Seven pairs of cows were chosen at parturition. In each pair, 1 member had had a twin pregnancy (T) and the other was a comparable cow with a single calf pregnancy (S). All cows were offered the same amount of feed over the first 28 weeks of lactation. In early lactation, intakes were equal; in mid-lactation the T group ate more. The T cows had a lower peak yield of milk than the S cows, but rates of decline in yield in mid-lactation were equal for the 2 groups. Lactose concentrations in the milk were equal throughout lactation, but the fat and protein concentrations were greater for the T group in early lactation. The T group yielded smaller amounts of milk solids. The T group lost less weight in early lactation and gained more weight subsequently. Three of the T group, but only 1 of the S group, did not conceive during the test lactation.

Half-udder comparisons were made using 72 and 59 cows in 2 experiments of 8 months duration to assess the incidence of pulsation failure, and its effects on teat condition and mastitis, as a consequence of milking with shortened teatcup liners. Diagonally opposite quarters of all cows were milked with control liners of 148 mm effective length or with the same liners shortened to an effective length of 110–130 mm by inserting a rigid sleeve in the lower barrel of the liner. Milking with shortened teatcup liners resulted in a high incidence of pulsation failure (defined for this experiment as failure of the liner to collapse fully beneath the teat in each pulsation cycle throughout milking). More than 50% of hind teats and 90% of front teats in the research herd penetrated deeply enough into the teatcup to prevent cyclic collapse of liners shortened to 130 mm. Such failure occurred with all teats in liners shortened to 125 mm for front teats or 110 mm for hind teats. The combined results for the 2 experiments showed that almost twice as many new mastitis infections occurred in quarters milked with the shortened liners (47 υ. 25, P < 0·01). This effect was confined to front quarters for which there was a 3-fold difference in the number of new infections (32 for the short liners υ. 11 for the controls, P < 0·001). In both lactations, significantly more teats had petechial haemorrhages when milked with the shortened liners and the electrical conductivity of milk from uninfected front quarters was higher. These results might reflect an interaction between pulsation failure and differences in the degree of overmilking that usually occur between front and hind quarters.

The surface tension (γ) of whole milk (4% fat) and skim-milk was determined in the temperature range 18–135 °C by the drop number method. Measurements were carried out in a temperature-controlled chamber under pressure. Above 90 °C, the mean residence time of the milk at the test temperature was 15 s. Surface tension values were calculated by the Harkins and Brown method, corrections being made to account for the kinetic energy of the drops. The results for whole and skim-milk were not significantly different and the combined data (66 values) could be represented by the equation:

γ =1·8 10-4θ2–0·163θ+55·6,

(where γ is in mN. m-1 and θ is the temperature in °C) with a residual s.d. of 2·1 mN. m-1 (mean relative error 9·6 % at the 5 % level of significance).

Nakai & Chi Le (1970) have described a rapid method for the determination of the N content of bovine milk based on the close correlation of absorbance at 280 nm with values for total N as determined by the Kjeldahl procedure. The applicability of the method to human milk samples has been investigated. The procedure is limited by: (1) the dependence of the relation between absorbance value and N content on both the mode of collection and the postnatal age of the donor, and (2) variable interference by non-protein components of the milk. In spite of these shortcomings, the method may be of value as a screening test for certain human milk banks.

The proteolysis of highly purified samples of αs1-, αs2-, β-and κ-caseins by porcine plasmin and by bovine plasminogen with urokinase has been examined principally by gel electrophoresis. The resulting peptide band patterns were compared with those of total proteose-peptone (TPP) samples prepared from fresh and stored raw and pasteurized milk, and also with those obtained during the natural course of proteolysis by indigenous enzymes in milk during storage. TPP was found to contain at least 38 components detectable by a single electrophoresis run. Apart from residual traces of whey proteins and intact caseins nearly all of these components were fragments of caseins produced by indigenous plasmin, with products from the breakdown of αs1- and β-casein predominating. Over 90 % of TPP has been accounted for in this way. A fragment consisting of residues 29–105 of β-casein was isolated and characterized from both stored milk and from plasmin digests of β-casein. This fragment was a relatively major product of the natural proteolysis occurring during storage of milk, but contrary to a report in the literature it was not the same as proteose-peptone component 8-slow. Since many of the components of TPP resulted from proteolysis, the composition of TPP was found to vary according to the time and temperature of storage of the milk from which it was prepared. Thus, while the proteose-peptone fraction of milk can easily be defined operationally it cannot be rigorously defined in terms of its composition unless the history of the milk is also defined.

