Proteolytic activity in mastitic skim-milk was often 5–10 fold higher than in normal milk, its level being related to somatic cell count but not precisely correlated with it. In milks with the highest levels of activity plasmin accounted for about one third of the total proteinase. A further third was sedimented with the micellar fraction together with the plasmin, but unlike plasmin, was not inhibited by addition of soyabean trypsin inhibitor (SBTI). The final third remained in the serum phase.

Polyacrylamide gel electrophoresis (PAGE) showed that αsl- and β-caseins were degraded at about the same overall rate. The plasmin produced the usual readily identified fragments from β-casein, but incubation of mastitic milk also produced changes in patterns in the γ-casein region differing from plasmin-induced changes which were also apparent when the micellar fraction was incubated. As they were inhibited by SBTI, a second trypsin-like enzyme in addition to plasmin may also have been present. Other proteinase(s) not inhibited by SBTI was also associated with casein micelles and produced at least 3 characteristic protein fragments seen on PAGE. The serum phase proteinase(s) was likewise not inhibited by SBTI, and did not produce any well-defined electrophoretic bands, suggesting a rather non-specific breakdown of caseins. After separation of mastitic whole milk, a considerable proportion of the proteolytic activity was found in the cream phase. The proportion was enhanced by freezing and thawing, and the enzyme appeared to be identical to the SBTI-resistant micellar proteinase.

Because of the considerable proteolysis likely to occur under the time and temperature conditions involved, our results may provide some explanation for the problems encountered in cheesemaking with mastitic milks (e.g. yield losses, poor curd strength and off-flavour development).

The vitamin content of ultra-high-temperature (UHT) processed milk was compared with that of the original raw milk. Three processes were used. In the first, which caused no change in oxygen content, the milk was heated and cooled in a plate-type heat exchanger. In the second, the milk was again heated indirectly and then evaporatively cooled, leaving in the milk about one-third of the initial oxygen content. In the third process the milk was heated by direct steam injection and cooled by evaporation and contained little or no residual oxygen.

On processing and during subsequent storage for 90 days there was no loss of vitamin A, carotene, vitamin E, thiamine, riboflavine, pantothenic acid, biotin or nicotinic acid. There was little or no loss of vitamin B6 or vitamin B12 on processing, but up to 50% of each of these vitamins was lost during 90 days' storage. All the dehydroascorbic acid (DHA) and about 20% of the ascorbic acid (AA) was lost on processing. There was no further loss of AA during 90 days’ storage when no residual oxygen was present, but in milks containing more than about 1 ppm oxygen all the AA was lost within 14 days. About 20% of the folic acid was lost on processing; thereafter, as with ascorbic acid, the extent of the loss on storage depended on the residual oxygen content of the milk: in the absence of oxygen the folic acid was stable.

The respiratory burst activity of polymorphonuclear leucocytes (PMN) was evaluated in eight Holstein cows from 8 weeks before until 6 weeks after calving by chemiluminescence (CL). The CL response started to decrease 1 week before parturition, reaching a minimum during the first 2 weeks after calving. From week 3 of lactation, CL increased again and returned to original levels by week 6 of lactation. Plasma concentrations of 3-hydroxybutyric acid, total bilirubin and bovine pregnancy-associated glycoprotein started to increase before parturition to reach a maximum during the first or second week of lactation. The concentrations of glutamic–oxaloacetic transaminase, lactate dehydrogenase, non-esterified fatty acids and bilirubin increased after calving, reaching a maximum during the second week. A small decrease in plasma cholesterol during the week before and after calving was followed by an increase. The CL response of the PMN showed significant temporal relationships with the plasma concentrations of 3-hydroxybutyric acid, bovine pregnancy-associated glycoprotein, bilirubin, glutamic–oxaloacetic transaminase, non-esterified fatty acids; that with cholesterol was nearly significant. This means that the change in the CL response with time coincided with the changes in plasma concentrations of these substances with time and that these changes were significantly related with each other. The results of this study show that the decreased respiratory burst activity of bovine PMN around parturition may be related to the extent of the metabolic and hormonal changes. Although the causative relationships are not proven, these results support earlier results suggesting that 3-hydroxybutyric acid and bovine pregnancy-associated glycoprotein may directly affect neutrophil function, whereas non-esterified fatty acids, cholesterol, bilirubin, and liver enzymes may have potential as diagnostic markers of impaired neutrophil function and consequently increased disease risk around parturition.

