A kinetic model of casein clotting caused in milk by rennet was defined by measuring changes in viscosity and in the complex modulus of rigidity. Three separate stages of the process were distinguished: the enzymic, the flocculation and the gelification. The kinetic equations and the main activation parameters were defined for each stage.

An experiment has been performed with 2 Jersey cows to determine the effect of a reduction in udder temperature on milk production. One half of the udder was covered while the other half was cooled by an increase in air movement at an air temperature of 6−10°C; for closely clipped udders it was estimated that the cooling conditions might be equivalent to an air temperature of −10°C with low air movement. Yields of milk, fat, protein and lactose were reduced significantly at the morning milking after 22 h cooling; the decreases which occurred at the afternoon milking after 9 h cooling, in all components except fat, were not significant. Wisconsin Mastitis Tests on the milk indicated that changes in udder health did not contribute to these effects.

Milk was added to a continuous fermenter to maintain a selected operating pH in the range 5·4–6·0, a fermentation system controlled in this way being called a pH-stat. Below pH 5·4 casein coagulated in the fermenter and on the pH electrodes. At pH 6·0 stirring inhibited the growth of the test culture, Streptococcus lactis C 10, but the inhibitory effect was reduced and the culture grew satisfactorily when the milk reservoir was sparged with 5% CO2 in N2 or with N2 alone. The dilution rate was highest near pH 6·0, which was close to the optimum pH for the growth of the organism. The rate of production of lactic acid and of bacterial cells in the fermenter was highest at pH 5·4.

A study of the hydrolysis of β-casein-B by crystalline rennin or rennet extract at pH 6·5, using a disk electrophoresis technique, showed that 3 bonds in β-casein are appreciably more sensitive than the others to rennin proteolysis, and that these bonds are probably located near the C-terminus of the protein. The most susceptible bond is hydrolysed, at 10°C, about 200 times faster than any other bond, whilst at 37°C it is hydrolysed 60 times faster. A study of the hydrolysis of this bond showed that its rate of hydrolysis at 37°C and pH 6·5 is decreased by either increased ionic strength or increased calcium ion concentration at constant ionic strength. Conformational changes in the substrate are probably responsible for these effects.

The specificity of the interaction between κ-casein, αs-casein and β-casein which forms the basis of micelle stabilization was studied by investigating the extent to which κ-casein could be replaced by other proteins. Of those tested, only gelatin replaced κ-casein and even it was only 2·5% (w/v) as effective and required a long pre-incubation period. The micelles formed by each of κ-casein and gelatin with αs-casein and Ca2+ were of a similar size to the casein—Ca complexes which compose natural micelles. Gelatin also formed complexes with αs- and with β-casein at 30°C in the absence of CaCl2. Evidence was obtained that the interactions between gelatin and the caseins had a much stronger ionic component than had those between κ-casein and the other caseins. It was concluded that the interactions between κ-casein and αs- and β-caseins which lead to micelle formation are highly specific and probably involve definite sites in each molecule.

The viscosity and adhesive strength of casein from the milk of 6 individual cows were determined on samples collected regularly over a period of 21 months. Three of the cows calved in autumn and 3 in spring. The viscosity of the casein from 5 of the cows showed a decline during lactation that was independent of the date of calving. It was therefore concluded that the decline in casein viscosity may be attributed to lactational rather than seasonal effects.

The feasibility of 2-stage continuous fermentation of skim-milk to produce sour curd cheese (quarg) has been established. The first stage was a stirred pH-stat fermenter, operated at pH 5·4, 5·6, 5·8 and 6·0, in which the operating pH was maintained by inflowing fresh milk. The remaining fermentation down to pH 4·7 was carried out by a procedure which simulated a second-stage plug flow fermenter (i.e. one in which forward velocity was as constant as possible at all points of the crosssection).

Skim-milk powder was dissolved to give 8, 10, 14 and 20% solids in the reconstituted milk. Maximum productivity of fermented milk solids was obtained with reconstituted milk containing 20% solids and operation of the first-stage fermenter at pH 5·4. This 2-stage continuous fermentation gave a total residence time of about 5 h, which represents a rate of production 4·8 times that obtainable in 24 h in a batch fermenter of the same fluid capacity filled once daily with the same reconstituted milk.

During 7 months' storage of heat-sterilized milk there were changes in the glycopeptides released by rennin (E.C. 3.4.4.3) action and soluble in 12% (w/v) trichloroacetic acid (TCA). The results showed a progressive decrease in the amount of carbohydrates attached to κ-casein. As these carbohydrates may contribute to the micelle-stabilizing power of κ-casein it was concluded that the loss of carbohydrates could be one factor in the development of gelation in heat-sterilized milks during storage.

The cationic detergent cetyltrimethylammonium bromide (CTAB), the anionic detergents sodium lauryl sulphate and sodium deoxycholate, and the nonionic detergents Tween 20, Tween 80 and Brij 35, and lecithin, starch, glycogen, chondroitin sulphate and polyethylene glycol, were tested for their ability to replace κ-casein in stabilizing αs- and β-caseins against precipitation by Ca2+. Of the materials tested, CTAB and Tween 20 stabilized both caseins and Brij 35 stabilized only αs-casein. Analytical ultracentrifugation of mixtures of CTAB with each casein and of Brij 35 with αs-casein indicated that both detergents acted by disrupting the casein aggregates and complexing with the monomers. Addition of CaCl2 did not aggregate the Brij 35–αs-casein complex. It is concluded that the basis of the stabilization phenomenon involves specific interactions between κ-casein and αs-casein or β-casein at mainly hydrophobic sites.

