Salivary and pancreatic lipases of the pre-ruminant calf have been studied using ion-exchange chromatography and gel filtration. In addition, pancreatic lipase has been fractionated using concanavalin A-affinity chromatography. The effects of 5,5′-dithiobis(2-nitrobenzoic acid), organic solvents and trypsin on pancreatic lipase have also been investigated. The effect of taurodeoxycholate on the lipolytic activity of the 2 lipases has been compared. Salivary lipase behaved as a single enzyme on ion-exchange chromatography, and gel filtration gave a mol. wt value of 52000 for the enzyme. Although pancreatic lipase appeared to be a single enzyme on gel filtration, with a mol. wt of almost 80000, the lipase was shown by ion-exchange and affinity chromatography to consist of at least 2 enzymes of mol. wts 72000 and 60000, and did not require colipase for maximum activity in the presence of high concentrations of bile salts. Colipase-dependent lipase, mol. wt about 45000, and probably amounting to not more than 10% of the total activity, was also present. This was the predominant form only after large losses in total lipolytic activity had occurred, as after treatment with a mixture of ether, ethanol and deoxycholate, or prolonged action of trypsin. When the concentration of taurodeoxycholate was increased from zero to 1 mM in a tributyrin substrate, the lipolytic activites of calf salivary and pancreatic lipases, and pig pancreatic lipase, increased. At a concentration of 4 mM-taurodeoxycholate, calf salivary lipase activity was higher, that of calf pancreatic lipase slightly lower and pig pancreatic lipase activity markedly lower.

A series of short duration experiments show that the way in which the milking machine cluster is removed from the udder influences the rate at which new udder infections occur when bacterial exposure is high at the end of milking. Shields fitted in the base of the liners controlled most of the infections caused by abrupt removal of the cluster.

During an outbreak of mastitis due to Streptococcus uberis the organism was frequently isolated from samples of straw bedding used by the cattle. More than one million Str. uberis/g straw were isolated from 36% of samples. The organism was also recovered from 34% of samples of bovine faeces, but usually in low numbers. An examination of straw samples collected from cattle yards on 32 other dairy farms demonstrated that this was not an isolated occurrence since Str. uberis was found in 24% of samples from 56% of the herds. No correlation was found between counts of Str. uberis and coliforms. The significance of these observations is discussed.

Swabs taken from the rectum and vulva of 14 dairy cows on 7 occasions at approximately weekly intervals revealed a pattern of isolations of Streptococcus uberis which were not random. Two of the 14 cows were responsible for 80 and 40% respectively of the swabs of the rectum and vulva which were found to be positive. It is suggested that these patterns are consistent with these 2 cows being intestinally colonized with Str. uberis. The implications of this work are discussed with reference to the sources of Str. uberis in the dairy herd.

Implantation of 1 μg Escherichia coli endotoxin into one papillary duct of a bovine udder frequently resulted in a mild inflammatory process in the treated quarter which was measured by use of the Wisconsin mastitis test (WMT) on quarter fore milk samples. Inflammation of a fore quarter was usually (18/24) accompanied by a transient elevation in WMT score from the homolateral hind quarter, but rarely (1/25) from the contralateral quarter. Inflammation in a hind quarter rarely (2/26) affected the homolateral fore quarter and never (0/25) the contralateral quarter. The pattern of responses suggests that absorption and initial removal of endotoxin from the mammary gland occurs via the lymphatic rather than the blood system.

Explants from lactating bovine mammary gland were cultured in vitro in serum-free medium through 1–9 d. Casein synthesis was determined by [32P] incorporation into newly synthesized Ca rennin precipitable fraction. High correlation (γ = 0·98) was found between incorporation of [32P] and [3H]amino acids in explants cultured under different hormonal regimes, thus indicating that post-translational phosphorylation is not a rate-limiting step in casein synthesis.

Hormonal effects on casein synthesis were studied by supplementing the incubating medium with insulin (I), prolactin (PRL), cortisol (F), thyroxine (T4) and triiodothyronine (T3). It was found that both PRL and I were required absolutely for maximal synthesis and almost maximal effect was achieved with 50 ng/ml. The effect of F was less clear, but some increase was achieved at the 200–1000 ng/ml range. T4 and T3 did not affect casein synthesis at a range of 10-11–10-7M while a significant inhibition was observed at 2 × 10-5 M. A time-course study of casein synthesis further substantiated the dominant role of PRL in maintenance or even elevation of the initial rate of casein synthesis in the explants, through the first 4 d of incubation.

