Fifteen lactating cows were milked throughout pregnancy, and the effects on milk performance were studied during this period and during the succeeding lactation, relative to 11 conventionally managed cows (2 months dry before calving) as controls. During the last 2 months of pregnancy, only nine cows did not dry off spontaneously. Protein and fat concentrations in milk increased rapidly, but the concentration of lactose, corrected for milk yield, did not change. The ratios of individual caseins to total protein decreased with the quantity of milk produced, but only for yields below ˜ 6 kg/d. The relative proportion of κ-casein tended to decrease in the last milkings. During the succeeding lactation (first 15 weeks after calving and first 6 weeks of grazing) continuously milked cows yielded 4 kg milk/d less than the cows of the other group. The protein content of their milk was higher (2–3 g/kg depending on the period) than that of the control group, and the lactose content tended (p < 0·10) to be lower. Changes in the relative proportions of nitrogenous fractions with time were not different in the two groups. Differences between the two groups in the concentration of protein in milk, and in the concentration of glucose and non-esterified fatty acids in the plasma, suggest a better energy balance for the continuously milked cows during the succeeding lactation.

Lactating heifers were treated for 4 weeks with recombinant bovine growth hormone (bGH, n = 9) or were untreated (n = 9). In addition, two mammary glands of each heifer were milked four times daily rather than the normal twice daily for the same 4 weeks, and for the following 2 weeks. Over the 4 weeks, milk yield was increased 12·8% by bGH, 14·0% by frequent milking and 28·5% by the combined treatment. The effect of bGH as administered here was slower in onset than that of frequency, but eventually produced a higher peak yield. ANOVA revealed significant effects of each stimulus independently and an additive, but not synergic effect of the combined treatment. The effect of the combined treatment tended to persist beyond the end of treatment; most of this response was related to the milking frequency component rather than the bGH. Mammary differentiation was assessed in biopsies of mammary tissue obtained prior to and at the end of treatment. Mammary enzyme activities (expressed on a per cell basis) indicated minimal differentiative response to either treatment, but synthesis rates for lactose and casein determined in vitro were increased by bGH treatment. Histological examination revealed a stimulatory effect of milking frequency on epithelial cell size. The results indicate that these two galactopoietic stimuli operate through independent mechanisms, and neither stimulus alone is sufficient to maximize milk yield in dairy heifers.

A mathematical model of the interactions within a milking machine teatcup has been developed, which describes the relationship between liner wall movement, pressures and flow rate when milking without an air inlet. It is based on equations of motion for a column of incompressible fluid and requires a second-order, non-linear differential equation to be solved. Incorporating a non-return valve allows hydraulic milking to be modelled, and the comparison between predicted and measured pressures, flow rates and liner wall movement when milking hydraulically is shown. The model can be used to optimize milking conditions to reduce vacuum peaks, improve liner opening and thus maximize flow rate.

Machine-induced changes in teat thickness were measured in a randomly selected group of 22 cows from a commercial dairy herd consisting of 110 cows during an experiment lasting 1 year. Half the cows used were free from mastitis whereas the remainder had at least one quarter infected by environmental pathogens. Teats were classified according to a threshold change in teat end thickness of 5%. The relationships between quarter infection or teat duct colonization and teat end thickness changes induced by machine milking were investigated. Teats with > 5% change in thickness had significantly increased teat duct colonization and a slight, non-significant, increase in quarter infection. We conclude that machine-induced changes in teat end thickness are predisposing factors for teat duct colonization by environmental pathogens. This may provid: an increased new infection risk, especially when hygiene is poor.

Whey samples were screened for the presence of the proteinase inhibitor α2-macroglobulin (α2M). From an enzymic test, α2M levels in normal whey varied in the range 0·49–0·84% of the serum level, whereas in mastitis whey the activity was markedly increased, reaching values between 0·91 and 138·5% (median 7·2%) of standard serum level. In mastitis milk samples but not in normal milk α2M was also detected by double immunodiffusion and Western blotting. The proteinase inhibitor was purified from a mastitis milk sample with high α2M activity (138·5% of serum level). In SDS-PAGE, native-PAGE and in double immunodiffusion analysis the inhibitor appeared indistinguishable from plasma-derived α2M. The α2M preparation from mastitis whey migrated essentially as native α2M, representing the ‘slow’ form of the molecule. Treatment with trypsin transformed the α2M preparation to the electrophoretic ‘fast’ form, but treatment with methylamine had only a minor effect. The receptor recognition sites were not exposed on the isolated α2M molecule but could be readily exposed by treatment of the proteinase inhibitor with trypsin, which further proved that the isolated α2M was in the entire native, functionally active state.