The effect of homogenization on the heat stability characteristics of milk was examined. The heat stability of homogenized milk, as determined by the time taken for protein clots to form when heated at 140 °C, was reduced with increasing pressure in the range 3·5–34·5 MPa. The heat stability of homogenized milks was greater for samples obtained in the summer months than for those obtained in the winter. The general destabilizing effect of homogenization could be partly offset by 2-stage homogenization (20·7 MPa followed by 3·5 MPa), addition of phosphate stabilizers (0·08% w/v) or homogenization at a high temperature (65 °C). Whilst homogenized and unhomogenized milks reacted similarly to the addition of Ca, phosphate stabilizers, sulphydryl-blocking and oxidizing agents, the effects of season, addition of urea or formaldehyde were different for homogenized milk.

The heat stability of homogenized concentrated milk was found to be more susceptible than that of unhomogenized concentrated milk to changes in the ratio of β-lactoglobulin to κ-casein which were deliberately designed to alter the extent of disulphide-linked complex formation during heating. When sulphydryl group interactions were prevented by the addition to the milk before homogenization of blocking or oxidizing agents, homogenization did not change the heat stability of subsequently concentrated milk.

The role of blood serum lipoproteins in stimulating milk lipolysis was investigated by adding blood serum and heparin to the milk of normal cows and cows in which elevated somatic cell counts were induced by intramammary infusion of Escherichia coli endotoxin and Staphylococcus aureus. There was considerable variation between individual milks in the extent to which lipolysis was stimulated. In some milks there was almost no response. For values obtained from 29 cows during the first 6 months of lactation a relationship was observed between free fatty acid (FFA) levels in the untreated and serum-stimulated samples (r = 0·776). FFA values were higher after 18 h at 4 °C in milks from infused quarters than in those from control quarters, but this difference was not entirely due to higher values from the infused quarters at 0 h. Both blood serum and heparin stimulated lipolysis in high cell count milks and control milks. The response to heparin was greater than that to blood serum, but the response to both was highly correlated with FFA levels in the untreated milks. There were no differences in the ability of skim-milks prepared from infused and control quarters to promote lipolysis in recombined milks containing sterilized homogenized milk fat globules as a source of substrate. Incubation of blood serum with 2 μg/ml trypsin for 1 h at 37 °C reduced the ability of the serum to promote lipolysis. The importance of lipase activation by blood serum lipoproteins in relation to milk lipolysis is discussed.

A quantitative method for rapid routine analysis of individual free fatty acids (FFA) in milk was developed. Lipid was extracted from milk in ether and FFA were recovered by shaking the extract with anion exchange resin Amberlyst 26. The resin-bound FFA were methylated directly and the individual acids quantified, using internal standards, by gas-liquid chromatography. The properties of the resin were measured. The validity of the method was established by extraction of FFA mixtures and milk. Individual acids were, on average, found to be within 6% of the actual concentration present in the mixture. An average coefficient of variation of 4·3% was achieved for the major individual fatty acids on repeated extraction of a single milk sample.

The rates of coagulation of completely renneted casein micelles have been measured as functions of ionic strength, temperature, and concentration of Ca2+. At 25 °C and below, the rate constants for the coagulation were found to be low, but increased with temperature so that at 60 °C the particles were coagulating at almost maximum rate permitted by diffusion. This maximal rate at 60 °C was achieved at nearly all of the ionic strengths and concentrations of Ca2+ used. At lower temperatures the rate constant decreased with increasing ionic strength, the dependence being more marked at lower temperatures. Increasing concentration of Ca2+ also increased the rate at low and moderate temperatures. The implications of these results are discussed in terms of specific and non-specific ionic interactions and of hydrophobic bond formation.

