Pressure processing of foodstuff has been applied to produce or modify proteinaceous gel structures. In real pressure processing the treatment is non-isothermal, due to the adiabatic nature of the process and the heat loss from the product to the vessel. In order to estimate the effect of pressurization on milk constituents pressure and temperature dependent kinetics were determined separately from each other. In a detailed kinetic study whey protein isolate was treated under isobaric (200 to 800 MPa) and isothermal conditions (−2 to 70 °C), and the resulting degree of denaturation of β-lactoglobulin A and B and α-lactalbumin was analysed. Kinetic parameters of denaturation were estimated using a one step non-linear regression method which allowed a global fit of the whole data set. The isobaric isothermal denaturation of β-lactoglobulin and α-lactalbumin was found to follow third and second order kinetics, respectively. Isothermal pressure denaturation of both β-lactoglobulin fractions do not differ significantly and were characterized by an activation volume decreasing with increasing temperature from −10 to about −30 ml mol−1, which demonstrates that the denaturation rate is accelerated with increasing temperature. The activation energy of about 70 to 100 kJ mol−1 obtained for β-lactoglobulin A and B is not dependent to a great extent on the pressure which indicates that above 200 MPa denaturation rate is limited by the aggregation rate while pressure forces unfolding of the molecule.

The effect of ultrafiltration and diafiltration on the distribution of the calcium phosphate clusters of the casein micelle was investigated using Small Angle Neutron Scattering (SANS). In the case of ultrafiltration, fresh skim milk was subjected to concentration using membrane filtration up to 5× its original volume, the retentate was rediluted with its corresponding serum and subsequently dialyzed against reconstituted milk powder dispersed in D2O/H2O (UF 5×(D)). In the case of diafiltered samples, the samples were concentrated adding water (diafiltration) at two different levels (DF 2·5× or DF 5×) and then redispersed with D2O/H2O. In the DF 5× case, the serum components were diluted to less than 1% of their original concentration. For analysis, all samples had the same volume fraction of dispersed casein micelles (φ=0·1), which is that of the control, unprocessed skim milk. A peak in the SANS data was observed in fresh skim milk at a scattering vector, qo, of 0·034 Å−1 (directly proportional to the reciprocal characteristic length), in agreement with previous literature results. Neutron data on the ultrafiltered, UF 5×(D) and diafiltered, DF 2·5× and DF 5× milk samples showed a progressive decrease in the intensity of the peak but invariance in qo. These results, combined with the determination of soluble and insoluble calcium in the samples, suggest a progressive and irreversible removal of calcium from within the micelle during membrane filtration of milk. Using SANS it was possible to clearly show changes in the micellar calcium clusters that may not otherwise be fully determined by only measuring the amount of total and insoluble calcium in milk.

The casein micelles of sheep (Ovis aries), goat (Capra hircus) and cow (Bos taurus) milks have been examined by electron microscopy. The ovine micelles were smaller than bovine micelles, most being below 80 nm diam. The caprine micelles were either large (c. 200 nm diam.), easily sedimented and electron-dense, or small (below 80 nm) and similar to the ovine micelles. Examination of the micelles using the freeze-etch technique showed, however, that they were all composed of subunits of similar dimensions (10–15 nm diam.).

Electrophoresis of the caseins showed that there were differences between them, with the caprine casein containing a smaller proportion of its protein in the more mobile components. The mineral contents of the milks were also different, but these were apparently related to the different pH and casein contents of the milks. It is likely that the diiferences in micelle size distribution are determined by the physical and chemical properties of the casein components.

1. Two levels of steaming-up and two levels of concentrates feeding during the first 84 days of lactation have been compared in a 2 x 2 factorial experiment over three winter seasons, using fifty-two Shorthorn and thirty-six Friesian heifers. The levels of steaming up were 2 cwt. and ½ cwt. of concentrates fed over the last 21 and 14 days of pregnancy, respectively. The levels of concentrates during lactation were 5 and 3 lb. per 10 lb. milk per day. Roughages were fed for maintenance. After the 84th day of lactation all animals were given the same treatment for the remainder of the lactation. Milk yield, milk composition and live weight were measured throughout the lactation.

