Six lactating dairy cows receiving a diet of grass silage and barley were given the following treatments as infusions into the abomasum, in a replicated 3 × 3 Latin square with 10 d periods: sodium caseinate (C), an enzymic hydrolysate of casein (CH) and soya-protein isolate (SPI). The C and CH treatments supplied ~ 185 g/d and the SPI treatment ~ 160 g/d of amino acids. Milk yield was similar for all treatments. Yields of milk protsein were 605, 604 and 582 (P > 0·05) g/d for C, CH and SPI respectively. For C, CH and SPI respectively, concentrations (g/kg) of fat were 39·6, 45·7 and 41·9 and yields (g/d) of fat were 790, 931 and 827; values for CH were significantly (P < 0·01) higher than for either of the other treatments. It is concluded that the form in which amino acid residues of casein are supplied to the abomasum (protein v. peptide–amino acid mixtures) can influence the partition of use of nutrients between body tissues and the mammary gland.

Skim milk was heated at 85 °C for different holding times. As a result of such heating, whey proteins, in particular β-lactoglobulin, denatured and associated with casein micelles. This led to an increase in size of the casein micelles but also to a different interaction between them. Both these changes could be described by using a quantitative model which was developed for the viscosity of so-called adhesive hard spheres. We applied the model successfully to skim milk and were able to describe on a quantitative basis the changes due to the heat treatment of milk. It was shown that after heating the casein micelles became larger and acquired a mutual attraction. The unfolding of the whey proteins and their subsequent association with the casein micelles appeared to be responsible for these changes. How this reaction influences the fouling of heat exchangers is discussed.

The formation of hydrophobic sites on the surface of casein micelles as a consequence of enzymic coagulation of industrially heat-treated milk was studied by following the distribution of a fluorescent hydrophobic probe between a free and an aggregated protein fraction. Results were compared with those obtained when coagulation of the same milk samples was followed rheologically in a Gelograph apparatus and by tristimulus colorimetry. Thermal treatment of milk appeared to affect the accessibility of casein to enzyme action, while homogenization influenced the rate of cooperative aggregation of casein subjected to proteolysis.

The effect of acidification method (microbiological with or without renneting, HCl addition) on mass transfer, fouling structure and the rheology of the retentate was studied in the ultrafiltration of skim milk coagula using a mineral microfiltration membrane. The increase in fouling with time appeared to determine permeate flow rates, which were higher in biological coagula, and the protein retention rates which were higher in chemical coagula. Fouling was investigated using scanning electron microscopy. The rheological study showed that at the same total solids, biological coagula were more viscous than chemical coagula. The initial coagula (total solids 97 g/kg) all displayed pseudoplastic behaviour at low shear velocities and Newtonian behaviour at high velocities. Ultrafiltration of fat-enriched milk coagulum to a dry weight corresponding to a soft cheese (total solids 334 g/kg; fat in total solids 60%) gave satisfactory permeate flow rates and protein retention rates. Performance was related to the composition of the product, the hydrodynamic parameters used and the resulting fouling. The rheological study showed that the initial coagulum behaved as a pseudoplastic body at low shear rate and for higher velocities as a Newtonian liquid. The concentrated retenate behaved as an ideal viscoplastic body (Bingham body).

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by 1H and 17O NMR spectroscopy. The proton transverse relaxation rate 1/T2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5·3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/T2, with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and ‘bound’ water contributed to proton relaxation in skim milk. The study of 17O relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by 31P NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.

Three major proteins from the proteose peptone of bovine milk were purified by Sephadex G-75 gel chromatography, Q-Sepharose ion-exchange and additional Sephadex G-75 gel chromatography in the presence of urea. From their mobility in a gradient SDS-PAGE, the proteins were found to have molecular masses of 17, 28 and 60 kDa. The N-terminal amino acid sequence of the 17 kDa protein was found to be homologous with a camel whey protein. This protein has not previously been described in bovine milk. From the SDS-PAGE results, the 28 kDa protein was judged to be the major protein of proteose peptone, contributing ~ 25% of the total. The N-terminal amino acid sequence showed no homology to any known protein sequence, but the amino acid composition indicated that the 28 kDa protein is identical with the PP3 component from the proteose peptone fraction of bovine milk, or part of it. The 60 kDa protein was found to be bovine osteopontin, a very highly phosphorylated protein with an Arg-Gly-Asp sequence which mediates cell attachment.

A comparative study was carried out on the flavour constituents of bovine, ovine, caprine and water buffalo fresh raw milks. The volatiles were isolated from milks by means of vacuum distillation and liquid–liquid extraction. Eighty neutral volatile compounds were identified using high resolution gas chromatography (HRGC) and HRGC-mass spectrometry. About 30 of these components have not been detected previously in milk. The volatiles in milk were similar for the four species, although several quantitative differences might explain the different odours. The volatiles consisted mainly of ethyl esters, especially those derived from butyric and hexanoic acids. Dimethylsulphone alone comprised ~ 25% of the volatile components in bovine, caprine and ovine milk, but only 4% in buffalo milk. Among the aldehydes, pentanal and nonanal were the most representative. Other aldehydes were present selectively: 3-methylbutanal was found only in buffalo milk; phenylacetaldehyde and benzaldehyde were present in large quantities in caprine milk, and in trace amounts in the other milks. The content of ketones, mainly 2-methylketones, was higher in buffalo milk than in the other three types. Alcohols varied between species, from 1·5% (bovine) to 5% (buffalo). Among these, 1-octen-3-ol and phenylethanol could be particularly important for the aroma of milk. The level of l-octen-3-ol in the milks was: buffalo > ewe > goat ≫ cow. Phenylethanol was not found in ewes' and goats' milk and its concentration in buffalo milk was 100 times higher than in bovine milk. Finally, indole and 4-methylphenol, interesting because of their odorant properties, were present in higher quantities in buffalo, ewes' and goats' milks than in cows' milk.

