Deposit formation from the milk of 2 individual cows was measured throughout lactation, using the hot-wire laboratory apparatus previously described. The amount of deposit fell slightly to a minimum at about the tenth week of lactation, and then rose to a maximum towards the end of lactation. The ratio of the maximum to the minimum was about 2:1. No correlation was found between amount of deposit and the fat, solids-not-fat, ash and calcium contents of the milks. The presence of mastitis in 1 cow and the administration to it of antibiotics did not affect the amount of deposit formed from its milk.

The dephosphorylation of β-casein, type A1, by dilute aqueous solutions of sodium hydroxide has been studied. It has been found that the mechanism whereby inorganic phosphate is liberated by the action of alkali is one of β-elimination from O-phosphoserine residues which are themselves converted initially to dehydroalanyl residues.

An investigation was made into the effect of subclinical mastitis on the accuracy of the Amido Black method for measuring the crude-protein percentage of the milk of individual cows. Values obtained by the dye-binding and Kjeldahl methods on a sample of milk from each quarter of 46 cows, 11 of which were sampled at regular intervals during lactation, were compared. It was concluded that, even in cases of severe subclinical mastitis, the effect is so small as to constitute no objection to the use of the dye-binding method. The accuracy of the Amido Black method, irrespective of disease, was less in milk produced in the first and last months of lactation.

The effects of feeding lactating cows a daily supplement of 1 kg formaldehyde-treated casein–safflower oil (1:1, w/w) on the milk composition are reported. Three breeds, Jersey, Sahiwal and Sahiwal × Jersey crossbred were used.

The formaldehyde treatment not only eliminated the suppressing effect on the milk, fat and protein yields and on the fat and protein contents usually encountered when such a level of unsaturated oil is fed to lactating cows, but also resulted, on the average, in 15 and 6% increases in the fat and protein contents respectively.

The formaldehyde-treated dietary supplement also caused a decline of 5% in the solids-not-fat content, presumably due to a decrease in the lactose content.

It is suggested that the formaldehyde treatment protects the casein from the ruminal proteolysis and this in turn prevents the encapsulated unsaturated oil from biohydrogenation and eliminates the diverse effects on rumen microflora and environment.

This study reports the effect of feeding formaldehyde-treated or untreated spray-dried casein–safflower oil (1:1, w/w) particles on the fatty-acid composition of milk and plasma lipids of cows.

When the formaldehyde-treated particles were given to Jersey, Sahiwal and Jersey × Sahiwal crossbred cows (1 kg per cow per day) the linoleic acid (18:2) present in the safflower oil was not hydrogenated by the rumen micro-organisms and was incorporated into milk fat.

When the untreated supplement was fed, however, the 18:2 fatty acid was hydrogenated in the rumen and there was an increased proportion of octadecenoic acid (18:1) in the milk fat.

The increased proportions of 18:2 (treated supplement) and 18:1 (untreated supplement) were associated with decreased proportions of palmitic (16:0) and myristic (14:0) acids in the milk fat. All 3 breeds showed similar responses.

The fatty-acid composition of plasma triglycerides, which are an important source of long-chain fatty acids for mammary-gland lipogenesis, was similarly affected. Furthermore, there were significant changes in the fatty-acid composition of other plasma lipids (e.g. phospholipids and cholesteryl esters). The interrelationships between dietary, plasma and milk fatty-acid compositions are discussed.

Domiati cheese from homogenized and unhomogenized pasteurized milk was pickled for 4 months at room temperature. During pickling, the soluble tyrosine and tryptophan contents of the cheese gradually increased, indicating progressive proteolysis.

Gel filtration of cheese extract on Sephadex G-25 revealed the presence of nitrogenous substances of various molecular weights. The maximum formation of amino acids and related substances was observed after 15 days storage.

The changes in the electrophoretic patterns of proteins in the cheese during pickling indicated that both αs - and β-caseins were attacked. β-Casein, however, was much less affected.

Homogenization was found to affect the formation of soluble nitrogenous substances, but was without effect on the electrophoretic pattern of the protein in the cheese.

Changes in the microflora and titratable acidity of bovine colostrum stored at 4 and 30 °C were monitored for 21 days. Colostrum did not inhibit the indigenous microflora, and increased titratable acidity may have accounted for the observed decrease in the numbers of Enterobacteriaceae after an initial rapid growth period.

Two chemical tests were used to measure the extent of lactose degradation by Maillard browning in casein and skim-milk powder during storage. At the same time these products were evaluated organoleptically. Highly significant correlations between age, flavour score and level of browning in skim-milk powder were in contrast to the lack of any relationship between the flavour score or age of casein and its level of browning. Maillard browning occurring during storage of skim-milk powder was accompanied by increasing levels of cereal and malty flavours. A stale flavour, which closely resembled the musty flavour common to casein, was not observed in skim-milk powder until after storage for 9 months. A distinct difference in the flavour stability of rennet casein and acid casein could not be accounted for in terms of the relative extent of lactose degradation. Evaluation of casein samples with different lactose contents suggested that Maillard browning does not contribute to flavour defects during storage unless the curd is so insufficiently washed during manufacture as to produce an abnormally high level of lactose in the dried product.

