The effect of vacuum fluctuations during milking on the somatic cell count of milk from cows free of intramammary infections was determined. Four vacuum fluctuation treatments were compared using a Latin square design with 20 cows being milked at each of the 4 treatments for a continuous period of 12 d. The vacuum fluctuations were designed to simulate the depressions in vacuum that occur when applying teatcups during milking. In the most severe treatment the vacuum level fell from 50 to 14 kPa. The somatic cell count was not affected by vacuum fluctuation treatment and it is concluded that vacuum fluctuations will not cause an increase in the somatic cell count of milk in the absence of mastitis infections.

The effect of milking vacuum levels and overmilking on the somatic cell count of milk from cows free of intramammary infections was determined. In the first experiment, 4 levels of vacuum ranging from 35 to 70 kPa were compared in a Latin square design with 20 cows being milked at each of the 4 vacuum treatments for periods of 19 d. In the second experiment 40 cows were milked for 16 weeks at either 50 or 70 kPa with or without 5 min overmilking. Neither vacuum level nor overmilking affected somatic cell count and it is concluded that these factors do not cause stress or irritation that will lead to an increase in somatic cell count in the absence of mastitis infections.

The effect of pulsator rates on the somatic cell count of milk from cows free of intramammary infections was determined. Four pulsator rates ranging from 30 to 120 cycles/min were compared in a Latin square design with 20 cows being milked at each of the 4 pulsator rates for periods of 19 d. Somatic cell count was not affected by any of the pulsator rates and it is concluded that varying pulsator rates will not cause stress or irritation that will lead to an increase in somatic cell count in the absence of mastitis infections.

The effect of different methods of teat washing on bacterial contamination and sediment levels of the milk of cows kept under 3 housing conditions was studied. By use of an in line milk sampler and steam sterilized clusters the bacterial contamination from the teats after udder preparation was determined directly.

The results show that, even under the same housing conditions and teat washing treatments, considerable variation in bacterial counts and sediment level occurred. Overall, washing with hypochlorite (600 mg/1 available Cl) and drying with a paper towel gave total counts that were significantly lower than were obtained with the other 4 treatments. The aerobic spore count, in which thermoduric flora predominated, was greatly reduced by drying the teats, but the inclusion of hypochlorite in the wash water had little effect. Coliform contamination was very low (∼ 1/ml) even where the cows' teats were heavily soiled and were not washed.

The sediment levels in the milk again showed wide variation and occasional high values (3 mg/l) were recorded for washed cows. In marked contrast to its effect on the bacteriological results, drying had little effect on reducing the sediment.

Samples of winter bedding and summer pasture, rinses of teats and milking equipment, and milk samples from bulk tanks from 4 farms were analysed bacteriologically for total count (TC), thermoduric count, total spore count (TSC) and psychrotrophic spore count (PSC).

In bedding and pasture samples the TSC comprised the main component of the thermoduric flora but was less than 0·1 % of the TC. The PSC formed ∼ 1 % of the TSC in winter bedding and ∼23 % of the TSC in pasture. For pasture, the PSC: TSC ratio was highest for the upper layer of soil and lower for the grass and deeper layer of soil. The TSC formed 5% of the thermoduric flora in rinses of milking equipment and only a very low PSC was obtained. The TSC of farm bulk tank milks was higher in the winter than the summer but the PSC was similar in both seasons. This was due to an increase of ∼9 times in the PSC:TSC ratio for milks in summer compared with winter. The PSC in raw and commercially pasteurized milks from a local creamery showed a similar seasonal trend.

A metalloproteinase, isolated from a shaken milk culture of Pseudomonas fluorescens AFT 36 by chromatography in DEAE and CM-cellulose and Sephadex G-150, was very unstable in 0·1 m-phosphate buffer, pH 6·6, being completely denatured above 70 °C in 1 min. It was also unstable in a Ca-containing buffer (synthetic milk salts, SMS) between 50 and 60 °C (minimum at 55 °C), but stability was very high above 80 °C in this buffer. D-values were determined at 10 °C intervals in the range 70–150 °C in SMS from which a Z value of 31·9 °C and an Ea of 8·82 × 104 J mol-1 were calculated; the half-life at 150 °C was 9 s.

Instability at 55 °C was due to autolysis as evidenced by gel electrophoresis, gel filtration and increase in 2,4,6-trinitrobenzenesulphonic acid-reactive amino groups. The extent of inactivation experienced at 80 °C was inversely related to the rate of heating to 80 °C, i.e. length of time spent in the neighbourhood of 55 °C. Addition of increasing concentrations of caseinate substrate reduced inactivation of the enzyme at 55 °C, presumably due to substrate binding. Attempts to stabilize the enzyme at 55 °C by addition of EDTA or by adjusting the reaction pH to 4·2, at which the enzyme has little proteolytic activity, were unsuccessful, although autolysis was prevented.

Unlike the proteinase from Ps. fluorescens MC 60, AFT 36 proteinase did not inactivate itself on cooling to 55 °C from 80, 100 or 150 °C, but did regain autolytic activity on cooling to below 50 °C to an extent dependent on the duration of holding at the lower temperature. It is suggested that on heating to ∼ 55 °C, a conformational change occurs which renders the enzyme susceptible to proteolysis by still active enzyme; at higher temperatures the enzyme, although susceptible to autolysis, is inactive; an active conformation is restored on cooling to below 50 °C.

