Freeze fractures of the tight junctions at the apices of sheep mammary secretory cells showed that the junction at 72 d of pregnancy was significantly wider than at any later stage. The number of ridges in the junction only increased significantly between 122 and 142 d of pregnancy. The initially even distribution of intramembrane particles across the tight junction changed gradually until in the fully lactating animal there were far more particles on the lateral surface of the plasmalemma. Possible correlations between these changes, the alterations in permeability of the mammary epithelium, and the differences in hormone levels are briefly discussed.

The relationship between nutrition, fertility and blood composition was studied in 400 cows from commercial farms. Fertility evaluated by overall conception rate (OCR) was shown to be significantly related to the relative proportion of concentrate and roughage feeds in the diet, and to its content of dry matter, digestible protein, Ca and P. The ratio of serum total lipids to glucose and a newly introduced parameter, ΔPi (a calculated value, obtained by subtracting whole blood levels of Pi from those of the blood serum), showed a highly significant relationship to OCR. No significant relationship was found between OCR and serum, total lipids, cholesterol, urea, Ca, Mg, organic and inorganic phosphate. The values and significance of the 2 blood parameters which showed a relationship to fertility are discussed.

Eight cows in mid lactation and receiving a diet of perennial ryegrass silage and barley (70:30 on a DM basis) were given intravenous supplements of l-methionine (8 g/d). The methionine treatment had no significant effect on milk yield, protein content or lactose content, but increased milk fat content and yield by approximately 10% (P < 0·05).

Experiments involving the depletion of milk calcium phosphate (MCP) with EDTA have shown that it has a Ca to phosphate ratio of 1·61±0·04, close to that of dental enamel. The calcium phosphate also contains small amounts of citrate (citrate/Pi = 0·097±0·011) and Mg (Mg/Pi = 0·044 ± 0·011). These experiments and model calculations indicate that the Ca binding capacity of casein is not reduced by mineralization with calcium phosphate and hence that ion binding sites and nucleation sites are distinct, or nearly so. Calculations of the ionic equilibria in the aqueous phase of milk show that there is an invariant ion activity product for a molecular formula close to that of dicalcium phosphate. It is suggested that MCP is a mixture of salts, with dicalcium phosphate a precursor of more basic salts such as octacalcium phosphate and hydroxyapatite.

The effects of feeding lipids protected against microbial degradation in the rumen, on the metabolism of β-carotene and cholesterol in the blood and milk of cows were studied. The diets fed to the cows consisted of a basal mixture of crushed oats and lucerne hay with a protected vitamin supplement containing α-tocopheryl acetate and β-carotene fed in conjunction with either (i) protected sunflower oil-seed rich in linoleic acid (PO), (ii) protected tallow (PT), or (iii) formaldehyde-treated casein (C) as a control. Diets PO and PT raised the concentrations of β-carotene and cholesterol in the blood plasma over that observed for diet C. Milk cholesterol concentrations were not affected by dietary supplements, but the level of β-carotene in milk of cows on diet PO showed a tendency to fall compared with milk from cows fed PT or C. The properties of the high density lipoprotein (HDLP) of the blood plasma which contained the β-carotene were affected by the PO diet. As a result of feeding this diet, the fatty-acid composition of the HDLP was altered and it emerged from a gel-filtration chromatographic column earlier than the control. This change in chromatographic behaviour was used as a measure of the effect of the diet, which, for some cows, was apparent long after the diet was changed. It is suggested that the altered lipid composition resulting from the PO diet affected the distribution of particle sizes of the HDLP and might interfere with the transfer of β-carotene from plasma to milk.

