Individual quarter milk samples were taken from 719 cows in 7 dairy herds and examined for mastitis-causing pathogens. Analyses were made to ascertain the prevalence of mastitis infection when the results from a single quarter were compared with those from all 4 quarters. There was a very close similarity between the 2 methods of assessment. When the number of samples is limited, sampling of a single quarter instead of all 4 quarters permits more cows to be examined.

An experiment lasting 42 d was performed in 4 consecutive stages on 6 healthy Friesian cows during mid-lactation. Mean values for the different components of mammary secretion during normal lactation were established. Milking was then suspended on all quarters from d 1–14. The mean values for lactate increased 20- to 30-fold over the mean value for normal lactation. Over the same period the leucocyte count (polymorphonuclear leucocytes and lymphocytes) also increased, whereas the mean values for glucose and the estimated redox potential decreased. From d 15–28 milking was resumed on one half of the udder and from d 29–42 milking was also resumed on the other udder-half. When milking was resumed the above-mentioned changes were reversed, taking 5–7 d to reach values obtained during normal lactation in the udder-half which had not been milked for 14 d, but at least 14 d in the udderhalf which had not been milked for 28 d. The changes in the levels of lactate, glucose, leucocyte count and estimated redox potential are discussed in relation to changes in the secretory activity of the mammary gland (lactose, β-lactoglobulin and epithelial cell count), permeability changes of the mammary epithelium (Na+, K+ and serum albumin) and the immune defence mechanism in the udder (immunoglobulins). The results indicate that lactate is formed during anaerobic glycolysis by the leucocytes in the mammary secretion, most probably due to reduced blood flow to the udder and the accumulation of secretion in the gland and teat cisterns on cessation of milking.

A method for the quantitative analysis of casein mixtures is described. The method uses ion-exchange chromatography at 4 °C on DEAE cellulose, Whatman DE52 in the presence of tris-chloride-urea buffer (pH 8·6), and a NaCl gradient to fractionate the alkylated casein mixtures and a micro-biuret technique to determine protein concentrations. Values for fractions representative of (1) γ-caseins, (2) κ-casein plus some unknown proteins, (3) β-casein, (4) minor αs-caseins and (5) αs1- plus αs0-caseins were obtained, and whole casein from herd bulk milk was found to consist on average of about 2, 13, 36, 11 and 38% respectively of these fractions. Agreement between duplicate fractionations was satisfactory and the recovery of material varied between 94 and 98%.

Progressive changes in the concentrations of milk protein components were followed after infusions of Streptococcus agalactiae or bacterial endotoxin into different quarters of individual cows. Both types of infusion produced similar increases in somatic cell count and resulted in similar changes in milk proteins, although the effects of the endotoxin infusion lasted for a shorter length of time.

The treatments had little effect on α-lactalbumin and β-lactoglobin concentrations, but serum albumin and immunoglobulin (Ig) concentrations increased markedly. The greatest effect on serum albumin was after the endotoxin infusion and on Ig after the Str. agalactiae infusion. Changes in the individual globulins indicated that passive transfer of blood proteins to milk could not account for the observed increases in IgM and IgA.

αs1-Casein and β-casein concentrations were reduced and inversely related to somatic cell count during the immediate post-infusion period, and this was accompanied by an increase in para-κ-casein. Para-κ-casein was not detected in pre-infusion or post-recovery milk samples. The decrease in β-casein was greater than that of αs1-casein. Casein concentrations returned to pre-infusion levels 2 d and 5 d after the endotoxin and Str. agalactiae infusions respectively.

The possibility that proteolytic enzymes are partly responsible for the changes in casein concentration is discussed.

This investigation was a first attempt to characterize the kinetics of the heat aggregation of the caseinate micelles in milk. The aggregation reaction subsequent to the beginning of visible aggregation in milk heated at temperatures in the range 110–140 °C appeared to be second order, with an activation energy of about 30 kcal/mol (124 kJ/mol). The kinetic parameters of the aggregation reaction were not significantly altered by the removal of serum proteins or of colloidal calcium phosphate, or by the inactivation of β-lactoglobulin.

Additions of urea progressively increased the heat stability of milk outside of its coagulation time (CT)–pH minimum. In the region of the CT–pH minimum larger amounts of urea were required before an increase in heat stability occurred. The effect of urea was observed over the temperature range 125–140 °Cfornaturalmilk, milk which had been dialysed against synthetic sera, and milk to which a sulphydrylblocking agent had been added. Urea additions did not affect the activation energy of the heat coagulation reaction or the stability of milk to rennet or ethanol.

Casein samples from untreated milk and stored ultra-heat-treated (UHT) milk were hydrolysed with pronase (protease ex Streptomyces griseus K-1) and subsequently with a mixture of peptidases prepared from the microsomal fraction of hog kidneys. Incubation of casein from unheated milk with pronase alone hydrolysed 70–80% of peptide bonds involving Ile, Leu, Tyr, Phe and His residues; other amino acids were released less well and proline hardly at all. The pronase/peptidase treatment resulted in 90–100% hydrolysis of peptide bonds involving all amino acids, including proline.

Caseins from stored UHT milks were more resistant to proteolysis than casein from unheated milk. Reduced release of all amino acids was observed from samples taken after storage at 37 °C for 12 months or longer and for Lys, Arg and Asn residues from samples taken after storage at 30 °C for 14 months. Resistance to proteolysis was attributed to the Maillard reaction between milk proteins and lactose during storage of UHT milk.