Using the Charm radioactive assay technique as a rapid test for the determination of penicillin in milk in experiments with milks containing known concentrations of penicillin, the coefficient of variation was found to be 15 and 11% for raw whole and reconstituted skim-milks respectively. Storage of reagents for up to 4 d increased the variance of the test by more than 20% and the estimated penicillin concentration 0·4 %/d of storage up to 25d. In comparison with the Bromocresol Purple Test, which was capable of 100% accuracy in defining a threshold of 0·02 i.u. penicillin/ml milk, the Charm Test falsely classified 15·6% of the samples.

The dynamic viscosities of non-homogenized and homogenized (2-stage) whole and half skim-milk and cream, of unhomogenized skim-milk and of water-diluted milks were measured at temperatures between 70 and 135 °C; this range includes those temperatures used in ultra-high-temperature (UHT) treatments. A tube viscometer was used at the outlet of a continuous flow indirect heater. Above 85 °C the mean residence time of a milk or cream at the test temperature was 3–6 s.

An equation relating viscosity μ and temperature θ of the form In μ = aθ2+bθ+c was derived for each product.

It proved possible to relate μ, θ and fat content (0·03–15·5%) in a single equation which was appropriate for all the homogenized milks and creams as well as the non-homogenized skim-, half skim- and whole milk; the equation is considered valid for these products under conditions of indirect UHT sterilization. Viscosity values calculated with the equation, even those relating to temperatures as low as 50 °C, were comparable with those reported in the literature.

The concentrations of dihydroxyacetone phosphate (DHAP), glycerol- 3-phosphate (GP), glucose and alkylglycerols were measured in bovine colostrum and milk. Concentrations of GP were higher in colostrum than in milk, and corresponded to intracellular concentrations at the onset of lactation. This suggests that GP equilibrates over the apical membrane at onset of lactation, which is believed to occur also in full lactation. The ratio of DHAP to GP showed a peak in colostrum that was more than twice that in milk, indicating that concentration changes in colostrum/milk were not caused by dilution. DHAP was positively correlated to metabolical compounds such as glucose and alkylglycerols. As DHAP is a required precursor for alkylglycerol synthesis, the positive correlation suggests that DHAP may be limiting for alkylglycerol synthesis and thus could explain the elevated levels of alkylglycerols found in colostrum.

In order to examine the influence of micelle size on the formation of soluble casein brought about by heating concentrated milk, casein micelles were separated into 3 fractions by differential centrifugation. The micelles were dispersed in the ultrafiltrate of skim-milk and the preparations were concentrated to 1/2·5 of the original volume. When the concentrated milk samples were heated at 135–140 °C, a larger amount of soluble casein was formed from smaller micelles than from larger micelles. Only a slight effect on the formation of soluble casein on heating was observed when colloidal calcium phosphate (CCP) was removed from the large micelles to a level lower than that associated with the small micelles or when the CCP in the small micelles was increased to a level higher than that of the large micelles. The soluble caseins formed on heating from different sizes of micelles were similar in composition. The fact that the formation of soluble casein on heating was affected by micelle size may be ascribed to variation in the κ-casein content of micelles rather than to variation in CCP content.

The detailed chemical composition of 25 milks from different breeds of cow, sheep and goat were related to their properties of rennet clotting, coagulum development and syneresis at pH 6·4. Experiments in which concentrations of fat and whey proteins in milk were manipulated, and in which milks were homogenized at different pressures, were also carried out and the effects observed were related to the above processes.

The composition of milks varied widely and many relations between concentrations of components could be related to their known modes of secretion from the alveolus or to their structural functions in the colloidal phase of milk. Rennet clotting was related to total Ca concentration and also to the proportions of αs- and β-caseins present. Coagulum strength was strongly related to total casein concentration in milk but was unaffected by total fat content or by casein/fat ratios. Syneresis was inversely related to fat content of milk and little affected by coagulum strength. Homogenization retarded syneresis, probably by altering coagulum structure and modifying entrapment of fat globules within the coagulum.

The effects of thermization (a sub-pasteurization heat treatment), followed by cold storage at 7 °C for up to 3 d on the post-gelation behaviour of rennet-induced gels from alternate-day collected milks, pasteurized before testing, were investigated. Observations of the time course of gel rigidity modulus development and of the syneresis of cut curds were fitted to a general exponential relationship to aid interpretation. Milk performance was assessed by measurements of rennet coagulation time at pH 6·5, various gel rigidity and syneresis parameters and losses of protein and Ca to the whey. A small but significant reduction in protein losses to the whey (mean 0·05%) was the only effect resulting from thermization. Cold storage gave rise to significantly weaker gels than those from unstored controls. The gels from cold-stored milks attained 50% of their maximum rigidity modulus in significantly less time and lost significantly more protein (mean 0·11%) to the whey. Samples cold stored for 1, 2 or 3 d were not significantly different from each other and no other parameters were affected. No significant interactions between thermization and cold storage were found.

A new method was developed for making Domiati cheese, based on blending ultrafiltered reconstituted milk and lipolysed recombined cream. The chemical composition of the new product was similar to the traditional cheese. The flavour intensity increased with the amount of lipase used in the cream hydrolysis. During storage the acidity and tyrosine content of cheese from different treatments increased at nearly similar rates. The level of total volatile fatty acids was affected by the level of lipase used. Organoleptic scoring showed that cheese made with cream treated with the intermediate levels of lipase (2 or 5 g/500 g cream) ranked the highest, after 1 and 4 months of storage respectively.