2. The treatments were annotated as HH, HL, LH and LL, the first letter indicating level of steaming-up and the second the level of concentrates feeding in the first 12 weeks of lactation.

3. Mean milk yields were as follows:

4. The LH treatment reqiuird 3 cwt. more concentrates than HL to produce the same amount of milk.

5. The response to additioal concentrates on the LH treatment after calving was l lb. additional milk per l lb. additional starch equivalent.

6. Butterfat percentages were higher (3·69%) in both groups on low-lactation feeding than in the two groups on high-lactation feeding (3·43%). The HL group (i.e. the group that received high steaming-up and low-lactation feeding) combined high milk yield with a higher butterfat percentage, and at current milk and feedingstuff prices and under the conditions of the trials, this group showed a greater cash return than the groups on the other three treatments.

Diagnosis of microbial infections in the udders of cows in commercial dairy farms for large experiments cannot be without error. Limitations of sampling method and routine prevent collection of the necessary information for sure diagnosis. However, with an organized method of repeated bacteriological examinations using consistent and proven methods of aseptic sampling the errors were shown to be very low. A method based on bacteriological tests on aseptic milk samples was used in 32 herds (approximately 2000 cows) for a 3-year period. This is described and examined in terms of other criteria to validate its use in experimental work. With this method it was not difficult to differentiate between those quarters which regularly shed pathogens and those which did not. Other evidence indicated that it was reasonable to assume that this classification accurately distinguished between infected and uninfected quarters. The errors using this method were quite small: when measuring the state of infection of all quarters in the herds the errors did not exceed 1 %. Some small modifications to the method described are suggested to improve further its diagnostic accuracy.

Coagulase-negative staphyloccoci were isolated regularly over a period of 39 weeks from quarter milk samples of 18 cows belonging to one herd. They did not produce a leucocytosis in the milk or high anti-haemolysin titres in the blood of these cows, but such responses occurred in 2 cows infected with pathogenic staphylococci. The coagulase-negative variety was found to colonize the teat duct and surface of the teat in preference to the milk and possessed greater resistance than pathogenic staphylococci to certain long-chain fatty acids. Infection with coagulase-negative staphylococci was thought to antagonize the colonization of the udder by a pathogenic variety present in the herd; 23% of the coagulase-negative strains were found to elaborate an antibiotic substance inhibitory to the growth of a coagulase-positive staphylococcus. Coagulase-negative strains were characterized by the production of ɛ-toxin and could be divided into 3 main groups according to their effect on gelatin, mannitol and phenolphthalein diphosphate as substrates.

The prevalence of bovine leukaemia virus (BLV) was determined in 113 Michigan dairy herds by ELISA testing for anti-BLV antibodies in milk. Additionally, an interview regarding management practices with cooperating herd managers identified farm-level variables thought to be associated with prevalence of BLV. Twenty-three risk factors (P ⩽ 0·1) were identified on one-way ANOVA or simple linear regression. Multivariate analysis identified several management practices whose predictive value for increased prevalence of BLV may relate to transmission among herd mates, e.g. reuse of hypodermic needles, lack of fly control, gouge dehorning and increased use of injections in dry cows. Additionally, exclusive breeding of heifers with artificial insemination was associated with decreased BLV prevalence, as compared with at least some use of natural service by a bull. Although intervention studies are needed before causal relationships can be concluded, and unaccounted variables related to transmission exist among dairy herds, these findings suggest management practices that may help dairy producers reduce the transmission of BLV within their herds.