Lactuloselysine (ε-N-deoxylactulosyl-L-lysine) and fructoselysine (ε-N-deoxyfructosyl-L-lysine), formed by Maillard reaction, were identified in enzymic hydrolysates of casein from stored ultra-heat-treated (UHT) milk. Furosine (ε-N-(2-furoylmethyl-L-lysine) and pyridosine (ε-(3-hydroxy-4-oxo-6-methyl-1-pyridinyl)-L-norleucine) were identified in the corresponding acid hydrolysates. Enzymic hydrolysis of the caseins, gel filtration and preparative scale amino acid analysis were used to isolate the compounds which were then identified by reference to synthesized authentic compounds. Lactuloselysine formation was extensive and involved 10–30% of lysine residues in UHT milks stored at 30–37 °C for 6 months to 3 years. Fructoselysine concentration was generally about 10% of the lactuloselysine concentration.

Four chemical methods were used in a comparison of the measurement of the extent of modification of lysine residues in casein during storage of ultra-heat-treated (UHT) milk. Determination of homoarginine after reaction with o-methylisourea appeared to be the best measure of total lysine modification. Reduction with borohydride before acid hydrolysis prevented recovery of lysine from lactuloselysine (ε-N-deoxylactulosyl-L-lysine) and fructoselysine (ε-N-deoxyfructosyl-L-lysine). This procedure gave values for these derivatives which were in good agreement with those obtained from the determination of furosine (ε-N-(2-furoylmethyl)-L-lysine) in acid hydrolysates. Determination of lysine after acid hydrolysis showed that lysine loss was accounted for by lactuloselysine and fructoselysine formation in most of the samples.

Browning was most apparent in UHT milk stored at 37 °C for 3 years and in this sample only part of the lysine residues was accounted for as lactuloselysine and fructoselysine, thus indicating that lysine had become involved further in sugar degradation products.

Changes in the appearance of casein micelles and fat globules in ultra-heat-treated (UHT) milk were comparatively slight within a 14-month storage period although closer inspection often revealed the beginnings of some of the changes observed after even longer storage. Within this period the milks remained liquid and no evidence for any proteolytic breakdown was seen on electrophoretio examination. After 34 months the milks had all gelled and there was evidence of a small amount of proteolytic breakdown. Electron microscopic changes were striking and depended on storage temperature. At 4 °C the casein micelles in the gel phase had become very ‘spiky’ and long tendrils often bridged micelles, forming extensive networks. At room temperature and at 30 °C this tendril formation was less marked and the average micelle size was considerably greater than in freshly processed milk. There was also clear evidence for the coalescence of micelles. After storage at 37 °C the micelles were much larger than in the other samples, often exceeding 0·4 mμ in diam.

Changes in the dependent variables flow velocity, inlet pressure and temperature affected the rate of concentration of separated milk by reverse osmosis; their values were measured for minimum fouling at the membrane, resulting in maximum rates of concentration. Most of the deposit formed at an early stage of concentration and its extent depended on the operational variables.

Electron micrographs showed that the major component of the deposit was casein micelles linked by bridges to form a lattice. Chemical analysis of the deposit confirmed a high casein content and showed that Ca phosphate was precipitated in the deposit throughout concentration. When fat globules and fat globule membrane were present in the feed, they appeared to be caught up in the deposit, but did not affect its initial formation.

Of 8 strains of Streptococcus cremoris tested, 5 grew almost as well in defined media in which various essential amino acids were supplied in dipeptides as they did in media containing the equivalent free amino acids. The remainder grew poorly or not at all in the peptide-containing media. Growth of peptide-utilizing strains was inhibited by also including structurally-related dipeptides in the medium, presumably due to competition for uptake by transport system carriers. Both types of starters produced cell-free dipeptidases recoverable from the medium of exponential phase cultures. Addition of the partly-purified extracellular dipeptidases to dipeptidecontaining test media initiated growth in strains unable to use peptides. Str. lactis grew in defined peptide media, but the further addition of structurally-related dipeptides did not inhibit growth, either bcause each dipeptide was transported by a specific carrier or because peptides were hydrolysed extracellularly. The presence of cell-bound extracellular dipeptidase was indicated by the hydrolysis of dipeptides with washed whole cells in buffer. This was not observed with Str. cremoris strains.

Porcine and bovine pepsins were very stable in 5·0 M-urea at pH 2·0 or 2·5 at 30 °C while chymosin and Mucor miehei and M. pusillus proteases were rapidly denatured under these conditions. These differences may provide the basis of a quantitative assay of the complement of individual milk clotting enzymes in a coagulant and a possible scheme is suggested.

Levels of Cu and Fe in fresh milk and in bulk pasteurized milk were measured. The endogenous levels varied during the lactation period. The level of Cu in bulk pasteurized milk was greater than the endogenous level, suggesting some contamination. This apparent Cu contamination was greater in winter than in summer. No similar contamination was observed with Fe. The levels of Cu and Fe in butter manufactured each month and in retail butter were measured. Higher levels of Cu were detected in ripened cream butter produced in winter compared to that produced in summer. No similar trend was evident for Fe in ripened cream butter. Very low levels of Cu, Fe and lipid oxidation were detected in Irish sweet cream butter and consequently Cu and Fe contamination and resultant oxidation are not a problem in this product.

Porcine pepsin was rapidly denatured in phosphate buffers, pH 6·4–6·7, in the temperature range 31–39 °C and was only slightly more stable in milk under similar conditions. However, the enzyme was considerably more stable in Cheddar cheese curd in which the extent of denaturation was very markedly influenced by the pH of the milk at setting. Under normal cheese-making conditions, porcine pepsin was about equally stable with chymosin. Two modifications of the cheesemanufacturing procedure were developed which permit the manufacture of cheese almost free of coagulant and suitable for the assessment of the contribution of starter proteinases to proteolysis during cheese ripening.