A new sampling method for the collection of fore and hind milk from the sow has been developed which resembles normal milk removal by the piglet, yet overcomes the difficulties of collecting milk that is available for only a short time (10–20 s) at each let-down. Samples of hind milk were collected immediately after the completion of a successful sucking, while the fore milk was collected at the beginning of the next natural let-down. Modification of existing assays for fat, protein and lactose was required to provide rapid analysis of the small volumes (< 0·5 ml) of milk collected using this procedure, and these methods were validated by comparison with reference methods. Total solids and the concentration of fat in whole milk, and protein and lactose in skim milk, were measured in fore and hind milk collected from 4, 20, 12 and 12 sows respectively, 6–11 d post partum. For fore milk, the results (mean ± SD (n)) were: total solids, 199·9±9·9 g/1 (8); fat, 96·7 ± 13·9 g/1 (41), protein, 47·7±4·5 g/1 (27) and lactose, 58·3 + 4±0 g/1 (27). In hind milk, there was a significant increase in the concentration of fat (15·3 g/1, p < 0·001, n = 41) which was reflected by a significant increase in total solids (14·7 g/1, P < 0·001, n = 8) and calculated energy (511 kj/l, P < 0·001, n = 27), but there was no significant difference in the concentration of either protein or lactose. This increase in milk fat during milk letdown is in contrast to the results of most previous studies in the sow and is discussed in terms of the possible mechanisms that may cause the concentration of fat to increase as milk is removed from the mammary gland.

A new αs2-casein variant was isolated from goat milk by means of covalent chromatography. With gel electrophoresis at alkaline pH, the αs2-casein variant showed a double band with the highest anodic mobility amongst the casein components. The mobility of the isolated αs2-casein variant was similar to that occurring in three individual samples of whole casein. Two dimensional electrophoresis revealed a more pronounced heterogeneity of this protein. Developing the two dimensional plate with polyclonal antibodies obtained against the four main bovine casein fractions, we demonstrated that the variant in question belongs to the αs2-casein family. Since the αs2-casein fractions, immunospecifically developed, spread along two different zones on the gel, it was concluded that a heterozygous αs2-casein form was present in the sample.

The moisture desorption isotherms of mineral acid, lactic and rennet caseins were determined at ˜ 28, 55 and 80 °C. The isotherms were all sigmoid type II according to the BET (Brunauer et al. 1938) classification. A modified Henderson equation was found best to describe the relationship of equilibrium moisture content, water activity (aw) and temperature for all three types of casein. BET monolayer moisture contents and heats of desorption of the three types of casein were calculated. Equilibrium moisture contents at aw 0·432 and 30 °C for mineral acid casein made under a range of manufacturing conditions varied significantly depending on the drying conditions. More severe drying conditions produced lower equilibrium moisture contents.

The effects of heat-induced interactions between milk fat globule membrane components and skim milk proteins in whole milk on the structure of the membrane were examined by isopycnic sucrose density gradient centrifugation and by using Triton X-100 as a membrane probe. Skim milk components were incorporated into all the lipoprotein fractions separated by density gradient centrifugation. High density complexes, higher in density than those found in the natural milk fat globule membrane, were formed during the heat treatment. Losses of natural membrane polypeptides from the medium and low density lipoproteins were observed on heating. Heating whole milk also altered the rate of release of membrane components by detergent, with decreases in protein released and an increase in phospholipid constituents released. Studies on washed cream indicated that some of the changes in the membrane on heating whole milk occurred due to the heat treatment alone, independent of the interactions with skim milk proteins.