Success in making a cheese acceptable in both flavour and texture depends partly on curd properties which determine the retention of fat and moisture and, thus, cheese yield and composition (Green & Grandison, 1993). When considering farmhouse and/or Appelation d'Origine Contrôlée cheese where milk treatments are not allowed, curd properties depend only on milk composition and clotting characteristics, which are subject to wide fluctuations over the year owing to the cow's stage of lactation and nutritional factors (O'Keeffe, 1984; Macheboeuf et al. 1993). These variations often result in seasonal differences in the chemical or sensory properties of the cheese (Kefford et al. 1995) which are sometimes difficult to understand (Grandison et al. 1985). Only a few studies have assessed the cheesemaking quality of milk by actually making ripened cheese; most studies report milk coagulation properties and curd firmness.

The object of this work was to study the influence of the chemical composition and clotting characteristics of the milk on the chemical and sensory properties of ripened cheeses when milk characteristics vary widely.

The detergents sodium dodecyl sulphate and octyl phenoxy polyethoxyethanol interact with casein and cause dissociation of the high-molecular-weight casein aggregates. It is presumed that the detergent binds with hydrophobic regions in the casein molecule. The size of the complexes formed between detergents and αs1-casein, β-casein and κ-casein, as estimated by gel filtration and sedimentation velocity experiments, suggests that the caseins were complexed as monomers.

During gel filtration under non-reducing conditions, detergent-κ-casein complexes were separated from other major components because of their conversion through formation of disulphide bonds into high-molecular-weight aggregates. This reaction, which did not occur in sedimentation velocity experiments, was presumably facilitated by the changes in the equilibrium between the individual caseins during gel filtration.

Sedimentation velocity experiments showed that a ratio of about 40 detergent molecules to 1 casein molecule was required to give the smallest casein-detergent complex.

Treatment of food proteins with heat and/or alkali may convert some of the constituent amino acids into the unnatural amino acid lysinoalanine (LAL), which has been found to cause kidney damage (nephrocytomegaly) when fed to rats. LAL may be produced by β-elimination of a phosphoseryl, of a glycosylated seryl or of a cystyl residue followed by a nucleophilic addition to a lysyl residue. The formation of LAL in αs0-, αsl- and β-casein is discussed in relation to the location of phosphoseryl residues immediately adjacent to lysyl residues. The influence of pH, temperature and exposure time on the formation of LAL in whole casein is described in detail. The formation of LAL can be prevented by the addition of reducing agents, dephosphorylation or chemical modification of the є-amino group of lysine. The LAL content of foodstuffs can be measured accurately by ion-exchange chromatography and a survey is given of the LAL content of commercial milk and milk products.

The biological effects of free and protein-bound LAL are discussed. It seems that renal changes due to the presence of LAL in the diet are species-specific to the rat although recent findings have shown that no effect is observed in rats fed a mixed diet containing proteins with and without protein-bound LAL. When LAL is removed from the diet, nephrocytomegaly disappears. From these results it may be concluded that the health risks for man, connected with the eating of proteins which have been subjected to treatment with heat and/or alkali, are small.

The decrease in lysine content of milk and milk products due to formation of LAL is small (0–1 or 2%) in comparison with the decrease in available lysine possibly caused by the Maillard reaction (0–24%) and so the decrease in nutritive value of food products due to LAL formation may be neglected.

With either Na caseinate or skim-milk powder, 1% (w/v) additions of pepsin, chymotrypsin or Alcalase and an incubation time of 24 h at 37°C gave the best hydrolysates for subsequent plastein production. In spite of the fact that peptide species which were produced by treatment with the 3 enzymes were entirely different, they were all capable of producing qualitatively similar amounts of plastein when concentrated and further incubated with proteinase, suggesting that the identity of peptide components was relatively unimportant. αs1-, β- and κ- caseins, Na-caseinate and skim-milk powder all led to plastein products with broadly similar properties, confirming the comparative unimportance of peptide composition. This also indicated that for most practical applications fractionation of initial protein mixtures (as in many food products) would not be justified. Optimum peptide concentration for plastein formation with pepsin and caseinate hydrolysates was 20–35% (w/v) with an optimum pH range of 4–5; the best peptide molecular weight varied between 400 and 800 and the optimum temperature was either 37°C for 24 h or 50°C for 4–6 h. Higher temperatures (70°C) gave more rapid plastein formation but poorer yields. Lower temperatures (e.g. 20°C) gave similar yields, but incubation times required to be extended to at least 48 h. The same proteinase should be used for plastein synthesis as that used in the initial hydrolysis stage, or else further hydrolysis (over hydrolysis) could occur during the plastein formation stage causing lower yields due to differences in the specificity of the 2 enzymes. Even under ideal conditions plastein yields never exceeded 27% of the weight of peptide taken and at the present time it seems unlikely that the plastein reaction will have anything more than curiosity value. Economics must preclude its application in low-cost, high volume situations such as generally exist within the food industry.

