An accurate and reproducible technique involving a continuous aqueous elution from a bed of dried-milk powder was developed to evaluate reconstitution characteristics. Quantitative data for the determination of reconstitution behaviour were obtained by analysis of the eluate for protein, lactose and Ca constituents. A reconstitution coefficient (P) was derived from the formula (W1+ΣWi)/T, where W1 was the amount reconstituted in the first eluate fraction, ΣWi the total amount reconstituted in 8 fractions, and T the time taken for 1/3 of the constituent to be reconstituted. Values obtained for P were thus related to the reconstitution properties and could be used to grade the powders. A freeze-dried skim-milk gave the ideal reconstitution behaviour with W1 values in the range 42·0–44·2, ΣWi values in the range 93·6–101·3, T values in the range 3·0–3·3 and reconstitution coefficients in the range 41·1–46·3 for the 3 constituents. All the other preparations tested gave lower values for W1 and P, although some gave ΣWi values in the same range, whilst all T values were higher than those for the freeze-dried sample.

Equipment and methods are described for the production, on the laboratory scale, of frozen concentrated cheese starters. A single-stage Porton-type fermenter with a working volume of 3–5 l was used for the continuous culture of the Streptococcus lactis and Str. cremoris starter strains. The cells grown in a trypsin-digested cheese-whey or trypsin-digested skim-milk, both containing autolysed yeast, were harvested with a Sharples laboratory super-centrifuge, resuspended in skim-milk and layer-frozen in liquid N2. The frozen culture was crushed to a granular free-flowing form facilitating direct addition to and ready dispersion in cheese milk. The cultures were stored at −196°C.

Frozen concentrated cheese starters were prepared from 7 Streptococcus cremoris and 2 Str. lactis strains by continuous culture. pH control with NH4OH and Ca(OH)2 increased the yield of Str. cremoris strains as compared with NaOH, but the resultant cells were smaller and less active. Maximum yields for both species were obtained at a dilution rate of 0·6 and pH 6·3, but with Str. cremoris cell activity was reduced above pH 6·0.

Degeneration in continuous culture was observed in the 2 Str. lactis strains, due apparently in one case to the action of a bacteriophage and in the other possibly to selection or to loss of adaptive enzymes.

Concentrated starters retained full viability and activity when stored for 10–12 weeks at −110°C, and 94% activity after storage for 4 weeks at −40°C.

Cheese made with the concentrated starters ripened normally and was comparable in quality with control cheese made with normal bulk starter.

Ultra-high-temperature (UHT)-treated milks containing a range of dissolved O2 levels were prepared using both direct and indirect UHT processes. The O2 content of directly processed UHT milk had a significant influence on the changes in the level of cooked flavour in the product during storage over a 12-week period. At storage temperatures of 2 and 20°C, a noticeable improvement in flavour could be achieved by O2 control. In general, samples with a head-space sufficient to yield an initial of 60–100 mm were most preferred. The effect of O2 control on samples stored at 38°C was only minor when compared with the marked decrease in acceptability of flavour occurring due to the high storage temperature.

Directly processed samples stored at 20°C all gelled at 14 weeks while those stored at 2°C showed no evidence of gelation after 36 weeks. Storage at 38°C gave rapid flavour deterioration, bitterness at 7 weeks and gelation at 10 weeks.

All indirectly processed samples were found to be severely cooked under the conditions employed, and any differences in flavour due to differing O2 contents were masked by the intensity of this flavour.

For radiographic studies of the teat and teatcup liner during milking, quarters were infused with a suspension of propyliodone in milk. Simultaneous recordings of vacuum within the liner, in the mouthpiece chamber and in the pulsation chamber, and recordings of liner-wall movement, were made in conjunction with the radiographs.

The studies indicated that the sudden decline in milk flow-rate at the end of the period of peak flow-rate from each quarter, and the concurrent changes in the appearance and position of the teat in the liner, result from a fall in teat sinus pressure. This fall in pressure follows the partial closing of the milk passageway between the teat and udder sinuses. The changes initiating the closing of this connexion seem to occur above the teatcup and are associated with the declining amount of milk in the udder.

The teat is elongated by 33–50% as it enters the liner and further stretching of the skin of the teat throughout milking is negligible. Thus, increase in the depth of penetration of the teat into the liner is due to more of the teat entering the top of the liner.

The force exerted on the teat by the closed liner, which is greatest near the end of the teat, usually appears to increase near the end of milking. The force acting against the inner surface of the open liner barrel is greatest during the peak flow-rate period and it appears to be fairly uniform over most of the area of contact.

