with øC2(W) resulted in marked inhibition of acid production for six cultures. Direct phage–nonhomologous ‘host’ contact was not required for inhibition. Lysates of øC2(W) contained a phage-induced lysin. Evidence is presented that the inhibitory effect of øC2(W) against paired and multi-strain cultures is due to the lysis of phage-insensitive strains by phage lysin. An isometric phage, ø712, was shown not to produce such a lysin.

Whey and deproteinated whey preparations with different carbon: nitrogen (C/N) ratios ranging from 7·5 to 73 were digested anaerobically in a downflow stationary fixed-bed reactor at 35 °C with a hydraulic retention time of 5 d. Effluent and biogas parameters indicated that no decrease in digestion and stability occurred at the highest C/N ratio. Chemical oxygen demand (COD) removal averaged 88%, while volatile fatty acids were maintained at a low level (< 500 mg l–1). Biogas yield averaged 0·423 m3 kg–1 COD and the methane content of the biogas varied between 57 and 63%. Ammonia toxicity occurred at a C/N ratio of 7/5 in the substrate feed. Virtually no ammonia N was detected in the effluent when whey substrates with C/N ratios of 50 and 73 were fed. Titration curves showed that buffer intensities in the effluent were not affected by a decrease in ammonia level. A decrease in the biomass content of the effluent which occurred as a result of the increase in C/N ratio of the substrate did not cause any rate limiting effect on biogas production. The removal of protein from the whey caused a reduction in the ratio of COD: total carbon in the whey. On average 59% of the carbon in the substrate was converted to biogas.

The lytic enzyme present in øC2(W) lysates was isolated by means of ion-exchange chromatography and further purified by gel filtration and ultrafiltration. The phage enzyme had an apparent pH optimum of 6·5–6·9 and was rapidly inactivated at temperatures > 47°C. The apparent temperature optimum was 37°C and Q10 and Ea values over the range 22–32°C were 2·5 and 69·2 kJ/mol respectively. Monovalent and divalent cations activated the enzyme. Reduced -SH groups on the enzyme were required for lytic activity. Gel filtration revealed a mol. wt of ˜ 46000. Strain-dependent differences in sensitivity of group N lactic streptococci to lysin were found. Group D streptococci were also lysed. Strains of three species of Leuconostoc, two species of Lactobacillus, one strain of Escherichia coli and of Micrococcus lysodeikticus were apparently resistant. Analysis of cell wall degradation products gave results which were consistent with the lysin having the specificity of an N-acetylmuramidase.

Interrelationships between concentrations of constituents of forages in diets fed to the cows, milks and cheeses, rennet coagulation properties of milks, manufacturing methods and the grades and textural properties of the cheeses produced by four farmhouse producers of Cheshire cheese were studied during the period December 1982 to June 1983. Many intercorrelations occurred in concentrations of constituents of the milks and ultrafiltrates. Rennet coagulation properties were related to concentrations of many of the constituents of the milks and forages in the diet. Many interrelationships in parameters of cheese texture and components of cheese composition were observed. Compositions and texture of the final cheeses were generally not related to the composition of the milks used in their manufacture.

To obtain information about the possible release of β-casomorphins from β-casein under in vitro conditions, cows' milk was incubated with 13 strains of Gram-negative or Gram-positive bacterial species isolated from bovine milk. After incubation periods of 1–24 d, milk samples were assayed for β-casomorphin-4, -5, -6 or -7 immunoreactive materials. In general, no β-casomorphin immunoreactive material was found in samples incubated with non-caseolytic strains, e.g. Pseudomonas putida or Streptococcus faecalis subsp. faecalis; however, high levels were found in several samples incubated with caseolytic strains, e.g. Ps. aeruginosa or Bacillus cereus. When such materials were present, the amounts of β-casomorphin-4, -6 or -7 greatly exceeded β-casomorphin-5 immunoreactivity. It was concluded that while some of these materials probably represented authentic β-casomorphins, others were more likely to be β-casomorphin precursors.

The involvement of toxic oxygen intermediates in the bacteriostatic effect of milk was determined by producing bacterial growth curves using turbidimetry in the presence and absence of oxygen radical-scavenging substances. Using whey as substrate, catalase, haemoglobin combined with ascorbic acid and xanthine oxidase inhibitors all provided protection against oxygen toxicity for a strain of Staphylococcus aureus and of Streptococcus agalactiae. Superoxide dismutase and mannitol were less effective. This was evident in whey alone and in the presence of oxygen radicals produced exogenously by the t-butylhydroperoxide, H2O2 and xanthine/xanthine oxidase systems.

The effect of preheating skim milk and artificial micelle milk on curd syneresis was studied. The inhibition of syneresis caused by heat was dependent on the presence of β-lactoglobulin (β-lg) and to a lesser extent α-lactalbumin. The degree of inhibition increased with increasing amounts of added β-lg and preheating temperature. This agrees with the hypothesis that the detrimental effect of preheating on syneresis is due to complex formation between β-lg and κ-casein. This complex appeared to be mediated via thiol–disulphide exchange and its formation appeared to interfere with the micelle–micelle interactions responsible for syneresis.