The variation in plasmin activity and proteose-peptone (PP) content in milks from 81 cows of four breeds was statistically analysed. Plasmin activity increased with parity and stage of lactation and differed between breeds, but the breed effect was removed when adjustment was made for differences in milk casein content. The negative correlation found between plasmin activity and casein content seemed to be expressed only under assay conditions and was thus probably an artefact. The linear regressions of proteose-peptones in fresh (PP) and cold-stored milks (ΔPP) on plasmin activity were highly significant (P < 0·001). However, the variation in plasmin activity explained only 38 and 33%, respectively, of the variation in PP and ΔPP. At apparent zero plasmin activity, PP and ΔPP were significantly different from zero (P < 0·001) and the intercepts represented 77 and 46%, respectively, of the averages. PP and ΔPP were significantly higher in milks containing the BB genotype of ²-lactoglobulin than in milks with the AA genotype.

The N distribution in terms of casein, albumin, globulin, proteose-peptone, and non-protein N is given for a number of samples of normal milk, of milk from cows with mastitis, and of milk persistently low in solids-not-fat content. Improved methods of determination have been used.

The average N distribution in normal milk is as follows: casein N 78·5, albumin N 9·2, globulin N 3·3, proteose-peptone N 4·0 and non-protein N 5·0% of the total N. The casein N figure is the least variable, ranging from 77·3 to 80·7. On this basis, normal milk contains, as an average: casein 2·63, albumin 0·31, globulin 0·11, and proteose-peptone substances 0·13%. The total protein content is 3·18%, and the casein: albumin: proteose-peptone: globulin ratios are 23·8: 2·8: 1·2: 1·0.

The milk from cows with clinical mastitis is low in solids-not-fat content, and characterized by a decreased content of casein and an increased content of albumin, globulin, and proteose-peptone substances, particularly of globulin.

The milk samples of low solids-not-fat content were of normal appearance and reaction, and produced by apparently healthy cows known to have yielded milk of low solids-not-fat content for some time. The samples were examined by strict bacteriological methods for the presence of mastitis streptococci, and formed two distinct groups: (1) samples from cows with subclinical mastitis, having the protein distribution of the mastitis samples described above, the cause of the low solids-not-fat being pathological; and (2) samples from uninfected cows, having the protein distribution of normal milk, the cause of the low solids-not-fat being physiological. The two groups of low solids-not-fat cases are discussed, and the conclusion drawn that subclinical mastitis may account for a large proportion of these widespread and puzzling cases.

Many cases of mild subclinical mastitis cannot be detected by the simple chemical tests which will reveal pronounced infection, and satisfactory bacteriological tests, when available, demand much skill and time. For the detection of milk from cows with subclinical mastitis, and for the differentiation of milk samples with pathological and physiological low solids-not-fat, the solids-not-fat, the figure is suggested, and the advantage of this “casein number” is illustrated.

In a feeding experiment with 6 cows in mid-lactation a portion of the starch in the concentrate part of the diet was isocalorically replaced by 8% soybean oil, and the effects on the yield and composition of the milk were studied. The oil was incorporated into the diet either as coarsely ground soybeans or by direct addition of the oil itself. The concentrate mixtures were given with a high-roughage diet that supplied 5·5 kg hay and 2·7kg sugar-beet pulp/day.

When the soybean oil was included in the diet either in the form of soybeans or as the oil itself there was an increase in the yields of milk, solids-not-fat (SNF) and lactose, and an increase also in the percentage of lactose in the milk. The percentage of protein, however, was decreased.

When the oil was included in the form of soybeans there was an increase in the yield of fat but a decrease in the percentage of SNF in the milk.

When soybean oil was included in the diet, the yield of protein was increased but the yield of fat and the percentage of fat in the milk were both decreased.

With both methods of incorporating the oil there was an increase in the relative proportion of propionic acid and a decrease in that of butyric acid in the total volatile fatty acids of the rumen liquor. When the oil was added directly, but not when it was added as soybeans, there was a decrease in the proportion of acetic acid in the total volatile fatty acids. The implications of these findings are discussed.