Changes in the concentrations of various metabolites in mammary secretions (organic acids, UDP-hexoses and nucleotides) have been observed at the onset and cessation of lactation. All metabolites, with the exception of galactose, increased dramatically in concentration just before and immediately after parturition. Galactose concentrations decreased with the onset of lactation. After regular milking ceased, the concentrations of these metabolites in milk contained within the udder fell, but the rates of decline varied.

Oxidized flavour developed in whole milk only through the catalytic effect of either Cu or light. The O2 requirement for the 2 processes differed as did the characteristics of the off-flavours produced. Cu-induced oxidized flavour was described as ‘cardboardy’ and light-induced oxidized flavour was ‘painty’. Light-induced oxidized flavour increased in intensity with O2 loss, and could be prevented in stored milk by restricting access of O2. In UHT milk with a dissolved O2 content of 6·6 mg/1, and in the absence of access of further O2, light-induced oxidized flavour did not develop; similarly, O2 uptake of 7·5 mg/1 in in-bottle sterilized milk exposed to fluorescent light did not result in flavour formation. When light-induced oxidized flavour developed consistently in whole milk none developed in skim-milk, indicating the lipid source of the flavour. In contrast Cu-induced oxidized flavour development was not associated with high O2 uptake. Although nearly complete deoxygenation of whole pasteurized milk contaminated with Cu prevented the formation of the flavour, moderate deoxygenation resulted in even greater flavour intensity than non-deoxygenation. The 2 oxidized flavours also differed in relation to ascorbic acid (AA) oxidation. Light-induced oxidized flavour developed only after AA oxidation was complete, whereas Cu-induced flavour developed with AA still present. AA oxidation was greatly accelerated through the effects of both Cu and light. In milk free from Cu contamination and protected from light, after AA oxidation (plus SH group oxidation in the case of UHT milk) was complete, no further loss of O2 occurred, even during prolonged storage at 5°C, despite the presence of large O2 concentrations. However, at 20°C, a small consumption of O2 was measured, and this was associated with stale flavour.

The effect of limiting the amount of O2 present in commercially pasteurized milk on bacterial growth and on the keeping quality of the milk was investigated. No increase in bacterial count of laboratory pasteurized milk during storage for 9 d at 5 °C was observed and O2 consumption due to bacterial activity was low. In commercial pasteurized milk containing post-pasteurization contamination (PPC) growth of strict aerobes was encouraged by high O2 content while that of coliforms was less dependent on O2 level. The amounts of headspace (HS) O2 consumed in milk with PPC growth were large. Reducing the dissolved O2 content of commercial pasteurized milk stored in O2-impermeable containers without HS had little effect on bacterial activity and shelf-life.

The effects of concentrations of 10–30 mM/1 of CO2 on the growth of psychrotrophic milk spoilage organisms were examined, both in untreated whole milk and in pure cultures of Pseudomonas fluorescens growing in skim-milk and broth. Spoilage of refrigerated untreated whole milk was found to be inhibited by the addition of CO2 and the effect was enhanced with increasing concentrations of CO2 and decreasing storage temperature. Experiments with pure cultures of Ps. fluorescens showed that the inhibitory effects of CO2 were not due to increased acidity or to displacement of dissolved O2, but to the presence of CO2per se which induced an increase in the duration of the lag phase of growth and had only a small effect on the logarithmic phase.

The effect of chemical modification of milk protein on the ethanol (EtOH) stability of skim-milk has been investigated. Modification of lysine residues by reaction with anhydrides increased the EtOH stability whereas amidation of carboxyl groups destabilized the milk. Lysine modification also counteracted destabilization induced by Ca2+ addition. By using multiple linear regression techniques these latter effects can be accounted for in terms of the changes they bring about in micellar charge. These results provide a further verification and an extension of the previously proposed mechanism for EtOH-induced coagulation of skim-milk.

Caprine skim-milk exhibits markedly lower ethanol (EtOH) stability than bovine skim-milk but can still be characterized by a sigmoidal pH profile. As with bovine milk, the position of this profile along the pH-axis was found to be sensitive to available Ca levels. Manipulation of salt levels, either by serum interchange, addition or diminution did not result in any significant increase in the EtOH stability high pH asymptote, Smax, and reduction of the colloidal calcium phosphate of caprine milk also had no significant effect. EtOH stability/pH profiles similar to those of bovine milk were achieved only by chemical modification of the caprine milk protein by reaction with aldehydes and anhydrides. It is concluded that the low EtOH stability of caprine milk as compared with bovine milk is due to the different proportions of the individual caseins present, in particular the lack of an αsl-casein homologue in caprine milk.