Eight cows in mid lactation and receiving a diet of perennial ryegrass silage and barley (70:30 on a DM basis) were given intravenous supplements of l-methionine (8 g/d). The methionine treatment had no significant effect on milk yield, protein content or lactose content, but increased milk fat content and yield by approximately 10% (P < 0·05).

The influence of lactic starter inoculation on La Serena cheese characteristics was investigated in duplicate experiments, each consisting of a vat inoculated with 1% Lactococcus lactis subsp. lactis skim milk culture and a control vat without lactic starter. Lc. lactis inoculation achieved a significant pH decrease in the interior and on the surface of cheese, which ranged from 0·09 to 0·25 units throughout ripening. Breakdown of αs1- and β-casein and formation of soluble N fractions were retarded by Lc. lactis inoculation. Levels of αs1-casein, β-casein and pH 4·6-soluble N after 60 d were respectively 30·4, 34·9 and 28·8% in cheese made with starter and 18·4, 23·0 and 34·2% in control cheese. More acidic conditions in the interior of cheese made with starter resulted in a reduction of Enterobacteriaceae, coliform and staphylococci counts, which after 60 d were respectively 2·55, 2·40 and 0·29 log units lower than in control cheese. The firmer texture obtained throughout maturation for cheese made with starter was attributed to retarded proteolysis and lower pH value. Cheese flavour quality and intensity were significantly impaired by Lc. lactis inoculation.

1. Pasteurized, sterilized, U.H.T. treated, U.H.T. treated-sterilized, evaporated, sweetened condensed, roller- and spray-dried milks were prepared simultaneously from one bulk of raw whole milk.

2. The preparation of any of these products entailed little or no apparent loss of riboflavin, pantothenic acid, nicotinic acid, vitamin B6 or biotin.

3. Thiamine was destroyed in amounts ranging from less than 10% to 40–50%, increasing with the severity of the heat treatment.

4. Prolonged high temperature used in the sterilization of evaporated milk and of in-bottle sterilized milk caused nearly complete loss of vitamin B12.

5. The new process of sterilization by the U.H.T. treatment alone when it was not followed by the, so far necessary, in-bottle sterilization caused little more loss of the vitamins measured than did H.T.S.T. pasteurization.

The effects of the isocaloric replacement of starch in a low-fat concentrate mixture by 8% soybean oil on the yield and composition of milk fat were investigated in a feeding experiment with 6 cows in mid lactation. Two high-fat concentrate mixtures were given to the cows. In one the requisite amount of soybean oil was given by including 50% coarsely ground soybeans in the concentrate mixture. In the other an equal amount of soybean oil was added directly to the concentrate mixture. The concentrate mixtures were given with a high-roughage diet that supplied 5·5kg hay and 2·7kg of sugar-beet pulp/day.

When the 2 high-fat rations were given to the cows there were reductions in the percentages and yields of 10:0, 12:0, 14:0, 14:1, 16:0 and 16:1 fatty acids and increases in the concentrations and yields of 18:0, 18:1 and 18:2 fatty acids in the milk fat. The direct addition of the soybean oil to the diet also reduced the percentage and yield of 6:0 and 8:0 whilst the inclusion of the soybeans increased the percentage of 4:0 and the yields of 4: 0 and 6:0 in the milk fat.