The flavours of the following compounds were determined in synthetic butter prepared from 84% steam-deodorized butter oil and 16% water: C6, 8, 10, 12, 14 δ-lactones, C12 γ-lactone, C2, 4, 6, 8, 10, 12, 14n-alkanoic acids, phenol, m- and p-cresols, o-methoxyphenol, indole and skatole. C6, 8, 10 δ-lactones, C2, 4, 6, 8, 10, 12, 14n-alkanoic acids, phenol, m- and p-cresols, o-methoxyphenol and volatile fractions from butter oil were also evaluated in bland butter. None of the compounds listed constitute basic butter flavour on their own, but C8, 10 δ-lactones, C10 acid, phenol, p-cresol, indole and skatole contribute to butter flavour.

Volatile sulphur compounds are major flavouring compounds in many traditional fermented foods including cheeses. These compounds are products of the catabolism of L-methionine by cheese-ripening microorganisms. The diversity of L-methionine degradation by such microorganisms, however, remains to be characterized. The objective of this work was to compare the capacities to produce volatile sulphur compounds by five yeasts, Geotrichum candidum, Yarrowia lipolytica, Kluyveromyces lactis, Debaryomyces hansenii, Saccharomyces cerevisiae and five bacteria, Brevibacterium linens, Corynebacterium glutamicum, Arthrobacter sp., Micrococcus luteus and Staphylococcus equorum of technological interest for cheese-ripening. The ability of whole cells of these microorganisms to generate volatile sulphur compounds from L-methionine was compared. The microorganisms produced a wide spectrum of sulphur compounds including methanethiol, dimethylsulfide, dimethyldisulfide, dimethyltrisulfide and also S-methylthioesters, which varied in amount and type according to strain. Most of the yeasts produced methanethiol, dimethylsulfide, dimethyldisulfide and dimethyltrisulfide but did not produce S-methylthioesters, apart from G. candidum that produced S-methyl thioacetate. Bacteria, especially Arth. sp. and Brevi. linens, produced the highest amounts and the greatest variety of volatile sulphur compounds including methanethiol, sulfides and S-methylthioesters, e.g. S-methyl thioacetate, S-methyl thiobutyrate, S-methyl thiopropionate and S-methyl thioisovalerate. Cell-free extracts of all the yeasts and bacteria were examined for the activity of enzymes possibly involved in L-methionine catabolism, i.e. L-methionine demethiolase, L-methionine aminotransferase and L-methionine deaminase. They all possessed L-methionine demethiolase activity, while some (K. lactis, Deb. hansenii, Arth. sp., Staph. equorum) were deficient in L-methionine aminotransferase, and none produced L-methionine deaminase. The catabolism of L-methionine in these microorganisms is discussed.

A high correlation was found between soluble calcium and citrate concentrations during a study of seasonal changes in the composition of the commercial milk supply to South-west Scotland. A simple physicochemical model of the ion equilibria in the aqueous phase of milk suggests that the observed correlation is causal. This hypothesis was corroborated by an analysis of data on individual cow milks and by experiments involving the addition of citrate to milk at constant pH.

Cheesemaking is initiated by specific hydrolysis of κ-casein, followed by random aggregation of casein particles to form a network. The structure of this curd determines its subsequent behaviour and the composition and texture of the cheese. It is affected by previous homogenization or concentration of the milk. The composition of the milk and the distribution of components in it, which are affected by season, mastitic infection and cold storage, also influence its curd-forming properties, and the composition and yield of cheese. During ripening of cheese, the final texture and the development of flavour occur. Both are influenced by the type and extent of proteolysis, catalysed by coagulant, bacterial and milk enzymes. These and other enzymic and non-enzymic reactions responsible for flavour development depend on the composition and environment within the product. Some of these reactions can be accelerated by incorporating enzymes and chemical reagents into the curd, resulting in the production of an enhanced flavour. A number of compounds responsible for the flavour of cheese and yoghurt have been identified and mechanisms for their formation and retention in the product have been postulated.

A comparison of three different media, yeast dextrose agar, tomato dextrose agar and acetate agar (which is selective for lactobacilli) showed that the acetate medium was satisfactory for the isolation and counting of lactobacilli from Cheddar cheese during ripening. Lactobacilli were isolated on this medium in the early stages of ripening when the large numbers of starter streptococci growing on the other media were completely suppressed. Lactobacilli isolated were identified biochemically and serologically. Serological typing was found to be a useful method of identifying large numbers of isolates and of assessing the incidence and distribution of particular types in the cheese.

We wish to thank Dr A. T. R. Mattick for his interest and helpful advice in this work; Dr L. A. Mabbitt for preparation of the cheese samples; Miss H. R. Chapman for making the experimental cheese; and Mrs P. Jarrett for technical assistance.

