Pituitary-derived bovine growth hormone (bGH) was administered to six lactating Friesland ewes for 7 d. There was no consistent galactopoietic response, with changes in milk yield varying from 0 to 33% during treatment compared with the pretreatment period. The major effect of bGH on the concentration of milk constituents was to increase fat by 14·2% (P < 0·05). Treatment resulted in significant increases in arterial plasma concentrations of growth hormone, insulin-like growth factor I and glucose, with decreases in the plasma arterial concentrations of acetate and certain amino acids. There was a marked reduction in haematocrit and in haemoglobin concentration which took at least 3 d to recover. The arterio-venous difference across the mammary gland decreased for O2 during treatment and the veno-arterial difference for CO2 decreased after treatment. Mammary respiratory quotient therefore decreased significantly after bGH treatment. The results suggest that bGH exerts effects at a number of separate loci.

Five lactating goats which had kidded normally in March were mated during seasonal anoestrus in May, at the time of peak milk production, after ovulation had been induced using gonadotrophin-releasing hormone (Knight et al. 1988). Milk yield was unaffected by the hormone treatment, and decreased at the same rate as that of control (non-pregnant) goats for the first 8 weeks of the pregnancy. Thereafter yield declined more quickly in the test goats and just before parturition (in October) was 57% of the control value. Following parturition in the test animals, yield rose rapidly as the second lactation was established. None became ‘dry’ at any stage. Yield continued to decline with advancing lactation in the controls, which were mated normally in October or November and dried-off in December. During their second (‘extra’) lactation in the winter the test animals produced 12% less than in a normal second lactation in summertime; during the year the extra lactation meant that the test animals produced 73% more milk than the controls. In some, a second concurrent pregnancy was established during the extra lactation, with the resuit that three lactations were obtained in the time normally taken for two. Mammary cell number and proliferation rate were both higher in the pregnant animals than in the controls in week 23 of the first lactation.

An experimental non-return valve was fitted in the short milk tube so that flow was permitted only in one direction, away from the teat. Liner movement was minimal because no air admission to the liner above the valve was provided. The extent of liner opening therefore depended on the amount of milk extracted within a pulsation cycle and on any air leakage past the liner mouthpiece. Milking experiments using four cows showed that the mean vacuum level in nominally open liners of valved teatcups during peak milk flow was 8·6 kPa higher than in conventional teatcups. Bench tests showed that the increase in liner vacuum (LV) depended on the nominal plant vacuum level, the pressure difference between initial LV and maximum pulsation chamber vacuum, the liner elasticity and mounting tension. The main source of energy available to raise LV above the nominal plant vacuum is the release of strain energy, stored within the liner during the collapse phase of each pulsation cycle.

The milking characteristics of conventional clusters were compared with individual teatcups or clusters fitted with valves in the short milk tubes without external air admission to the liners. Individual teatcups fitted with non-return valves had peak milk flow rates 13% higher than conventional teatcups. A daily cross-over experiment involving 36 cows compared clusters fitted with non-return valves to conventional clusters. Valved clusters milked 18% faster, showed significantly better teatcup stability and 3% higher machine milk yields, but 48% higher strip yields were recorded. Clusters with non-return valves were difficult to remove after milking because the valve closed when the vacuum supply was shut off, thereby maintaining the vacuum level under the teats. When the nominal plant vacuum level for teatcups with non-return valves was reduced by 10 kPa to compensate for the higher effective milking vacuum level, the valved teatcups had peak flow rates 20% lower than the conventional teatcups. Clusters fitted with a modified valve, which allowed some reverse flow when closed (a ‘leaky’ valve), milked significantly faster than conventional clusters, had 33% lower strip yields and were easy to remove after milking.

The partial pressure of O2 in milk from normal cows and from cows with mastitis was measured and the concentrations of O2 calculated. Oxygen levels of milk from normal cows were similar to those in venous plasma, but inflammation of the mammary gland led to a dramatic drop in O2 concentration to < 10% of control values. Intracellular survival of Staphylococcus aureus strain M60 in bovine neutrophils was greater under anaerobic than aerobic conditions. The implications of low O2 concentrations in milk from infected mammary glands for the bactericidal activity of bovine neutrophils is discussed.

A detailed calculation of the growth of molecular weight during the renneting of milk is given, based on a first-order breakdown of κ-casein followed by development of instability caused either by a decrease in the intermicellar repulsive potential or by the formation of holes in the stabilizing surface layer of the micelles. Unlike most of the models which have been described, this model allows a complete analytical solution. The solution is, however, complex and difficult to use simply, although it is shown that the calculations are in accord with experimental observations of the dependence of the coagulation process upon the enzyme concentration and the concentration of the milk. The calculations are also compared with those from other models of the reaction.

The dissociation of caseins of different types from casein micelles in milk, acidified to different pH values in the range 4·9–6·7, and at temperatures of 4, 20 and 30 °C, has been studied. In contrast to a number of previous findings, it was shown that caseins of all types were dissociated from the micelles, although in all cases β-casein was in highest concentration. The amounts and proportions of all of the caseins were found to be pH- and temperature-dependent, especially the former. Studies of the proportions of the different caseins liberated suggested that, at a defined temperature, the proportions of κ;- and αs2-caseins were independent of pH, while the proportions of β- and αsl-caseins were variable, changes in one being compensated by changes in the other. The manner in which the proportions of the αsl-casein and β-caseins changes with pH was found to be temperature-dependent.