Whole milk was concentrated by ultrafiltration in a plant causing some homogenization of the fat. Comparisons were made with milk concentrated in a plant causing little homogenization and with milk homogenized conventionally. None of the processes appreciably affected the casein micelle size distribution. On rennet treatment of homogenized milk, casein micelle aggregation occurred more slowly, the protein network in the curd was less coarse and the rate of whey loss was reduced, compared with non-homogenized milk at the same concentration. In using concentrated milks for cheesemaking homogenization improved the composition of Cheddar cheese, because of increased fat and moisture retention, but curd fusion was poorer. Some aspects of the texture of the mature cheeses were improved, but the free fatty acid levels were higher. Values for the firmness of curds, formed from milks processed in different ways, did not relate to the extent of aggregation of the casein micelles. It is suggested that the complete cheesemaking process is driven by the tendency of the casein to aggregate.

Variations in the biochemical performance of 2 DL-type mixed strain starters CH 01 and FDn grown in a sample of skimmed goat's milk I (obtained 2–3 weeks after parturition) and of goat's milk II (8 months after parturition) were studied, and results compared with those of the cultures grown in skimmed cow's milk. The initial levels of citrate differed in the 3 milks, with goat's milk II containing 0·7 mg/ml, goat's milk I 1·4 mg/ml and cow's milk 1·8 mg/ml. The production of diacetyl, α-acetolactic acid and acetoin was very low in goat's milk II. In goat's milk I the production of these compounds was higher, but still considerably lower than in cow's milk. Production of ethanol (EtOH) was similar in the 3 milks incubated with starter CH 01, but with FDn the production of EtOH was distinctly higher in goat's milk II. The total amount of CO2 produced corresponded with the initial level of citrate; consequently the highest production was found in cow's milk and the lowest in goat's milk II. The use of different starters for fermented products made from goat's milk in early and late lactation is suggested.

Generation times and acid production after 6 and 24 h by 20 strains of Streptococcus lactis of dairy origin were determined in heat treated (80 °C for 30 min) and sterilized cow's and ewe's milk. Ewe's milk enhanced growth of the streptococci, with significantly (P < 0·001) shorter generation times and higher acid production after 6 h incubation than cow's milk, probably due to its higher vitamin content. The stronger buffer capacity of ewe's milk allowed a higher (P < 0·001) acid production after 24 h than cow's milk. A stimulatory effect of sterilization on generation time and acid production after 24 h was observed in cow's milk. However, the heat treated ewe's milk was shown to be a better substrate than sterilized ewe's milk for Str. lactis.

An extracellular proteinase-and lipase-deficient mutant of a psychrotroph, Pseudomonas fluorescens strain 32A, has been isolated and the absence of the proteinase enzyme confirmed by growth on differential media, enzyme assay and polyacrylamide gel electrophoresis. Competition between the parent and the mutant was observed when equal numbers of the 2 strains were inoculated together into raw skim-milk at 6 °C. Bitterness was detected at 6 °C in pasteurized skim-milk inoculated with the parent cells concurrent with the detection of proteolytic activity. In the case of the mutant, slight bitterness which did not increase with increasing cell numbers was detected in the absence of proteolysis. Mutant cells failed to grow on Na caseinate as the sole source of carbon. It was concluded that the extracellular proteinase, while not essential for growth in milk, does provide a selective advantage to the producer organism. This enzyme is, however, essential for growth on milk proteins and contributes to the development of bitterness in pasteurized milk.

Cell-free extracts of both Lactobacillus bulgaricus and L. acidophilus demonstrated threonine aldolase activity, the end product of which was acetaldehyde, the major flavour compound of yoghurt. L. acidophilus also possessed an alcohol dehydrogenase activity capable of reducing acetaldehyde so that little yoghurt flavour was present in milks fermentation with this organism. Addition of threonine to fortified milk before fermentation with L. acidophilus increased acetaldehyde production and resulted in a well flavoured product similar to that of yoghurt made with L. bulgaricus. The contribution of these 2 enzymes to flavour production is discussed.