The effects of stage of lactation and of age on milk and composition yield of cows kept under controlled conditions of feeding and management were investigated. Two groups each of 3 Friesian heifers were used: one group (A) was given a daily diet of 18 lb hay and concentrates at the rate of 3·8 lb/10 lb of milk and kept on experiment throughout the first 3 lactations; a second group (B) was given the same amount of hay and a constant daily amount of concentrates of 10·7 lb and was kept on experiment throughout the 1st lactation only.

The total yields of milk and peak yields for the 1st lactation were similar in both groups. In all the cows, fat content was high early and late in lactation and varied little throughout the mid-lactation period. With cows of group A, the content of solids-not-fat declined until about the 5th week of lactation and then remained steady until a marked rise occurred late in lactation. With cows of group B, however, the content of solids-not-fat was at a minimum between the 5th and 10th weeks of lactation and then increased progressively until late lactation when a more marked rise occurred. Lactose content increased rapidly in the first few days of lactation and then remained constant for several months, eventually declining late in lactation. Characteristic changes were also observed in the contents of total protein, and of the various protein fractions, and of the major minerals of milk.

The concentrations of all major constituents in milk secreted from the 2nd–5th months of lactation tended to decrease from lactation to lactation.

The results are discussed in relation to the development of knowledge of the changes in milk composition throughout lactation.

When crude к-casein was precipitated at pH 3 in the presence of 0·4m-NaCl the supernatant contained up to 80% of the total sialic acid but no detectable cystine or cysteine. Two fractions were obtained from this supernatant by chromatography on DEAE cellulose; one containing 4–6% sialic acid and the other only onetenth of this amount.

Most of the sialic acid of the sialic acid-rich fraction was soluble in 12% trichloracetic acid following treatment with rennin. It is suggested that the glycopeptide released by the action of the enzyme on casein originates from this fraction.

A simple, quick, batch-type method for the laboratory-scale HTST pasteurization of milk is described and details of construction, operation, cleaning and sterilization of the apparatus are given. The heating and cooling curves approximate closely to those of commercial HTST milk-pasteurizing plants and the end-product, as measured by residual phosphatase, is similar. The method has a relatively high sample throughput and is suitable for the routine pasteurization of large numbers of milk samples of up to 300 ml in volume. Other possible applications of the method are discussed.

Whole casein, β-casein, β-lactoglobulin and euglobulin were labelled with 131I. The conditions under which iodination was carried out were chosen so as to avoid any modification of the original characteristics of the proteins. This was checked by starch-gel electrophoresis and determination of sedimentation constants, apparent molecular weights and, for euglobulin, the clustering effect on fat globules.

As was shown by autoradiograms of the starch-gel plates, the radioactivity was incorporated in all zones of the electrophoresis pattern.

A laboratory method is described which it is hoped will be of value in the search for some of the chemical factors in milk which control the formation of deposits on heated surfaces during commercial processing. The method uses an electrically heated platinum wire as the surface on which deposits are formed under controlled heat transfer conditions. Preliminary results are given on the effect of milk pH on deposit formation, differences in the sensitivity to deposit formation of different bulk milk supplies, and the effect of forewarming on the reduction of deposition.

The occurrence of coagulase-positive staphylococci in 954 samples of raw or heat-treated market milk and in 910 samples of cheese of different varieties, taken at 40 cheese factories or farms in England and Wales was determined. Only 9% of the cheeses contained more than 500 000 staphylococci/g, and these were all made from raw milk. All but the mildest of the heat treatments used were effective in greatly reducing the numbers of staphylococci present in milk and usually resulted in cheese virtually free from staphylococci. The effect of sublethal heat treatment on multiplication of these organisms in cheese curd is discussed.

Similar, marked variations in the freezing-point depressions of jugularvenous blood and of milk throughout a day were observed in cows when drinkingwater was offered for a single, short period each day. Values for milk were found to agree more closely, however, with those for mammary-venous blood than with those for jugular-venous blood. It appears that milk is in osmotic equilibrium with the blood flowing through the udder continuously throughout the period the milk remains within the udder and not only during its formation, and that milk secretion causes a slight alteration in the osmotic pressure of fluids within the immediate locality of the mammary gland. Changes in the milk composition that occurred in association with the observed changes in freezing-point depression were consistent with a movement of water into or out of the udder in response to any change in the osmotic pressure of blood.

k-Casein and whole casein when photo-oxidized in the presence of methylene blue lose the ability to clot when treated with rennin. Two effects are involved—first the photo-oxidation alters the k-casein fraction so that the rennin is unable to split off the glycopeptide fragment, and secondly, in whole casein the photo-oxidation interferes with the aggregation in the presence of Ca++ that normally follows rennin action. As a result of amino acid analysis and specific treatments which affect other photo-oxidizable side chains, it is concluded that both of these effects are caused by the alteration of histidines.

A method is described by which the microbial flora of a raw milk sample was reconstituted on different occasions. After segregation on selective media into groups consisting of lactic acid bacteria, Gram-negative rods including coli-aerogenes, staphylococci, streptococci, and microbacteria, and propagation by replica plate techniques on optimal media, suspensions of these groups of organisms were then added in approximately the original proportions to 40 gal batches of heated milk of low bacterial count. When this was repeated on 18 different occasions over a period of 11 weeks, the milk flora was found to be almost identical with the original one.

Milk deposits from an ultra-high-temperature (UHT) plant were analysed for protein, fat and minerals. The physical appearance and composition of the deposits was found to vary with modifications of the plant. For each particular arrangement of the plant, a characteristic pattern of deposits occurred. Possible mechanisms involved in the formation of the deposits are discussed.

The β-lactoglobulin types of 812 individual cows of various breeds were determined by paper and polyacrylamide gel electrophoresis. The results support the hypothesis that β-lactoglobulins A and B are controlled genetically by codominant autosomal alleles (LgA and LgB).

Milks from 5 heterozygous cows (LgA/LgB) were examined by free electrophoresis and column chromatography to estimate the relative quantities of β-A and β-B produced by these animals. The results indicate that β-A and β-B are produced in approximately equal amounts, with β-A being slightly in excess of β-B.

Lacteal secretions were obtained from 2 castrated males and typed for β-lactoglobulin by means of paper electrophoresis. These animals were found to produce β-lactoglobulins (B and AB) consistent with their expected genotypes as deduced from pedigree analysis.