Effects of long-term deep-freeze storage on the stability of fat in raw sheep's milk were assessed by measuring free fatty acid (FFA) concentrations, lipoprotein lipase activity and the amount of solvent-extractable fat. Portions of milk from three herds were frozen and stored at – 12, – 20 and – 27 °C. There was a gradual increase in FFA during 6 months storage, the rate of lipolysis being affected by storage temperature and differences between herds. After 6 months storage the residual lipase activity was only 2% (– 12 °C), 11% (– 20 °C) and 24% (– 27 °C) of the initial activity in fresh milk. The potential for lipolysis in the stored samples, after they were removed from storage and allowed to thaw to 4 °C, gradually fell as storage time was prolonged. This may be due to the loss of lipase activity.

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.

Lysozyme (LZ), lactoferrin (LF), lactoperoxidase (LP), immunoglobulin G and secretory immunoglobulin A were extracted from camel milk. The activity of these protective proteins was assayed against Lactococcus lactis subsp. cremoris, Escherichia coli, Staphylococcus aureus, Salmonella typhimurium and rotavirus. Comparative activities of egg white LZ, bovine LZ and bovine LF are also presented. The antibacterial activity spectrum of camel milk LZ was similar to that of egg white LZ, and differed from bovine milk LZ. Bovine and camel milk LF antibacterial activity spectra were similar. The camel milk LP was bacteriostatic against the Gram-positive strains and was bactericidal against Gram-negative cultures. The immunoglobulins had little effect against the bacteria but high titres of antibodies against rotavirus were found in camel milk. The LP system was ineffective against rotavirus.

Cells were obtained from the mammary glands of sheep and cows by collagenase–hyaluronidase digestion. Characterization of cells as epithelial was by reaction with a monoclonal antibody to cytokeratin. A subpopulation of spindle-shaped or stellate cells reacted with a monoclonal antibody to desmin and may be related to myoepithelial cells. The development is described of a simple serum-free culture system for these cells on gels of rat tail (type 1) collagen. A commercial medium (M199) was used, buffered with Hepes and with bovine serum albumin as the sole protein supplement, plus fibronectin for the first 18 h only as an attachment factor. The cell cultures showed stimulated DNA synthesis in response to mitogens on attached gels and also responded as floating cultures to lactogenic hormones with production of α-lactalbumin.

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 nature and composition of the saturated oxo fatty acids in Cheddar cheese have been re-evaluated following isolation by moern chromatographic procedures, including low-pressure column chromatography and HPLC (adsorption and silver ion modes). The oxo fatty acids were identified and quantified (for the first time) by gas chromatography–mass spectrometry in the form of the methyl ester derivatives and many of the structures were confirmed similarly by preparing picolinyl ester derivatives. Thirty-six saturated oxo fatty acids were identified, ranging in chain length from 9 to 22 with an oxo group on carbons 4–17 (except for 12), of which twenty-one had been identified in a previous study and fifteen were new. Fifteen fatty acids tentatively identified by others did not in fact appear to be present.

Milk production of dairy cows in 14 herds was increased by 3·8–32·1% by the administration of recombinant methionyl bovine somatotropin (bST) in a sustained-release vehicle at 14 d intervals at 40–94 d post partum. A greater response in multiparous than in primiparous cows was found in cows turned out to graze spring pasture. Administration of recombinant bST resulted in elevated plasma bST during the first 9 d after injection. Clinical characteristics such as respiration, heart rate and body temperature were unaffected by bST treatment, as were blood erythrocyte and leucocyte counts, haemoglobin concentrations and haematocrit values. Plasma levels of glucose, free fatty acids, urea and P, and serum alanine aminotransferase and aspartate aminotransferase were not affected by bST treatment, and acetone was not detected. No adverse effects of bST on general health, infection status of mammary glands, mastitis incidence and reproduction were found.