The effect of the covalent binding of reducing sugars such as galactose, glucose, fructose, lactose and maltose on the flow properties of casein solutions and on the voluminosity of casein molecules was investigated, voluminosity being calculated from viscosity measurements. Rheological parameters appeared to be efficient indices of structural changes occurring in proteins as a result of chemical modifications. Results showed an increase in voluminosity of casein after the binding of sugars, possibly explained by an increase in the net negative charge or by an increase in the steric hindrance of the molecule. At high concentrations (above 0·03 g/ml), viscosity of the solutions depended on the nature of the attached sugar and on the level of the modification. Thus galactose- and glucose-modified caseins were more viscous than the control. With disaccharides, the level of modification appeared to be more important than the amount of sugar bound.

A peptic hydrolysate of proteinaceous material of whey from Mozzarella cheese manufacture was incubated with α-chymotrypsin under conditions leading to the formation of plastein products. The influence of incubation time, concentration and molecular size of substrate was studied. The most favourable conditions for the plastein synthesis were 12 h incubation time and 35% (w/v) concentration of peptides having mol. wt in the range 451–1450.

Deposit formation was measured in a model laboratory plant in which whole milk was in contact with a heated surface at 100 °C for 1 h. The effect of the interfacial properties of various poly mer-coated surfaces on the amount and the adhesion strength of deposit was determined. The nature of the surface influenced the formation of deposit only slightly, but had a large effect on its adhesion strength. From correlation analysis, the polar contribution to surface energy was identified as the main factor influencing the deposit adhesion strength. These results suggest that the type of interactions at the surface govern the ease of removal of deposit.

The influence of preheating intensity was examined on deposit formation in an indirect ultra high temperature (UHT) plate heat exchanger and on some quality parameters of the UHT treated milk, such as taste, proteinase inactivation and protein destabilization during storage. Response surface methods were used to define the relationship between the forewarming conditions and the responses. More intense forewarming reduced the fouling rate in the UHT plate section, resulting in longer operation times. While a greater inactivation of the proteinase(s) in the milk was obtained, the taste of the UHT milk was adversely affected, as well as the physical stability during storage owing to higher sedimentation. An optimum temperature/time range for the preheating process could be defined within the temperature/time range of 76–80 °C/40–70 s at which adverse effects were minimal, and positive effects were maximized.

Continuous production of concentrated cells of Streptococcus salivarius subsp. thermophilus was achieved in a continuous stirred tank reactor coupled with an ultrafiltration module. A cellular productivity nine times higher than that obtained by conventional methods was observed. The viability of the cells produced was as satisfactory as that obtained by classical culture, but an increase in latency time and a slight decrease in acidification rate during downstream tests in milk were observed. Specific lactic acid productivity decreased as biomass increased. This process produced concentrated starters and lactic acid which could be purified by a downstream processing such as electrodialysis.

Different selective agar media were compared for the recovery and isolation of five species of Listeria from raw milk and cheese. The selective media examined were Beerens medium, MacBride medium and that described by Dominguez et al. (1984) with 6 mg/1 acriflavine, listeria selective agar medium (LSAM), and LSAM with 12 mg/1 acriflavine (LSAM × 2A); a non-selective yeast glucose Lemco agar was included for comparison. When the difference between listeria and the natural microflora of raw milk and cheese was 102 cfu/ml, listeria could be isolated by direct plating on all media tested. When it was lower than 103–104 cfu/ml, listeria were isolated by direct plating only on LSAM and LSAM × 2A. When the difference was greater than 104 cfu/ml, a previous enrichment was necessary to isolate them. LSAM and LSAM × 2A media performed better than the other media tested for isolating listeria by direct plating and improved their isolation from dairy products. This superior performance was evaluated by the ability of these media to support colony formation of different species of Listeria tested, the easy recognition of these colonies from those formed by other microorganisms and by their capacity to inhibit the natural microflora of these foods.

A simple, rapid and sensitive spectrophotometric assay was developed and evaluated for monitoring proteolysis during cheese ripening, based on the fact that α-amino groups released by hydrolysis of cheese proteins react with trinitrobenzenesulphonic acid to form products that absorb strongly at 420 nm. A linear relationship was shown to exist between A420 and concentration of free α amino groups up to 0·5 HIM (r = 0·999, 38 df, P < 0·001). Repeatability of the method was satisfactory. The coefficient of variance was 0·53% for amino acid solutions and 1·19% for cheese extracts. Average recovery of glycine added to the cheese was 104 ± 2·9%. A comparison of the above method with that of determination of water-soluble N to total N ratio showed that there was good agreement between these two methods of assessment of proteolysis in cheese (r = 0·857, 32 df, P < 0·001). Mainly Feta and Teleme cheese were examined, but a similar correlation was obtained with hard Greek cheeses. Analytical conditions of the procedure are discussed.

Effects of the addition of a proteinase (Neutrase 1–5S) and a peptidase (aminopeptidase DP-102) as agents for accelerating the ripening of Cheddar cheese on the survival of some non-starter bacteria (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli and a Salmonella sp.) were studied throughout a 4-month ripening period. The enzymes were found to have no significant effect on the survival of the Gram-positive bacteria but some significant effects were observed, at some stages of the ripening period, with the Gram-negative bacteria in that lower levels were recovered from cheeses treated with the enzyme system.

Infections of the bovine mammary gland are considered to be ascending infections through the teat canal (Forbes & Hebert, 1968). With respect to skin and wound infections, staphylococci in particular adhere to and colonize traumatized areas (Mertz et al. 1987). Staphylococcus aureus and coagulase-negative staphylococci can colonize the teat canal for weeks before they invade upper parts of the mammary gland (Forbes & Hebert, 1968). Such teat canal infections predispose the mammary gland to mastitic infections, particularly with the so-called ‘contagious’ mastitis pathogens, namely Staph. aureus, Streptococcus agalactiae and Str. dysgalactiae.