A preliminary examination of strains of leuconostocs collected from many different laboratories has resulted in a division into six groups. Some of these groups are heterogeneous. The name Leuconostoc cremoris is used in preference to Leuconostoc citrovorum for the non-sucrose fermenting species. Reasons are given. Other arguments lead to the conclusion that Streptococcus kefir (Evans) should be regarded as a synonym of Leuconostoc mesenteroides, and the name Leuconostoc lactis is used in preference to Streptococcus kefir (Abd-el-Malek & Gibson).

The effect of a switch level of 200 or 400 g/min for automatic cluster removers on milking performance and udder health was measured with 71 first lactation cows in their first 36 weeks of lactation and with 64 older cows in their first 12 weeks of lactation. Early removal of the milking unit decreased machine-on time by 0·5 min, increased average milk flow rate slightly, improved teat condition significantly and reduced the change in teat end thickness during milking of first lactation cows. Early removal of the milking unit did not affect milk yield or composition, and the incidence and prevalence of subclinical mastitis were the same in the two groups. Fewer older cows developed clinical mastitis in the group switched at 400 g/min, but this was not significant. It is concluded that the milking unit can be detached at a milk flow rate of 400 instead of 200 g/min without having a negative influence on milk yield. Machine-on time is shortened and teat condition improved, and udder health does not seem to be affected.

The influence of pH, temperature and Ca depletion on bovine casein micelle suspensions in D2O containing simulated milk ultrafiltrate was studied by 1H-NMR spectroscopy.

In the pH range of 5·8–7·5 the spectrum of the micelles showed very little pH dependence, indicating that no changes occurred in the dynamic behaviour of the proteins constituting the micelle.

The NMR spectrum of casein micelles was strongly temperature dependent, particularly in the temperature range of 60–98 °C. Increase in temperature resulted in a strong increase in spectral intensity concomitant with changes in the spectral characteristics. In micelle suspensions these changes were reversible, and indicated that at clevated temperatures the rigid structure of the casein micelle started to melt, leading to an increased mobility of appreciable parts of the proteins in the micelle.

Ca depletion of the casein micelles by addition of EDTA resulted in an increase in spectral intensity, which arose from the presence of casein components in the serum phase, The spectrum of these serum phase particlcs resembled closely the spectrum of a solution of total casein in simulated milk ultrafiltrate and was quite different from the spectrum of casein micelles.

The implications of these results with respect to models of the structure of bovine casein micelles are discussed.

The odour impact compounds of raw, pasteurized and UHT bovine milk were investigated using vacuum extraction and extract dilution sniffing analysis using CharmAnalysis™. Fifteen odour peaks with Charm values between 10 and 3443 were detected. Of these peaks, twelve were identified as hexanal, ethyl butanoate, 2-heptanone, heptanal, dimethyl sulphone, l-octen-3-ol, ethyl hexanoate, 2-nonanone, nonanal, benzothiazole, 2-undecanone, indole and one as a mixture of 2-tridecanone and δ-decalactone. Dimethyl sulphone, indole and one unknown compound (retention index 1154) were the only ones detected as odour impact compounds in all three types of milk. Ethyl butanoate and ethyl hexanoate were found as powerful odorants only in raw milk. A further predominant odorant for this milk was dimethyl sulphone, which was the most important odorant in pasteurized milk. 2-Hexanone and 2-nonanone were identified as the most intense volatile flavour compounds of UHT milk. The powerful odorants described can be used as indicators of the aroma quality of heated milk.

Two feeding systems trials were carried out to determine the antioxidant activity of soft goats' milk cheeses, and also to evaluate the presence of bioactive polyphenolic compounds. Two groups (A and B), each one with 20 (BW 50±5 kg) French Alpine goats were employed. All animals had between 70 to 80 milking days and were milked once a day. Group A included daily grazing on shruby rangeland vegetation, and group B was kept in full indoor confinement. Thirty kg of goats' milk from each group were collected; 15 kg from each were pasteurized and the remaining 15 kg were not pasteurized (raw), resulting in four treatments (two feeding system×two milk treatments). Soft goat's cheese antioxidant activity can be modified by the animals feeding system; grazing management represents a better option than indoor feeding to produce a healthy profile of bioactive compounds; providing an increase of total polyphenol, hydroxycinnamic acids and flavonoid concentrations. Pasteurization did have a significant effect on these metabolites, and diminished total polyphenol concentration. More research is needed to elucidate the potential of soft goat's cheese as a functional food.