Four studies are described of the role of friction in maintaining the teatcup stable on the teat. Measurements of the coefficient of friction between teats and pieces of liners, in which most values for the coefficient fell between 0·5 and 1·0, indicated that friction between skin and rubber-like materials was consistent with the general frictional behaviour of elastic solids. Studies during milking showed that the sudden restriction of milk flow that normally occurs near the end of milking is accompanied by a marked fall in the frictional force between the teat and barrel of the open liner. During the period of peak milk flow-rate, the major source of friction maintaining the teatcup stable on the teat is the large area of contact between the teat and liner barrel. The frictional force is derived from the pressure difference across the teat wall which presses the teat against the comparatively rigid liner. Frictional force between the teat and barrel increases after the start of milking because the coefficient of friction rises as one surface gradually moulds to the other. In addition, the total frictional force increases because of the increasing area of contact whenever the teat moves deeper into the liner, until the end of the peak flow-rate period. When this period ends, friction between the teat and open barrel is reduced suddenly because the teat sinus pressure falls. After this stage, the main source of friction appears to be derived from the force between the teat and mouthpiece lip.

The fatty-acid composition of 17 samples of New Zealand milk fat obtained throughout one dairy season is reported.

The weight percentage of butyric (C4:0) acid was significantly correlated with that of caproic (C6:0) acid and that of caprylic (C8:0) acid. Percentages of C6:0 and C8:0 showed a highly significant correlation with each other and with weight percentages of capric (C10:0) and lauric (C12:0) acids.

There was a highly significant negative correlation between palmitic (C16:0) and stearic (C18:0) acids and between C4:0 and C16:0 acids. Oleic (C18:1) acid showed significant negative correlations with C8:0, C10:0, C12:0, myristic (C14:0) and C16:0 acids.

When heat treatments of the same sporicidal effectiveness were given, directly heated ultra-high-temperature sterilized milk gave twice as much sediment as indirectly heated milk after storage at room temperature for 100 days. Both types of process reduced the rate of clotting of the milk with pepsin and rennin, but the effect of the indirect process was markedly greater than that of the direct process.

A study was made, using Holstein cows, of the relationship between (1) the dose of oxytocin administered; (2) the time between milking and intravenous (i.v.) injection and (3) the time between injection and withdrawal of residual milk. Effectiveness of each dose for removal of residual milk was expressed as the percentage of residual milk in the quarter which was withdrawn by a test milking. The total amount of milk in the quarter was determined by giving a large dose of oxytocin and a second test milking, which then indicated the volume of milk missed by each combination of dose and delay times. It was found that delay before injection had no consistent influence on the effectiveness of the test dose; however, an increased delay after injection required an increase in the test dose for removal of a constant percentage of residual milk. The mean dose required for 75% removal increased from 6·3 mu/kg bodyweight to 57 mu/kg bodyweight as the time from injection to removal of residual milk increased from 5 to 30 min. The doses which produced removal of 75% of the residual milk in at least 50% of the quarters tested were 5, 10 and 40 i.u. for delays between injection and removal of residual milk of 5, 15 and 30 min respectively.

The susceptibility of the components of various caseinate systems (skim-milk, β-casein-depleted milks, colloidal phosphate-free (CPF) milk, sodium caseinate and isolated β-casein) to proteolysis was investigated. Isolated αs1- and β-caseins were quite susceptible to proteolysis, but their susceptibility decreased in heterogeneous soluble systems and even more so in heterogeneous aggregated systems. In skim-milk and β-casein-depleted milks only about 50% of both αs1- and β-casein was hydrolysable by high levels of rennin, and in CPF milk all αs1- and 70% of the β-casein was hydrolysable. It is suggested that about 50% of micellar β-casein is firmly fixed within the micelle and is unavailable for proteolysis, while the remainder can dissociate from the micelle on cooling and is then readily hydrolysable.

The compatibility of the data with the various published models of the casein micelle is discussed, and a modification of Rose's (1969) model is proposed.

The development of a rapid, highly sensitive double antibody–solid phase (DASP) radioimmunoassay for ovine/caprine prolactin is described. The assay was applied to the measurement of basal levels of prolactin in the blood of 4 anoestrous lactating female, 4 anoestrous virgin female and 4 castrated male goats. For several hours after milking there was a chronic gradually decreasing release of prolactin in the lactating goats, such that the circulating levels of prolactin were, for 14–16 h of the day, at a much higher level than those in the virgin female or castrated male goats. The basal levels of prolactin in the castrated male goats (87–200ng/ml) were somewhat higher than those in the virgin females (47–77ng/ml). The effects of feeding and stress on prolactin levels are discussed.

Forty-five Friesian cows were assigned to one of 3 milking management treatments: (1) cows milked 14 times weekly, (2) cows milked 13 times weekly (Wednesday morning milking omitted), and (3) cows milked 12 times weekly (Tuesday evening and Wednesday morning milkings omitted). Cows were removed from the trial at the end of their lactations, and newly calved cows were added as they became available. The trial was continued for 12 months.

