Experiments were performed on 3 lactating goats, in which mammary arterial plasma amino-acid concentrations were elevated by the infusion of a solution of essential amino-acids into the carotid artery supplying a transplanted mammary gland. In 2 experiments there were marked elevations in the arterial concentrations of most essential amino acids, but in one case only did this result in significantly increased uptake of amino acids by the gland, the arterio-venous difference being significantly correlated with arterial concentration for all except one amino acid. In the experiment in which increased amino-acid uptake was observed, infusion also resulted in a significantly increased milk yield and increased milk protein yield. The results are discussed in relation to data from other laboratories and lead to the suggestion that milk protein synthesis may be limited by the availability of either methionine or tryptophan.

Three experiments were carried out on 2 lactating goats, in which mammary arterial plasma amino-acid concentrations were elevated by the infusion of a solution of non-essential amino acids into a carotid artery supplying a transplanted mammary gland. In a fourth experiment a solution of glucose was similarly infused. In some cases the increased arterial concentrations of amino acids resulted in their increased mammary uptake, and in a depression of glucose uptake. However, infusions of neither amino acids nor glucose resulted in increased milk protein yield. Infusion of [U-14C]glutamic acid in one experiment demonstrated gluconeogenesis from glutamate carbon within the mammary gland. The results are discussed in relation to data obtained in another laboratory, from which it has been claimed that non-essential amino acid supply may limit milk protein synthesis. The present results provide no confirmation for the claim.

The addition of small amounts of phosphate salts reduced the amount of deposit formed from heated milk, both in laboratory experiments and during the operation of a practical ultra-high-temperature heat-exchanger. Sodium pyrophosphate was much more effective than the other phosphates tested. Reasons are suggested for these observed effects.

Milk gels varying widely in firmness were examined by both scanning and transmission electron microscopy, and a close correlation was found between their ultrastructure and firmness. In gels containing 40 and 50% total solids (14 and 17% protein respectively), casein micelles appeared as individual entities linked by some bridging material. At 60% total solids (20% protein), the micelles were fused and offered a considerably higher resistance to a penetrometer probe. Likewise some additives to gels, such as ammonium persulphate and CaCl2, caused the micelles to fuse. These chemicals appreciably increased firmness of such gels. In gels containing hexametaphosphate, a known inhibitor of gelation, most of the casein micelles were disintegrated; these gels were soft.

Intracellular dipeptidase of Streptococcus cremoris NCDO 924 has been investigated as a marker enzyme for detecting the release of starter enzymes into Cheddar cheese during maturation. Dipeptidases were produced extra- and intracellularly by Str. cremoris in broth cultures, but the intracellular enzyme was distinguishable by its relatively high activity against L-alanylglycine at low temperatures. The presence of dipeptidase activity of intracellular origin was demonstrated in extracts of Cheddar cheese freed from whole starter cells, opaque material of high molecular weight and free amino acids by a combination of centrifugation and gel filtration on Sephadex G200. The enzyme was not detected in cheeses made without starter (δ-gluconic acid lactone cheeses). In fresh curd with a high viable count of starter the dipeptidase activity was low and mostly extracellular. As the count declined the proportion of intracellular enzyme activity increased, reaching a maximum when 90% of the starter cells had died. The total dipeptidase subsequently decreased until the level remained 30% higher than that of the original curd, and changed little on further ripening up to 120 d. There remained a significant proportion of extracellular dipeptidase activity throughout the maturation period studied.

Rennin acts on a specially sensitive bond in αs1 B-casein to produce a basic peptide containing residues 1–23 of the original protein. At pH 6·4 and 30°C, the action is specific and rapid, the kinetic constants being Km 4·5×10−4M, Kcat 3·8 s−1, and kcat/Km 0·85×104 s−1 M−1. Pepsin, and a protease impurity in the acid phosphatase from wheat germ, have a similar action.