The influence of mastitis and early lactation, and the effect of treating milk with heparin, blood serum and trypsin, on the proportion of lipoprotein lipase (LPL) activity in milk serum was investigated. The relative importance of milk serum LPL and LPL bound to micellar casein in promoting lipolysis was also examined. Colostrum contained LPL activity, 45% of which was found in the serum phase in samples obtained from the first milking post partum, but this value fell to 34% in samples taken 24 h later. The proportion of serum LPL was also increased in milks from quarters infected with Staphylococcus aureus, but not after overnight treatment of normal milk at 4 °C with 5% (w/v) blood serum or 2 µg/ml trypsin. The addition of 5 µg/ml heparin resulted in a consistent increase in serum LPL which varied between 14 and 50% of total milk LPL. Heparin did not release all the enzyme bound to casein micelles even after a second heparin treatment of resuspended micelles. Serum LPL was more effective in promoting lipolysis and was more responsive to blood serum activation than LPL bound to casein micelles. Lipolysis increased after heparin treatment but the increase was not related to serum LPL activity.

A method capable of determining the interfacial tension (γ) of individual emulsified liquid drops has been investigated and applied to a study of the surface properties of milk fat globules. Measurements on several organic liquids in bulk against water were made to validate and standardize the technique. With one exception, results agreed well with literature values, but the reproducibility of measurements was not as good as that of other, standard, methods. Measurements performed on small drops were affected by several factors and precision was not high.

The influence of different milk treatments upon the interfacial tension of milk fat globules was studied. Agitation of milk had the effect of increasing γ while the effect was reversed upon allowing agitated milk to stand. Dilution of normal milk with water resulted in an increase in γ while acidification and alkalization produced a decrease. Pasteurization of raw milk caused a slight, non-significant decrease. Values of γ were low, 1–2 mN m-1, irrespective of the nature of the milk treatment and were an order of magnitude smaller than those obtained for interfaces of butter oil and milk protein solutions. The results support the concept of a 2-layer membrane and suggest that changes in γ follow mainly the adsorption/desorption of surface active components in the outer hydrophilic layer of the globule membrane. An inner, firmly bound lipophilic layer, resistant to many different milk treatments, appears to determine the overall low γ value under most conditions.

A sample of milk fat was fractionated into high, medium and low molecular weight triglycerides by silicic acid column chromatography. Each fraction was further separated by silver ion adsorption thin-layer chromatography into triglyceride classes which were subjected to stereospecific analysis. Trans-monoene fatty acids were incorporated into triglycerides in the same manner as their cis-isomer. The specific positional distribution for fatty acids in intact milk fat triglycerides was not always maintained in the molecular weight fractions or in the various classes of unsaturation. The significance of these findings is discussed in relation to biosynthetic mechanisms for milk fat synthesis.

Buffalo milk fat was first separated by thin layer chromatography (t.l.c.) into high (HMT), medium (MMT) and low (LMT) molecular weight triacylglycerols. The triacylglycerol (TG) fractions thus obtained were further resolved by argentation t.l.c. into saturated, trans-monoene, cis-monoene, diene and polyene TG species. Whereas 18:1 was concentrated in HMT, 6:0, 8:0 and 10:0 tended to be located in MMT. LMT was unique in containing almost all 4:0. HMT, MMT and LMT showed great variations in their levels of different TG species. The distribution of fatty acids among the TG species was found to be characteristic of each TG species.

The effect on the composition and properties of casein micelles of the binding of ionic materials which accelerate the coagulation of milk on rennet treatment, was investigated. When considered in terms of their relative charge concentration, all the materials tested caused similar effects. The casein, inorganic phosphate and Ca contents of the micelles increased slightly. Micelle hydration decreased as additive binding increased. Casein and Ca dissociation on cooling increased at low concentrations of bound material, then progressively decreased at higher concentrations. The mean size of micelles and their electrophoretic mobility was little affected by bound ionic materials. The aggregation of the casein complexes in colloidal calcium phosphate-free milk was markedly increased by adding ionic materials, the efficiencies of these additives paralleling their efficiencies in accelerating the coagulation of milk by rennet. The results suggested that the ionic materials were bound in the interior of the casein micelles and promoted aggregation after rennet treatment by shielding charged groups, thus increasing the micellar hydrophobicity.

