The detailed chemical composition of 25 milks from different breeds of cow, sheep and goat were related to their properties of rennet clotting, coagulum development and syneresis at pH 6-4. Experiments in which concentrations of fat and whey proteins in milk were manipulated, and in which milks were homogenized at different pressures, were also carried out and the effects observed were related to the above processes.The composition of milks varied widely and many relations between concentrations of components could be related to their known modes of secretion from the alveolus or to their structural functions in the colloidal phase of milk. Rennet clotting was related to total Ca concentration and also to the proportions of

Supplements of protected soya-bean-tallow were incorporated at 0, 1-7, 3-0 or 5-0 kg/day into a hay-concentrate diet and fed ad libitum to Friesian cows during the first 13 weeks of lactation.Intakes of supplement during weeks 1-6 were 0,1 • 34, 2-25 and 3-18 kg/day and during weeks 7-13, 0, 1-64, 3-10 and 4-24 kg/day on each of the respective treatments.As the lipid supplement increased there was a reduced intake of hay and concentrates, giving rise to reduced intakes of total, protein, and particularly 'carbohydrate' (non-protein, non-fatty-acid) energy. Apparent digestibilities, particularly of fibre, decreased as fatty-acid intake increased. After correcting for endogenous losses, true digestibilities of organic matter, carbohydrate, protein, non-fatty-acid lipid, total energy, calcium, magnesium and phosphorus were unaffected by the amount of supplement given, whereas fibre digestibility was reduced by increasing the supplement. Fatty acid digestibility was little affected up to an intake of 1-4 kg/day but above this there was a sharp reduction.Yields of milk, milk energy, protein, fat and lactose were increased at low lipid supplementation but reduced at the highest supplementation when amounts of protein and carbohydrate digested were lowest.Lipid supplementation reduced concentrations of rumen total volatile fatty acids in association with reduced, proportions of butyrate and. increased proportions of propionate. Plasma ^-hydroxybutyrate and magnesium were reduced and plasma free fatty acids increased in cows given lipid supplemented diets. There were no differences between treatments in plasma glucose, lactate or calcium nor in live-weight changes.

1. Concentrations and compositions of liver, serum and milk lipids of cows were measured during 6 days' starvation and serum lipids during 60 days' re-feeding. 2. The concentration of free fatty acid in serum increased fivefold during starvation. 3. The content of total lipid in liver (g/100g of liver dry matter) doubled owing to a 20-fold increase in triglyceride, an eightfold increase in cholesterol ester, a three fold increase in free fatty acid and a 20% increase in cholesterol. There were no changes in the content or composition of liver phospholipids. 4. Starvation lowered the concentrations of total lipid, phospholipid and cholesterol ester of dextran sulphate-precipitable serum lipoproteins. Total lipid and cholesterol ester concentrations in lipoproteins of d greater than 1.055 and in lipoproteins not precipitable by dextran sulphate decreased from day 4 of the starvation period and during the first 20 days' re-feeding. 5. During starvation there were decreases in percentages of stearic acid and increases in oleic acid in serum free fatty acids and triglycerides and in liver neutral lipid. 6. Throughout starvation total milk lipid yield decreased, yields and percentages of C4-14 fatty acids decreased and percentages of C18 fatty acids increased. 7. It is suggested that accumulation of triglyceride in liver may be caused by increased uptake of plasma free fatty acids without corresponding increase in lipoprotein secretion.

I . Two experiments are described in which the effects of dietary fat on the synthesis of milk fat in the dairy cow have been investigated. In the first experiment a change-over design was used with two cows to study the effect of removing a mixed-oil supplement to a basal diet low in fat on the concentration and composition of plasma lipids and on the yield and composition of milk fat.2. Reducing the dietary fat intake from about 430 to 170 g/day caused falls in the concentrations in plasma of phospholipid and of free and esterified cholesterol and, in one cow, also of triglyceride. The changes in composition and concentration of the fatty acids in the plasma triglyceride fraction also reflected changes in dietary intake of fatty acids. No effect of dietary fat intake on the tots1 synthesis of milk fat was observed, but the composition of the milk fat reflected that of the dietary fat, and the yields of lauiic, myristic, stearic and oleic acids were decreased in association with a decreased dietary intake of these acids.3. I n the second experiment the effect of supplementing a basal diet low in fat with either coconut, red palm or groundnut oil on the composition and yield of milk fat was studied in four cows using a 4 x 4 Latin square design balanced with respect to residual effects.4. With each oil, increasing the dietary fat intake from about IOO to 400 g/day significantly increased the total yield of milk fat. Also, coconut and red palm oils significantly increased the fat content of milk, and groundnut oil the yield of milk.5 . W,ith the exception of linoleic and palmitic acids, the increased dietary intake of the major fatty acids characteristic of the various oil supplements led to increased yields of these acids in the milk.Tritium-labelled stearic acid administered orally, as either free acid or triglyceride, to lactating goats and cows appears in the blood plasma mainly in the triglycerides of the /3-lipoproteins and is then selectively removed from the blood by the udder and secreted in milk fat (Glascock, Duncombe & Reinius, 1956;Glascock, McWeeny & Smith, 1957;Glascock, Welch, Bishop, Davies, Wright & Noble, 1966). The intravenous infusion of tritium-labelled chylomicra into conscious lactating goats, and the perfusion of isolated goat mammary glands with similarly labelled chylomicra, have confirmed that the udder of the lactating ruminant rapidly removes, for the formation of milk fat, a large proportion of the chylomicra entering the circulation (Lascelles, Hardwick, Linzell & Mepham, 1964). Furthermore, there is a considerable difference in the arteriovenous concentrations of plasma triglycerides across the udder of lactating, but not of dry cows (Hartmann & Lascelles, 1964), and in the goat similar more detailed studies have shown that it is mainly stearic and palmitic acids in the triglycerides of the chylomicra and lipoproteins of density 1.005-1.019 which are taken up by the mammary gland (Barry, Bartley, Linzell & Robinson, 1963 Although there is, therefore, conclusive evidence that the m...

SUMMARYEffects of pH, added Ca, rennet concentration, temperature, concentration and dilution of milk, on rennet clotting and coagulum development have been investigated. Rennet clotting time (RCT) was decreased by reduced pH, by increased temperature and by increased concentrations of rennet or added Ca. Only extremes of concentration or dilution of milk increased RCT. Strength of milk coagulum at 1 h after RCT was increased by reduction in pH and by increased concentrations of added Ca. Increased temperatures decreased coagulum strength whereas increased rennet concentrations had little effect. A curvilinear relationship between total casein concentration and coagulum strength was found for Friesian and Jersey milks. Two phases of coagulum development after clotting were observed. Changes in pH particularly affected the first phase whereas changes in temperature particularly affected the second. Effects of rennet, casein and Ca were similar for both phases.