The production of milk, fat and protein of 37216 mothers of twins was compared with that of 1691 127 mothers of single calves of Black and White breeds of cattle recorded in England and Wales. At every parity, the mothers of twins produced more milk, fat and protein than the mothers of single calves. The proportion of twin-bearing cows retained increased with parity, from 0·67% at first calving to ∼ 4·0% at the tenth or eleventh calving. More mothers of twins achieved 305 d of lactation subsequently than mothers of single calves, although their inter-calving intervals were not generally longer.

Data from experiments in 55 commercial herds have been examined to study the patterns of Corynebacterium bovis infection. Such infections are most common in herds that are not subjected to regular control methods but their commercial significance appears to be small. In herds using teat disinfection and dry cow therapy levels of infection with this organism are low. There is no evidence from these data to suggest that C. bovis infections protect the udder against invasion by a major pathogen. On the contrary these infections do not persist when major pathogens invade.

Experiments are described in which lactating cows were exposed to Corynebacterium bovis either by dipping the teats in a suspension of the bacteria or by inoculating the bacteria into the teat duct or the teat sinus. All three methods readily led to ‘infection’ being established. The effect of these infections on somatic cell count was minor and no clinical mastitis resulted during the course of the experiments although some cases occurred subsequently. There was evidence that 44% of these infections were confined to the teat duct. Quarters excreting C. bovis in milk continued to do so during endotoxin-induced inflammation and showed a similar cellular response to that of uninfected quarters.

In addition to a control diet, lactating cows were offered saturated fatty acid mixtures in three forms, free acids, free triglycerides and protected triglycerides, i.e. triglyceride encapsulated within a protein matrix which was cross linked by exposure to formaldehyde. Relative to the control diet, all three supplements increased milk yield. However, only the free fatty acids gave rise to increased yields of the three major milk components. The free fat and the protected fat caused significant increases only in the lactose yield. The different effects of the supplements on the yield of milk fat are suggested to be due to the types of long chain acid reaching the mammary gland rather than to any change in rumen activity. Changes in the concentrations of the soluble multivalent ionic constituents of the milks were consistent with this conclusion.

The effects of κ-casein genetic variants, lactation number, pH and casein content on the renneting properties of individual milks were studied. Rennet coagulation time (RCT) and curd firmness were determined with the aid of a lactodynamograph and the results statistically analysed. Renneting properties were strongly related to κ-casein variants and lactation number, with κ-casein B and higher lactation numbers giving shorter RCT and increased curd firmness, the effects on curd firmness being more pronounced. The effect of κ-casein variant on RCT could be eliminated by reducing pH and adding CaCl2 to the milk samples. The effect on curd firmness was not altered by this treatment, however. The results show that the renneting properties of individual milks are to a considerable degree genetically controlled.

The detailed composition, pH, and the properties of rennet clotting, coagulum development and syneresis at pH 6·4 were estimated in milks from 4 Friesian cows during the first 9 weeks of lactation. The concentrations of casein, fat, citrate, Ca, Mg, Pi and Na decreased significantly during early lactation whilst the pH and concentration of lactose increased. Levels of whey protein and K and the casein:fat ratio were unaltered. Considerable variation occurred in the relative proportions of α-, β- and κ-casein. These changes in composition were associated with decreasing coagulum strength and an increased rate of whey drainage although rennet clotting time (RCT) did not follow a significant trend. RCT was positively related to levels of Na and whey protein but negatively correlated with Pi, K and lactose. There were significant positive correlations between coagulum strength and casein, fat, non-protein nitrogen, Pi, Ca, Mg and citrate. The correlation with α-casein was stronger than that with β- or κ-casein. Syneresis time was positively related to fat, Ca and coagulum strength, but negatively related to the concentration of lactose.

The action of rennet on homogenized milk was studied using turbidimetric and light scattering techniques, and compared with results obtained previously for skim milk. The time required for the onset of coagulation was shorter for homogenized milk than for skim milk. The rate of coagulation of fully renneted particles increased with increasing temperature, and with increasing Ca2+ concentration, but was only slightly influenced by changes in ionic strength. The von Smoluchowski rate constant for the coagulation reaction was two orders of magnitude smaller for homogenized milk than for skim milk. Results suggest that coagulation of homogenized milk is controlled in general by the same factors as skim milk, and that the reaction may be inhibited owing to a reduction in the amount of casein available for mutual interaction, rather than to disruption of the micelles on homogenization.