When both high-fat rations were given to the cows there were increases in the yields and concentrations of as-9–18:1 in the milk fat, whilst inclusion of the soybean oil in the diet increased also the concentration and yield of trans-11–18:1.

The implications of these findings are discussed.

κ-Casein was purified from a single batch of whole acid casein (κ-A variant) using different methods in order to compare their merits in producing a purified material with a carbohydrate and phosphate heterogeneity representative of the whole κ-casein complement in milk. Ion-exchange methods of purification gave products of higher purity than precipitation techniques involving final purification by ethanol fractionation, but all methods resulted in κ-caseins of apparently similar heterogeneity and chemical composition. The purified κ-caseins were hydrolysed with chymosin and the derived macropeptides isolated. These were all virtually identical as determined by reversed-phase chromatography and gel electrophoresis. Some observations on chymosin hydrolysis of κ-casein were made. In addition to formation of the major para-κ-casein (Glu1–Phe105) and macropeptide (Met106–Val169), chymosin hydrolysis at pH 6·6 also resulted in two minor para-κ-caseins with N-termini corresponding to Phe18 and Ser33 of κ-casein. At pH 5·5 and 4·5 para-κ-casein was rapidly hydrolysed into at least six fragments, one of which had an N-terminus corresponding to Trp76 of κ-casein. At pH 6·6, 5·5 and 4·5 the κ-casein macropeptide was stable to chymosin, but at pH 2·3 it was hydrolysed by chymosin into fragments with N-termini corresponding to Met106, He125, Ala138, Val139, Thr145 and Glu147 of κ-casein.

Microstructural and textural characteristics of rennet curd made from rehydrated non-fat dried milk standardized to 40 g fat/l were compared with those of non-fat curd made with capsule-forming non-ropy lactic culture. Microstructure examination using confocal scanning laser microscopy indicated that the presence of a capsule-forming culture produced an open casein network with large pores. Distribution of fat globules was observed within the undisturbed casein structure after staining with a fluorescent lipophilic dye which was added to the milk before coagulation. Bacterial capsules and milk fat reduced curd tension and firmness of rennet curd to similar levels.

Xanthine oxidoreductase (XOR) was purified from goats' milk. The u.v.–visible absorption spectrum was essentially identical to those of the corresponding bovine and human milk enzymes and showed an A280/A450 ratio of 5·20±0·12, indicating a high degree of purity. Like bovine and human milk XORs, enzyme purified from goats' milk showed a single band on SDS-PAGE corresponding to a subunit with approximate Mr 150000. On Western blotting, mouse monoclonal anti-human XOR antibody cross-reacted with purified caprine and bovine XORs. The specific xanthine oxidase activity of goats' milk XOR, however, was very much lower than that of bovine XOR, although NADH oxidase activities of XOR from the two sources were similar. In these respects, the caprine milk XOR mirrors the human milk enzyme, in which case the kinetic effects have previously been attributed to relatively low molybdenum content. The molybdenum content of goats' milk XOR also was shown to be relatively low, with 0·09 atoms Mo per subunit, compared with 0·55 atoms Mo per subunit for the bovine enzyme. A parallel purification of human milk XOR showed 0·03 atoms Mo per subunit. The possible physiological significance of the low molybdenum content of the caprine milk enzyme and of its correspondingly low enzymic activity is discussed.

Clones of Streptococcus lactis and of Str. cremoris resistant to some bacteriophages that could virtually annihilate the parent strains were selected by plating very large numbers of the bacteria in the presence of the phage. Some of the resistant strains were shown to form acid rapidly in milk and to be satisfactory for cheese-making. They were not lysogenic. Adsorption of the phage to which the parent culture had been sensitive still occurred with 3 strains but not with 4 others. Rates of mutation from phage sensitivity to phage resistance were determined for 2 original strains of Str. lactis and were found to be of the same order as the rates quoted in the literature for other mutations. The mutation rates of phages to forms which could attack the resistant strains were too low to be measured.