The proportion of materials which adsorb to casein micelles and accelerate milk coagulation, and remain bound on dilution of the micelle suspension with milk dialysate was determined. This was higher than expected from the postulate that equilibration took place between the solution and many equivalent binding sites on the micelle. This suggests that binding involved either or both hydrophobic interactions and multipoint attachment to charged groups. The location of the additive binding sites in the micelle was investigated using models Na caseinate and hydroxyapatite at pH 6·6. Materials which accelerated coagulation bound to either or both models. Those with the greatest effect on milk coagulation probably bound primarily to the casein rather than to the colloidal calcium phosphate moiety of casein micelles.

The calcium-induced aggregation of αs1-casein at pH 7·0 has been studied in the presence of citrate and phosphate anions. The effects of the citrate can be reconciled with the suggestion that the citrate, acting as a Ca chelator, reduces the Ca available for the aggregation reaction, but in no other way interferes with the mechanism of that process. In the presence of phosphate, precipitation is induced below the normal critical Ca concentration. This reaction proceeds to completion at low and high concentrations of phosphate. There is, however, an intermediate range of phosphate concentrations, whose limits depend on the protein concentration, where a colloidally stable precipitate is produced. The significance of these observations is discussed in terms of the currently held theories of micellar structure and formation.

The heat stability of skim-milk powder varied throughout the season, but was highest during the period April–August. Addition of urea increased heat coagulation times during March–August, but the extent of increase was dependent upon urea addition either before or after preheating for 30 min at 90 °C during manufacture. During March–June, higher heat stabilities were recorded when urea was added after preheating, while in July–August urea added before preheating was more effective. Partial ultrafiltration before dehydration decreased the heat stability of reconstituted milks due to a loss of natural urea in the permeate together with a disruption in the salt balance.

Continuous measurements of curd-firming using a commercial vibratingreed viscometer and a pressure transmission system (PTS) developed at this Institute were compared both in the laboratory and during Cheddar cheesemaking. The coefficient of variation about the mean of the curd-firmness measured in reconstituted skim-milk 40 min after renneting was 3·5% with the viscometer and 2·6% with the PTS. The effects of changes in conditions on the curd-firming rate of raw whole milk were determined. As the rennet concentration was raised the rates increased linearly as measured by the viscometer, but non-linearly as measured by the PTS. Addition of CaCl2 increased the rates measured by both instruments rapidly at first then more slowly at higher levels. Raising the temperature increased the rate exponentially in the case of the viscometer but more slowly with the PTS. With both instruments the curd-firming rates of milk at pH 5·8 were 6·5 times those at pH 6·7 but the rates fell slightly at pH 5·6. In experimental cheesemaking, the mean readings of both instruments at the subjectively determined cutting time of the curd had the same coefficient of variation.

The commercial viscometer is probably more suitable for laboratory investigations of the earlier stages of the coagulation of milk by rennet whilst the PTS may prove the more useful in indicating the optimum cutting time of the curd during cheesemaking.

The development of typical flavour in Cheddar cheese was accelerated by the addition of bacterial neutral proteinase to the curds, but not by the addition of fungal acid proteinase which produced a bitter defect even at low concentrations. An optimum level of neutral proteinase addition was established at which general proteolysis was increased by about 20% in 2 months and treated cheeses of this age had flavour intensities typical of 4-month-old untreated cheeses. Enzyme-treated cheeses were softer-bodied and more brittle than untreated cheeses of the same age.

Yoghurts were made with single strain starter organisms Lactobacillus bulgaricus and Streptococcus thermophilus, L. bulgaricus being cultured in milk + 50 ppm Na formate or in autoclaved milk and Str. thermophilus in milk treated with the proteinase Maxazyme(R) or with added casein hydrolysate. They were assessed chemically and organoleptically and compared with conventional mixed starter yoghurt. The single starter yoghurts all had acceptable acidities and acetaldehyde levels and keeping quality was good. The coagulum of each was firm and when stirred no syneresis occurred; the yoghurts remained smooth and all had an excellent appearance.