The use of a commercial laser Doppler electrophoresis apparatus to measure electrophoretic mobilities of milk constituents is demonstrated, and calculations of apparent ζ-potentials from these mobilities are given. Systems studied were: native and renneted casein micelles suspended in milk ultrafiltrate and different buffers containing Ca2+, particles from homogenized milk before and after renneting, fat globules, and homogenized fat/caseinate systems. The results confirmed previous estimates of the ζ-potentials of these materials, but also extended to cover the effect of changing the concentrations of Ca2+ and other salts on the electrokinetic properties of the particles. Increasing concentration of Ca2+ caused considerable decrease in the ζ-potentials of native and renneted casein micelles, and temperature increase caused the apparent ζ-potential to increase in both cases. Particles from homogenized milk showed ζ-potentials very similar to those of native casein micelles, and renneting of the homogenized milk gave behaviour similar to renneted casein micelles. The ζ-potential of the homogenized materials could be distinguished clearly from those of native fat globules. Likewise, milk fat which had been homogenized using whey protein showed distinct differences in its surface properties from particles based on caseinate or casein micelles.

The heat coagulation time (HCT) of concentrated whey-protein-free (WPF) milk measured at 120 and 130 °C was reduced to by addition of 5–20 mM-2-mercaptoethanol (ME). However, although the amount of soluble casein formed on heating was doubled by addition of ME, the shape of the HCT–pH profile was affected only slightly. The proportion of κ-casein in the soluble casein from heated concentrated WPF milk containing ME was very high, though it was somewhat lower than that of the soluble casein from heated concentrated WPF milk containing no ME. No solubilization of colloidal Ca phosphate was observed in either unheated or heated concentrated WPF milk on addition of ME. These facts suggest that ME probably promotes the formation of soluble casein with release of κ-casein from micelles on heating, thus destabilizing the casein micelles.

Milk with 1·5% fat was treated to increase its viscosity to that of whole milk on the assumption that the sensation produced in the mouth, its mouthfeel, would be improved. Selected treated 1·5% fat milks were assessed for similarity to whole milk in taste panel trials. Panelists were chosen because of their ability to distinguish between whole milk and that containing 1·5 % fat by taste in at least one of two tests. Only 34% of the persons screened were suitable. Throughout the trials, the proportion of panelists able to distinguish between the two milks served alone and in tea remained at about 50%, but rose to 80–90% for the milks in coffee. In general, addition of KOH to 1·5% fat milk to pH 7·4 and of 6 mM-citrate overcame about 75 and 50%, respectively, of the difference in mouthfeel between whole and 1·5% fat milk. Whole and 1·5% fat milks were easily distinguishable visually, but the appearance of the latter was improved by adjustment to pH 7·4.

Different concentrates from sheep's milk cheese whey, with up to 26·5% solids, were prepared in the laboratory by reverse osmosis, and their viscosities and heat capacities determined over the temperature range 5–80 °C. These products behaved like other liquid milk concentrates previously studied, so that similar analytical expressions could be used to predict these physical properties in cheese whey as a function of the temperature and solid content. A nomogram is also given for an easier computation of each property. The water in liquid dairy products and their concentrates up to 40% solid content appears to behave as pure, free water with respect to its partial contribution to the heat capacity of these products, its apparent heat capacity being 4·208±0·016 kJ. kg-1 °C-1.

The ability of 42 microorganisms isolated from cheese, including strains of yeasts, Geotrichum candidum, Arthrobacter, Moraxella spp., Brevibacterium linens and Staphylococcus saprophyticus spp. to produce phenethyl alcohol (PEA) from L-phenylalanine was studied. All the yeast strains produced labelled PEA from L-[U-14C]phenylalanine. Phenylpyruvic acid was detected as an intermediate of PEA production and CO2 was produced by decarboxylation of this acid. For five strains tested, the level of PEA which had accumulated in the culture at the end of exponential growth phase represented 39–52% of the L-Phe added. None of G. candidum strains nor bacterial isolates produced PEA from L-Phe. Strains of Moraxella spp. and four strains of the S. saprophyticus group produced phenylacetaldehyde. For three strains of Arthrobacter spp., five of B. linens and all the Moraxella, 14CO2 produced from uniformly labelled L-Phe represented more than one carbon atom of the L-Phe molecule, suggesting that Phe was catabolized beyond the stage of phenylacetic acid. Production and disappearance of PEA during Camembert cheese ripening is probably related to the metabolic activity of microorganisms present at the cheese surface.

Zabadi was manufactured using reconstituted skim milk powder prepared with different ratios of water and sweet whey and the organoleptic properties and chemical composition of the milks were measured. Sweet whey could be used successfully to replace up to 50% of the water added for reconstitution, whilst with 75 or 100% whey a sweet off flavour appeared. However, these higher levels could be used for fruit flavoured zabadi, and about 40–60% less sucrose could be used because of the additional glucose + galactose contributed by the whey.