The main compounds responsible for the aroma of bovine, ovine, caprine and water buffalo freshly secreted milk have been identified by means of a gas chromatography–olfactometry technique. Of the fourteen odour-active volatile compounds detected, eight were present in all milks studied. Ethylbutanoate and ethylhexanoate (fruit-like aroma), among the neutral odorants, were the major contributors to the odour of bovine, ovine and caprine milk. The aroma of water buffalo milk was less dependent on ethylhexanoate and was also due to l-octen-3-ol (aroma of raw mushrooms), nonanal (freshly cut grass), indole (stable, animals) and an unidentified constituent, characterized by a retention index of 828, with a typical odour of warm milk and/or smoked cheese. Other aroma components were specific for some types of milk, contributing to the complexity and richness of flavour.

A rapid method of performing the Limulus amoebocyte lysate (LAL) test in milk is proposed using the Toxinometer ET-201. This instrument measured the increase in turbidity due to the interaction between the endotoxins of the Gram-negative bacteria and the LAL reagent, monitored the ratio Rt of the sequential to the initial transmission at 12 s intervais and quantified endotoxins by determination of the reaction time Tr required to obtain a 5% decrease in Rt. There was a good correlation between the toxinometrically determined endotoxin concentrations and the number of Gram-negative bacteria (SD, 0·18 log(plate count units)), and the repeatability (CV, 6·10%) was high. The assay may be useful for screening raw materials for UHT milk production, as the endotoxin content of the raw material is related to the rest proteinase activity in the UHT milk.

The extracellular proteinase from Pseudomonas fluorescens No. 33 was purified to electrophoretic homogeneity by a procedure including precipitation with HC1 and (NH4)2SO4, and column chromatography. The enzyme was purified 170-fold giving a yield of 7 % of the original activity. The molecular mass of the purified enzyme was 48000 by SDS-PAGE. The optimum pH and temperature for the hydrolysis of casein were 8·0–9·8 and 30–35 °C respectively. The enzyme was more thermostable in synthetic milk salts solution than in 0·1 M-sodium phosphate buffer, but was heat-labile at 50 °C in both buffer Systems. The activity was inhibited by o−phenanthroline, Hg2+, Cu2+, Fe2+ and, to a lesser extent, Ni2+. Caseins were susceptible to the proteinase, but degradation patterns were dependent on the form of the casein.

Part of the raw cheese base used for manufacturing high-fat processed cheese spread Was replaced by cows' (C) or buffalo (B) butter residue at levels of up to 20%. These residues were produced during the conversion of butter to butter oil by traditional moderate (T) or severe (M) heat treatments. The hypocholesterolaemic properties of butter residues incorporated into processed cheese were tested with rats (serum and liver cholesterol) and human beings (serum cholesterol). When rats were given the control cheese with no butter residue there were significant increases in both serum and liver cholesterol compared with rats given stock diet. However, when part of the raw cheese base was replaced by butter residue these rises were reduced by amounts proportional to the level of butter residue. The hypocholesterolaemic properties of the residues were in order CT > BT, CM > BM, and incorporations of 10% CT, 15% BT, 15% CM and 20% BM were found to nullify the hypercholesterolaemic effects of control cheese and restore cholesterol levels to their normal values. A small human trial gave similar results. Incorporation of butter residue slightly affected the chemical composition of the cheeses. Fat content and pH were little different; however, cheeses with butter residue tended to have higher total protein but lower soluble protein, tyrosine and tryptophan, and total volatile fatty acids. Rheological properties were not altered significantly. Incorporation of butter residue improved the organoleptic properties, particularly flavour intensity. Storage at 5 °C for 2 months did not alter the organoleptic properties and changes in chemical composition followed the normal pattern.

Using polymer producing (ropy) strains of lactic acid bacteria it was possible to reduce considerably the syneresis of yogurt, even with 12% total milk solids. The viscosities obtained with these strains were also similar to those obtained using normal strains and milk with 17% total solids content. The concentration of milk and the polymer produced by ropy starters had a synergic effect in increasing viscosity. Polymer production was not affected in most cases by milk concentration. One type of ropy culture (Wiesby) seemed to produce a different kind of polymer as it could not be determined by alcohol precipitation, in spite of being able to reduce syneresis and increase viscosity in yogurt. A limited number of yogurts were evaluated organoleptically, one prepared with a ropy starter strain (NCFB at 12, 14·5 and 17% total solids) and one prepared with a non-ropy strain (LL-I at 17% total solids). The results suggest that the ropy strain yogurts had different mouthfeel from the non-ropy strain yogurts; the most acceptable product overall was the ropy strain made with 12% total solids.