Effects of pH, added Ca, rennet concentration, temperature, concentration and dilution of milk, on rennet clotting and coagulum development have been investigated. Rennet clotting time (RCT) was decreased by reduced pH, by increased temperature and by increased concentrations of rennet or added Ca. Only extremes of concentration or dilution of milk increased RCT. Strength of milk coagulum at 1 h after RCT was increased by reduction in pH and by increased concentrations of added Ca. Increased temperatures decreased coagulum strength whereas increased rennet concentrations had little effect. A curvilinear relationship between total casein concentration and coagulum strength was found for Friesian and Jersey milks. Two phases of coagulum development after clotting were observed. Changes in pH particularly affected the first phase whereas changes in temperature particularly affected the second. Effects of rennet, casein and Ca were similar for both phases.

The effect of aqueous algal extract, yeast extract casein peptone and meat peptone on acid production by 4 strains of lactic acid bacteria was tested manometrically using a milk–bicarbonate medium. A strain-dependent stimulation of the bacteria was detected after 4 h incubation. After fractionation of an aqueous extract of the green alga Scenedesmus obliquus on a Sephadex G25 column, 2 fractions with stimulatory activity were found using Streptococcus lactis as the test organism. One of the peaks of activity, confined to shortening of the lag phase, was due to hypoxanthine which, however, had only a slight stimulatory effect in conventional milk souring tests. The manometric method provides a sensitive and rapid test for detecting compounds with biological activity in µg amounts, but it should be accompanied by milk souring tests for interpretation of the type of stimulation involved.

The aspartyl proteinases of Penicillium caseicolum and P. roqueforti acted identically on β-casein; both enzymes split at least 3 bonds: Lys29–Ile30, Lys97–Val98 and Lys99–Glu100. From αsl-casein, these proteinases released 6 main degradation products which arose from the splitting of 4 bonds; P. roqueforti aspartyl proteinase was found to cleave 1 bond at a higher rate than P. caseicolum aspartyl proteinase. A hypothetical sequential hydrolysis mechanism of αsl-casein by these 2 enzymes is proposed from a study of the degradation by isoelectric focusing and by 2-dimensional electrophoresis.

An unfortunate error arose in our previous paper ‘Electrofocusing and twodimensional electrophoresis of bovine caseins’ by P. Trieu-Cuot & J.-C. Gripon, Journal of Dairy Research (1981) 48 303–310. A paragraph describing the 2-dimensional electrophoresis technique in ‘Materials and Methods’ (p. 304) should read as follows:

The running gel was a linear gradient gel obtained with 2 starting solutions: (i) 28% acrylamide.0·7% bisacrylamide,4·9 M-urea,0·675 M-Tris-HClpH 8·8,0·1% SDS, 6·0 % sucrose, 0·04% TEMED, 0·012% ammonium persulphate; (ii) 16% acrylamide, 0·4% bisacrylamide, 4·9 M-urea, 0·675 M-Tris-HCl pH 8·8,0·1% SDS, 0·04% TEMED, 0·012% ammonium persulphate.

Isoelectric focusing and 2-dimensional electrophoresis were used to study the development of the pH 4·6-insoluble fraction during Camembert cheese ripening. Modifications of this fraction were due mainly to the action of 5 proteinases: rennet (chymosin + bovine pepsin), plasmin and Penicillium caseicolum aspartyl-and metalloproteinases. Rennet was inactive on β-casein, but acted very early on αs1-casein. However, rennet and P. caseicolum aspartyl-proteinase had a very similar action on the latter substrate, which prevented clear definition of the respective actions of these proteinases on αs1-casein after 7 d of ripening. Plasmin action on β-casein was important from 21 and 35 d of ripening at the surface and in the centre of the cheese respectively, suggesting an important influence of pH changes during maturation. The respective activities of the metallo-and aspartyl-proteinases of P. caseicolum were characterized and followed using β-casein degradation products as markers. The metallo-proteinase activity was detectable immediately after the development of the Penicillium (7 d), while that of the aspartyl-proteinase was observed 3 d later. Thereafter, the amount of β-casein degradation peptides resulting from the metalloproteinase decreased while that resulting from the aspartyl-proteinase increased, suggesting a more important role of the latter enzyme.