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.

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.

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.

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.

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.

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.

Samples of liquid and solid fractions obtained by a commercial process from anhydrous milk fat of softening point (S.P.) 33·5–34°C had S.P. values of 22–23°C and 36–38°C respectively.

Determinations of fatty acid composition by GLC showed that unsaturated and short chain fatty acids were present in increased concentration in the liquid fraction (average 37·8 and 12·4% as compared with 35·1 and 10·8% in the original milk fat) and long chain saturated acids in the solid fraction (average 57·8 as compared with 53·8%). There was some concentration of carotene and vitamin A, and to a lesser extent of cholesterol, in the liquid fraction.

Crystallization and melting curves determined by a differential scanning calorimeter (DSC) showed that while the liquid fraction was completely melted at 25°C, the solid fraction contained an increased proportion of fat melting between 30 and 40°C. In addition to these high melting triglycerides the solid fraction also contained some 65% of fat in the liquid phase at the original crystallization temperature of 25°C.

Storage of aseptically packed UHT milk produced changes in the electrophoretic pattern of the milk caseins when the milk was stored at ambient or higher temperatures. Lower temperature storage at 4°C did not give rise to these changes.

The alterations in electrophoretic properties of the caseins appeared to be due to the action of carbonyl compounds, produced by a Maillard type of reaction, which led to changes in the charge of the protein together with some degree of polymerization. These conclusions have been drawn from results obtained on model systems of casein-lactose subjected to various heat treatments, and on casein and milk treated with acetaldehyde.

Changes in the sensitivity to calcium ions of individual caseins, whole casein and milk that had been subjected to various heat treatments or to treatment with acetaldehyde showed that all these different treatments gave rise to modified casein which, in general, became less sensitive to calcium. κ-Casein when treated alone rapidly lost its ability to protect αs1-casein from precipitation by calcium ions, while αs1-casein treated alone only gradually became more soluble in the presence of calcium. Thus, on treating whole casein there was evidence for a stability minimum when the protective ability of the κ-casein has been destroyed without a compensating gain in the stability of the αs1-casein.

The importance of these changes in relation to the stability of UHT milks has not yet been elucidated but the results indicate that cross linking between protein chains and changes in calcium sensitivity occur during long-term storage.

Between August and May-the butter-making season in New Zealand-the average vitamin Dcontent of the butterfat varied between 10 and 64 i.u./100g and during the corresponding months in Great Britain (February-November) the values were between 15 and 39 i.u./100g. At the beginning and end of the season the values in both countries were similarly low but in mid-season New Zealand values were up to twice those for Great Britain. Within each country the vitamin D values were highly correlated with hours of sunshine and, for both countries, with total radiation, which includes the diffuse radiation in ‘skyshine’ and also allows for variation in sunshine intensity. The higher content of vitamin D in New Zealand butter in mid-season was attributed to a higher level of total radiation.

A process for the removal of cationic fission products from milk is described in which no prior acidification is necessary. The milk is treated at 40°C by 2 beds of suitably charged cation exchange resins, one of which is of the carboxylic acid type and the other of the sulphonic acid type. On the first bed all the metallic cations of the milk are replaced by K+ and H+, and on the second the ioniccomposition is restored. During processing the pH is reduced from 6·8 to approximately 6·2 and is restored to 6·8 by the addition of alkali metal carbonates to the milk after treatment.

On a pilot plant of capacity 1600 1 (30 resin bed volumes) per 5-h day it was found that the radiochemical efficiency was approximately 80% for 85Sr and 97% for 137Cs.

The process caused some losses of vitamins, especially of thiamine, and an increase in the lead, copper and iron contents all of which, however, were at an acceptable level. Other changes in chemical composition were negligible and enzymic clotting time remained unchanged. Flavour and appearance of the product were satisfactory and it was satisfactorily both roller- and spray-dried. The process is discussed with reference to an earlier method in which prior acidification is necessary. It is concluded that the double-bed process is somewhat more efficient for the removal of fission products and that a full-scale plant would be little or no more expensive to erect and only slightly more expensive to operate.