A lactating mammary gland of a goat was perfused for 9 h in the presence of [U-14C]-l-threonine and received adequate quantities of glucose, acetate and amino acids. Two lactating sheep udders were likewise perfused in the presence of [U-14C]-l-phenylalanine: the plasma levels of phenylalanine in the first of these experiments were 4 times higher than in the second.

In the [14C]threonine experiment, 4 % of the casein and 0·4 % of the expired CO2 were derived from threonine; 85 % of the threonine and 1·6 % of the glycine residues in casein originated from plasma threonine. Small 14C levels were found in glutamic acid, aspartic acid and serine of casein. The relative specific activities amongst the casein amino acids and the appearance of appreciable labelling in plasma glycine are consistent with the view that threonine is split by threonine aldolase.

In the [14C]phenylalanine experiments virtually no radioactivity was detected in CO2, lactose or citric acid, indicating that this substrate is not broken down by mammary tissue. In the second experiment, 96 % of the phenylalanine and 0·3 % of the tyrosine of casein originated from plasma phenylalanine. In the first experiment, a 30-fold higher 14C incorporation into casein tyrosine relative to phenylalanine was observed. The possible significance of the phenylalanine concentration in the plasma on the degree of conversion of phenylalanine into tyrosine within the mammary gland is discussed.

The total solids (TS) content of milk was estimated using freeze-drying followed by a Karl Fischer (KF) titration. The average solids-not-fat (SNF) value for 12 milks calculated from the TS results by this method was 0·003±0·039 % lower than that calculated from the TS results by the British Standard (1963) gravimetric method (BS). The average SNF value calculated from the sum of the contents of lactose anhydride, ‘total’ protein and ash was slightly higher (+0·017±0·068 %) than that obtained from the BS method.

Certain modifications of the BS method for TS gave erroneous results. Heating 1 or 2 g milk at an oven temperature of 100 °C for 5 h, however, gave results agreeing fairly closely with the BS values.

For lactic (starter) casein whey the average SNF content from freeze-drying and KF titration was 0·075 % higher than that obtained by following the BS procedure (for TS in milk). Average values for the sum of the SNF constituents (including lactic acid) were 0·08–0·11 % lower than those obtained from BS method.

In many of the values recorded in the literature there is an unexplained discrepancy between the SNF content calculated from the sum of the constituents and that calculated from the TS content estimated gravimetrically by an alternative British Standard (1951) method. The need for a uniform method for estimating TS is stressed.

Three enzymes were assessed as rennet substitutes for cheese-making. The bovine and chicken pepsins used were relatively crude extracts of bovine stomach mucosa and chicken proventriculae respectively; the swine pepsin was a partially purified commercial product. The ratios of milk-clotting activity to general proteolytic activity were high for rennet and bovine pepsin and low for swine and chicken pepsins. Both bovine mucosa and chicken stomach gave low milk-clotting activities compared with calf stomach. For all the enzymes the chemical reactions causing milk clotting appeared to be the same. The milk-clotting activity showed a decrease with increase in substrate pH for all the enzymes, although they were all still active at pH 6·81.

Duplicate cheeses were made from each of the swine, bovine and chicken pepsins, with rennet as a standard in each trial. The cheese-making process was similar with each enzyme, but differences appeared during ripening. The chicken-pepsin cheeses had poor body and weak Cheddar-cheese flavour, with many and intense off-flavours. The cheeses made with bovine and swine pepsins were only slightly inferior in quality and intensity of Cheddar-cheese flavour to the rennet cheeses. From a simulated cheese-making experiment it was concluded that 30–40 % of the added rennet, bovine pepsin and chicken pepsin was probably inactivated during the cheese-making process and that most or all of the swine pepsin was lost. These results provide an explanation for the variations observed in cheese ripening.

It was concluded that chicken pepsin would not prove a suitable rennet substitute for making Cheddar cheese because of the quality of the cheese produced, and that bovine pepsin would not prove suitable because of the cost of preparing a suitable extract. Swine pepsin would appear to be suitable if the ripening time were to be lengthened or if another enzyme were to be added to assist ripening; it is cheaper than rennet and other rennet substitutes.

The temperature coefficient of electrolytic conductivity of normal milk decreased with temperature from 0·0241 ± 0·0003/°C at 15 °C to 0·0173 ±0·0003/°C at 40 °C. The temperature coefficient for a sample of mastitic milk lay within this range.