The effects of diets high in concentrates and low in roughage on the yield of milk and milk constituents were studied in 32 cows and heifers throughout lactation. A high level of concentrates in the ration caused an increase in milk yield, a decrease in fat percentage but not in total fat yield, and an increase in protein production.

Milk produced under such feeding conditions is capable of yielding greater quantities of curd than would be expected from its fat content, and should be evaluated accordingly. Feed requirements of the cows should be assessed not solely on the basis of the fat content of their milk but also in accordance with the protein content.

TWO levels of roughage, 3 FU (feed units) and 5 FU daily, were fed to Israeli?–Friesian cows and heifers receiving either a normal (Normal) or a high-energy (High) diet.

Milk yield and composition were examined for 12 weeks following parturition. The animals receiving 5 FU of roughage daily showed a higher milk yield during the first 8 weeks. With the Normal diet the amount of roughage had no effect on milk composition. With the High diet, milk yield was higher than with the Normal, and a depression in fat percentage was observed in the milk of cows and heifers receiving 3 FU daily. No corresponding fall in milk protein percentage was observed – the cows of this group (High-3 FU roughage) showing, in fact, a rise in protein percentage. No differences in milk total solids were found. With these high-energy low-roughage diets no correlation was apparent between milk fat and protein percentages.

(1) Two wether sheep fitted with ruminal and duodenal re-entrant cannulas were used to study the influence of the physical form of a barley grain and barley straw diet and intraruminal addition of ammonium salts (mainly acetate) on digestibility of dietary constituents, the flow of digesta to the duodenum and the composition of digesta from the rumen and duodenum.

(2). Grinding and pelleting of the diet depressed the digestibility of crude fibre and increased that of the nitrogen-free extract and addition of ammonium salts increased the digestibility of crude fibre. The effects of the physical form of the diet on the composition of the short-chain fatty acids of rumen liquor were not consistent and the addition of ammonium salts produced changes over and above those attributable to the small amounts of acids in the mixture. The extreme values observed for the molar proportion of propionic acid were 12·3 and 38·1% and the corresponding values for η-butyric acid were 28·1 and 9·0%.

(3). Variations in the flow of nitrogenous materials to the duodenum were related more to the pattern of fermentation established in the rumen than to the experimental treatments. There was a highly significant relationship between the molar proportion of propionic acid and the abomasal output of nitrogen and also the abomasal output of α-ε-diaminopimelic acid and α-linked glucose polymers. The amino acid composition of duodenal digesta differed from that of the diet—in particular the proportion of glutamic acid was decreased and the proportions of aspartic acid, alanine, lysine and histidine were increased—but differences in composition between treatments and between animals were small.

(4). The faecal output of nitrogen differed little between animals and between treatments, but nitrogen retention was significantly increased during the intraruminal infusion of ammonium salts.

Analytical electrophoresis of native and of rennet-treated casein micelles was carried out at 26 and at 5°C in a sucrose density gradient in a medium of the same ionic composition as milk. Under normal circumstances, the micelles were negatively charged and showed little heterogeneity in the electric field, the fastestmoving ones having at 26°C a mobility not more than 30% greater than that of the slowest. Native micelles were more highly charged at 5 than at 26°C; at each temperature, the mobility was approximately halved by rennet treatment. The results suggest that the effects of rennet treatment and of alterations in temperature on the tendency of the micelles to clot can be entirely explained as being caused by changes in charge.

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.

1. Proteolysis of β-casein by rennin and by pepsin was completely inhibited in the presence of 10% NaCl and was very significantly reduced by 5% NaCl. The rate of proteolysis of αs-casein was maximal in the presence of 5–10% NaCl. 2. The inhibitory effect of NaCl on the proteolysis of β-casein was independent of pH and incubation temperature. 3. The effectiveness of NaCl in controlling the development of bitter flavour in Cheddar cheese may be due to its inhibitory effect on the proteolysis of β-casein. 4. Rennin hydrolysates of β-casein were bitter in flavour whereas those of αs-casein were not.

Bloat in identical twin lactating cattle grazing clover-rich pastures was controlled by twice-daily drenching with 7 ml ‘ Pluronic’ L 64 in water. The drench did not alter milk composition and there was no significant difference in milk production between control and treated groups. A significant (P <0·05) linear relationship between bloat protection and butterfat production was found between twin sets. Drenching increased milk production in high-bloating cows but slightly depressed production in low-bloating animals.

An experiment lasting 4 years has been carried out with 4 groups (3 test and a control) of 10 sheep. Non-protein nitrogen (NPN) (as urea, ammonium sulphate or a mixture of the two) replaced 25% of the dietary nitrogen in the first 2 years and 50% in the second 2 years. The yields of milk, protein and fat were generally depressed in the test groups but not all of the differences reached significance. In the group receiving ammonium sulphate, the milk fat percentage was higher than in the control group in all years and the protein percentage was higher in the last 2 years. Contrary to results elsewhere, the yield of lactose was depressed in all test groups. In the urea group, the content of minerals and of calcium in the milk was significantly less than in the control groups. The total amount of NPN and its components (ammonia, urea and creatine) in milk were not affected by the substitution of NPN in the rations.

The type and distribution of esterases in milk has been investigated using selective inhibitors during normal assay procedures and during histochemical staining of polyacrylamide gels. Enzyme solutions were obtained from skim-milk by acid and alkali precipitation, followed by ammonium sulphate fractionation, ultra-filtration and Sephadex G-100 chromatography. The major type of esterase present was an aryl-esterase (E.C. 5.1.1.2) while a smaller amount of a choline-ester hydrolase (E.C. 3.1.1.7; 3.1.1.8) was detected. The significance of these findings is discussed.