Two types of lecithin, namely egg and soya lecithin, were investigated as potential stabilizers of recombined milk. They were incorporated into recombined milk both before and after homogenization (20·7 MPa; 60 °C). Their presence at homogenization changed neither mineral equilibria nor homogenization efficiency. However, heat stability varied significantly irrespective of batch of low-heat skim milk powder used in recombined milk. The variation in heat stability depended on type of lecithin. Soya lecithin proved to be a very effective stabilizer. It improved heat stability over a wide pH range (6·3–7·1) and the effect occurred even when the lecithin was added after homogenization. In contrast, egg lecithin destabilized the system to heat at pH < 6·7 by converting a Type A into a Type B heat coagulation time-pH profile if it was incorporated before homogenization; after homogenization it had no effect. The effects of both egg and soya lecithin on the heat stability of recombined milk strongly suggest that interactions occur between phospholipids and milk protein.

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 presence of 3-deoxypentulose in heated milk is reported. This compound has been isolated from alkaline solutions of lactose and characterized by spectroscopic and Chromatographie techniques. Gas Chromatographic analysis of free monosaccharides of milk revealed two peaks corresponding to 3-deoxypentulose in commercial sterilized milk samples, in a concentration range of 30–50 mg/1. Its concentration increased with temperature and length of time of processing.

Mesophilic lactic starters and thermophilic lactobacilli but not Streptococcus salivarius subsp. thermophilus grew on the sodium lactate agar (SLA) used for estimating the numbers of propionic acid bacteria (PAB) in cheese. The addition of cloxacillin (4 μg/ml) to SLA inhibited the starter bacteria but had no effect on the PAB. It was possible to count low numbers of PAB in the presence of high numbers of starter bacteria. A correlation coefficient of 0·9 was obtained between the level of propionic acid and the counts of PAB in cheese (n = 40). A disadvantage of the medium is that other bacteria found in cheese (mesophilic lactobacilli, enterococci, Clostridium tyrobutyricum) also grow on it; however, these bacteria are easily distinguishable from PAB on the basis of size, colour and absence of catalase.

Heating raw milk at 80 °C for 2·5–20 min was found to result in compositional changes in the milk fat globule membrane (MFGM). The yield of protein material increased with the duration of heating, owing to incorporation of skim milk proteins, predominantly β-lactoglobulin, into the membrane. Lipid components of the MFGM were also affected, with losses of triacylglycerols on heating.

A recently described nuclear magnetic resonance (NMR) method was evaluated for its usefulness in studying thermal effects on milk proteins. The increase in water proton T2 relaxation rate observed during thermal treatment of aqueous whey protein solutions above the denaturing onset temperature paralleled results obtained with the standard Rowland (1938) method. The influence of milk constituants on NMR characteristics was analysed. The NMR response increased with the ionic strength and the addition of caseinate or casein micelles. The relevance of the T2 relaxation probe for studying thermal modifications of milk proteins is discussed. It is proposed to apply the NMR method for determining either reversible or irreversible thermal denaturation of whey proteins in model Systems.

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.

Acid-set milk gels containing up to 30 mM added hexametaphosphate, oxalate, citrate, EDTA or orthophosphate were prepared using gluconic acid-δ-lactone and their properties investigated. Increases in the apparent shear modulus were observed in the gels containing hexametaphosphate, oxalate, citrate and EDTA. The effect was maximum for hexametaphosphate-containing gels at ˜ 8 mM. Increases were also observed in the force required to break the gels, and for added hexametaphosphate the effect peaked at ˜ 5 niM. The ability of the gels to resist syneresis was investigated by a cut, tip and drain type test and by a centrifugation method. In both tests, resistance to syneresis was increased by added hexametaphosphate, oxalate, citrate and EDTA. The additions of anions were shown to cause disintegration of the micelles and release caseins to the serum. It is suggested that the improved homogeneity of casein distribution that results allows greater opportunity for protein-protein interaction during gel formation and leads to improved properties. Controlled micelle disintegration would therefore appear to offer promise for the development of improved or novel products.