When one milking was omitted weekly, the total lactational milk yield was reduced by 3·5%, fat yield by 2% and protein yield by 1·5%. These reductions were not statistically significant. When 2 consecutive milkings were omitted, the yield of milk and of its main components was reduced by 14% (P < 0·001). The effects were greatest for highly productive cows in early lactation.

When one or 2 milkings were omitted the milk yield on the following day was nearly 50% greater than the mean daily yield. No increase in the incidence of subclinioal mastitis was observed in either milking omission treatment.

It is concluded that it is practicable to omit one milking each week though it is suggested that the technique should not be used in highly productive herds unless most of the cows are past the sixth week of lactation.

Forty-five Friesian cows were assigned to one of 3 milking management treatments: (1) cows milked 14 times weekly, (2) cows milked 13 times weekly (Wednesday morning milking omitted) and (3) cows milked 12 times weekly (Tuesday evening and Wednesday morning milkings omitted). The trial continued for 12 months.

Milking omission led to compositional changes in herd milk similar to those described for subclinical mastitis, including increases in pH, chloride level and cell count. Alterations in processing properties consistent with these changes were encountered. Heat stability and curd firmness were affected by extending the milking interval, but the magnitude and direction of the effects varied.

Where only one milking was omitted weekly, herd milk arriving at cheese or powdered-milk factories would not be expected to cause any serious technical difficulties. Omission of 2 consecutive milkings a week would cause processing difficulties where milk was to be condensed or dried, since the maximum heat stability obtained by pH adjustment was reduced.

The biochemical properties of rennin purified from calf stomach, Mucor pusillus and Endothia parasitica have been compared.

The specificities of the enzymes have been studied by separating by disk-gel electrophoresis the macropeptides released on incubation with acid casein and κ-casein. The products formed on incubation with rennin from calf stomach and M. pusillus showed some similarity although several bands were found which did not correspond to one another. The products formed with rennin from E. parasitica differed considerably from those with the 2 other enzymes.

The kinetic properties of the enzymes have been investigated in stopped-flow experiments on the enzyme-induced aggregation of κ-casein. The aggregation has been characterized by 2 parameters—the lag time prior to the increase in extinction and the maximum rate of change in extinction during aggregation.

The specific activity measured from the inverse value of the lag time was under optimal conditions nearly 2·5 times higher for rennin from calf stomach than for rennin from the 2 micro-organisms. The fact that with increasing enzyme concentration the maximum rate of change of extinction during aggregation was identical for all 3 enzymes indicates that the same product is responsible for the formation of the aggregates. The data suggest that the Km value for the aggregation of κ-casein by the enzymes is about 100 times smaller than Km for the proteolytic activity of calfstomach rennin on κ-casein. The possibility is suggested that the high specificity of the enzymes in milk clotting is due to the very low Km values for the peptide bond modifications involved in the formation of para-κ-casein, the first enzymic step in the clotting of milk.

Published data are available for the fatty-acid compositions and molecular-weight distributions of triglycerides in milk and adipose tissue from the guinea-pig, rat, mouse and rabbit, and in milk from the human, cow, mink and hamster. These data have been used to calculate the extent of non-random associations of fatty acids of different chain lengths in these triglycerides.

The results support, in general, the concept of non-correlative acylation, i.e. that during the biosynthesis of these triglycerides, fatty acids already present on the glycerol backbone do not influence the selective acylation of subsequent fatty acids. However, milk triglycerides from these animals show varying degrees of intermolecular specificity which can be explained in terms of positional specificity of fatty acids on the glycerol backbone.

From a series of short-term experiments under accentuated milking machine conditions predisposing to new mastitis infections, it appears that the cow is at increased risk near the end of a milking. The observations recorded are consistent with the view that bacteria implanted in the teat during milking as a result of the action of the machine are the less likely to be washed out the nearer implantation occurs to the end of milk flow.

The effects of 2, 4, 7 and 10% supplements of tallow to a basal diet low in fat on the composition of volatile fatty acids (VFA) in the rumen and on the secretion in milk of fat and its component fatty acids are reported. There were no significant effects of the tallow on rumen VFA or on the secretion of total milk fat. The yields in milk of stearic (18:0) and oleic (18:1) acids were positively correlated and those of palmitic (16:0) and myristic (14:0) acids negatively correlated with their respective dietary intakes. Apart from butyric acid which increased, the yields of other short-chain acids synthesized within the mammary gland decreased with increased intake of tallow. Possible factors influencing the output of total milk fat through a balance between increased transfer to milk of dietary fatty acids and decreased intramammary synthesis of fatty acids are discussed.