The three aspartic acid residues that form part of the Ca-binding site of mares' milk lysozyme have apparent pK values of 4·9, 4·3 and 4·1. The fluorescence of tryptophan has been used to compare the denaturation of mares' milk lysozyme by guanidinium chloride at various concentrations of Ca with that of hens' egg-white lysozyme (EC 3.2.1.17) and α-lactalbumin. Fluorescence revealed an intermediate stage in the denaturation of mares' milk lysozyme. The Ca-free form of mares' milk lysozyme is slightly more stable than that of α-lactalbumin, but its interaction with Ca is similar to that of α-lactalbumin, since only the native state binds Ca. Three-state models of denaturation can usefully be displayed on a ternary diagram.

The transport of K+ (Rb+) and Cl− by membrane vesicles isolated from bovine milk has been studied using ion-exchange column chromatography. K+ (Rb+) and Cl− accumulation by the vesicles was time-dependent and was almost abolished by 0·1% Triton X-100, suggesting that uptake represents ‘real’ transport rather than binding. K+ (Rb+) uptake was influenced by the anion in solution in a manner suggesting that influx is sensitive to changes in vesicle membrane potential. Similarly, Cl− uptake was found to be sensitive to vesicle electrical potential: Cl− influx was enhanced by inside positive potentials. Cl− uptake was not saturable with respect to external Cl−. The results suggest that K+ (Rb+) and Cl− cross the apical membrane by way of conductance pathways. The similarity between ion transport by skim milk membrane vesicles and that of the apical aspect of the intact mammary epithelium suggests that the former may be a good model to study solute transport by the apical membrane of mammary secretory cells.

Streptococcus salivarius subsp. thermophilus strain NST5 exhibited a temperature-dependent defence mechanism against the virulent bacteriophages φ.B1.2 and φA1.1. It was active at 42 °C but not at 30 °C as demonstrated by a significant increase of both plaque size and efficiency of plaquing. This defence mechanism did not affect host-dependent phage replication and did not interfere with phage adsorption to NST5. These results suggest that it interfered with phage development. The phages φT33, φT58, φD1, φT21 and φT9, belonging to the same phage type as φB1.2, were examined for their ability to infect NST3 and NST5. Restriction modification systems of different specificity were detected in NST3 and NST5; host-dependent phage replication was detected at 30 and 42 °C; an abortive defence mechanism was detected in NST5 which was active at 42 °C, but not 30 °C, and was independent of restriction modification action or interference with phage adsorption. Our investigations of phage-host interactions showed that the two Str. salivarius subsp. thermophilus strains studied avoided attack by related bacteriophages by evolving at least three different resistance systems.

The kinetics of lactate dehydrogenase from Leuconostoc lactis NCW1 were studied. The pH optimum for the enzyme depended on the concentration of pyruvate used in the assay and the enzyme displayed an ordered mechanism with respect to substrate binding. The Km for pyruvate and NADH and the Vmax of the enzyme decreased 20–, 30– and 6-fold respectively as the pH decreased from 8·0 to 5·0. No activators were found and none of the intermediates of the phosphoketolase pathway tested inhibited the enzyme. ATP, ADP, GTP and NAD+ were inhibitory. The intracellular volume (Volin) and intracellular pH (pHin) decreased as the extracellular pH (pHex) decreased. Co-metabolism of citrate and glucose affected the Volin but did not affect the pHin, which decreased by 0·6 units per unit change in pHex; at pH 7·0, the pHin and pHex were equal. The results suggest that pHin may play a role in determining the production of diacetyl and acetoin at low pH by Leuconostoc.

Bovine colostrum was investigated as a source of biologically active molecules capable of stimulating the growth of mammalian cells in culture and modifying the immune response in a murine model. An extract prepared from bovine colostral whey by cation exchange and re versed-phase chromatography stimulated the growth of L6 rat myoblasts, Balb/c-3T3 mouse fibroblasts and BHK-21 baby hamster kidney cells with equal or greater potency than fetal bovine serum. Fractionation of the bovine colostral extract by gel-permeation chromatography in M-acetic acid identified a number of cell-growth factors for each cell type. Bovine colostral extract was compared with an ovine colostral whey preparation for its ability to modulate IgE antibody responses in mice. Doses of 8 and 4 mg/d of ovine colostral whey or bovine colostral extract specifically suppressed IgE antibody responses, whereas at lower doses suppression did not occur. We conclude that bovine colostrum contains cell-growth factors as well as immunomodulatory factors that are able to regulate the IgE response in a heterologous species.