Lysogenic strains of Streptococcus salivarius subsp. thermophilus were studied using induction with mitomycin C (MC). The induction and presence of temperate phage were investigated carrying out tests on sensitive strains, electron microscopy and phage DNA analysis. Forty-five Str. salivarius subsp. thermophilus strains were subjected to induction with MC and growth of the various cultures was evaluated. Only one strain of those tested showed lysis after adding MC, thus indicating a possible lysogenic state, 0·5 μg MC/ml being the optimal dose. Two Φ18 phage-sensitive strains out of 45 were isolated in which this phage behaved as virulent, causing lysis of the culture in broth, but no lysis plaques on agar medium were detected. The St18 strain was cured by u.v. irradiation but no mutants sensitive to the Φ18 phage were found among the clones non-inducible by MC. The presence of phages having a hexagonal isometric head and a long non-contractile tail in the lysate obtained after inducing the St18 strain was confirmed by examination under the electron microscope. Study of the phage DNA showed a genome size of 40·9 ± 0·5 kb without cohesive end fragments. In addition, the restriction map of the phage genome was constructed. This study has demonstrated lysogeny in Str. salivarius subsp. thermophilus and also that several phage infections of Str. salivarius subsp. thermophilus starters may have an ‘endogenous’ origin.

Plasmin (EC 3.4.21.7) was added to cheese milk to assess its contribution to Cheddar cheese ripening; the activity of plasmin in the cheese was increased by levels ranging from 1·5 to 6 times that in the control cheeses. Even at the highest level of added plasmin, no activity was found in the whey. β-Casein was degraded faster in the experimental cheeses than in the controls, and the concentration of γ-caseins increased concomitantly. The total N in the water-soluble extract was up to ˜ 20% higher in the experimental than in the control cheeses but phosphotungstic acid-soluble N was not affected by the plasmin activity in the cheese. Several differences were apparent in the gel electropherograms of the water-soluble extracts of the experimental and control cheeses; some peptides were present at higher concentrations in the former, others in the latter, suggesting that plasmin contributes to both the formation and degradation of water-soluble peptides in cheese. The organoleptic quality of the plasmin-enriched cheeses was judged superior to that of the controls and ripening was considerably accelerated; a plasmin level 3–4 times the indigenous value appeared to be optimal. No bitterness was detected in any of the cheeses.

Correlations were identified between levels of the native whey proteins, β-lactoglobulin and α-lactalbumin and the surface and total hydrophobicities of cheese whey in response to different heat treatments. Heat-induced changes in the native βlactoglobulin content and surface hydrophobicity of whey exhibited the most significant linear relationship while correlations between total hydrophobicity and the native proteins were less significant because of an atypical rise in the n−heptane-binding capacity of whey after high-temperature treatment. The content of native β-lactoglobulin in whey was more sensitive to heating than the content of native α-lactalbumin, while heat-related changes in the total hydrophobicity of whey were generally greater than similar changes in surface hydrophobicity.

Ewes' milk coagulation was significantly influenced by the type of rennet when recombinant chymosin produced by Kluyveromyces lactis was compared with two commercial calf rennets with declared chymosin contents of 55 and 70%. Recombinant chymosin and calf rennet with 70% chymosin exhibited similar thrombelastographic characteristics, and coagulated milk more rapidly than calf rennet with 55% chymosin. The type of rennet had no effect on Manchego cheese dry matter, pH or recovery of dry matter in cheese. N soluble at pH 4·6 was the only N fraction influenced by the type of rennet, with slightly higher levels for cheese made with recombinant chymosin or with calf rennet containing 70% chymosin than for that made with rennet containing 55% chymosin. No significant differences in rheological or sensory characteristics between Manchego cheese made with recombinant chymosin and that made with the calf rennets were detected.

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.