Psychrotrophic bacteria isolated from raw milk were tested for their ability to adhere to steel, two types of rubber, and glass, materials employed in the construction of milking equipment. The adherence assays were carried out by exposure of the materials to radioactively labelled bacteria in both a buffering solution (Ringer's) and milk. The degree of adherence of Gram-positive bacteria was lower (P < 0·001) than that of Gram-negative bacteria. Glass was the material least prone to bacterial adherence (P < 0·001); there were no significant differences between the other three materials. Milk was found to inhibit adhesion significantly (P < 0·05), this inhibition being more evident with the most adherent bacteria. There was no statistically significant correlation between bacterial surface hydrophobicity and adherence. Our results suggest that intrinsic bacterial adherence cannot be considered a relevant factor in the contamination of milking equipment.

Lactose-negative mutants of Lactococcus lactis subsp. lactis and Lacto-coccus lactis subsp. cremoris with good autolytic properties were used at two different levels in addition to the normal starter in Gouda-type cheesemaking experiments. Increased numbers of bacteria were observed in fresh cheese, and the pH changes during ripening were as normal. A more rapid development of soluble N compounds was observed in cheeses with mutant addition, especially with Lc. lactis subsp. lactis IMN-L2–3 and Lc. lactis subsp. cremoris IMN-C12–1, compared with the control cheese. Cheese with added Lc. lactis subsp. lactis IMN-L2–3 and Lc. lactis subsp. cremoris IMN-C12–1 showed the most extensive ripening and the highest product quality of the mutants tested. The experimental cheeses were graded higher than the control cheese. The ripening time was significantly reduced, and the quality was retained at an acceptable level during storage.

The isolation and identification of low molecular mass peptides formed during the ripening of Parmigiano-Reggiano cheese is described. A strategy was used based on the fractionation of nitrogenous material using chemical methods followed by HPLC to isolate peptides and fast atom bombardment-mass spectrometry to identify them. It was found that the majority of cheese oligopeptides arose from the proteolysis of β-casein. Several phosphopeptides and oligopeptides known in vivo to be biologically active have also been identified during the ripening of cheese.

The relationships between the composition, texture and microstructure of chhana (an Indian-style soft cottage cheese analogue) and rasogolla (a sweetened dairy product made from chhana) were analysed. Chhana contained a significantly higher proportion of fat, protein, lactose and minerals than did rasogolla. Cooking of chhana in 60% sucrose solution introduced sucrose and changed the texture and structure to that typical of rasogolla. As chhana was transformed to rasogolla, the Instron textural properties, hardness, gumminess and chewiness, fell significantly, whereas springiness increased dramatically. Market rasogolla had a slightly different composition from the laboratory-made samples, but both had similar Instron textural and sensory textural attributes. Scanning electron microscopy showed that chhana had a structure like cottage cheese or cream cheese, a compact, coalesced casein matrix with fat globules embedded in it. Rasogolla had a conglomerated, ragged and porous casein matrix with uneven surface and numerous large void spaces. The collapsed and ruptured fat globules were found embedded in the agglomerated protein particles of rasogolla. The textural attributes of both chhana and rasogolla could be explained by the microstructure.

α-Lactalbumin (α-La) is a protein found in the milk of all mammalian species and is involved in lactose biosynthesis in association with galactosyltransferase (Brodbeck et al. 1967). The primary structure of α-La from several species such as bovine (Brew et al. 1970), human (Findlay & Brew, 1972), and goat (McGillivray et al. 1979) has been determined. Hall et al. (1982) have established the nucleotide sequence and the organization of the human α-La gene (Hall et al. 1987).

The keeping quality of properly refrigerated pasteurized milk and cream is primarily determined by post-pasteurization contamination by Gram-negative psychrotrophic bacteria (Phillips et al. 1981; Schröder et al. 1982). Reliable and rapid methods of assessing the levels of contamination by these organisms are therefore of commercial interest.