Since Indian law requires curds to be made from heat-treated milk, a means for distinguishing between curds made from raw and from heat-treated milks was sought. Curds contain whey proteins that are partly or completely denatured by heat treatment; the kinetics of the reaction are first order. The concentrations of native whey proteins in raw and heat-treated milks and in curds made from those milks were measured. No difference was found between the concentrations in heattreated milk and in the curds made from it, so that identical values were obtained for the half-life on heating and for the activation energy of denaturation. The acidity and microbial growth occurring during the production of curd did not affect the whey proteins. On electrophoresis, curd from raw milk gave a few whey protein bands, but curd from milk boiled for 10 min gave none, showing complete denaturation of the whey proteins.

The host–phage reactions of 20 strains of lactic streptococci currently used in cheese manufacture, and the phages active on them, were studied under a variety of conditions. The phages of the lactic streptococci conformed in their behaviour with the general behaviour of phages for other bacterial species. The burst size of phages which attack one strain of starter culture varied widely and also varied when one phage attacked different host strains. The burst size of a phage was also influenced by the host on which it was previously propagated. An increase of temperature from 30 to 37 °C the latent period, but it increased, decreased, or had no effect on the burst size. This increase in temperature also usually decreased the percentage adsorption of phage. Rennet at a concentration of 0·02 % significantly reduced the percentage adsorption of phage in sterile reconstituted milk and also in tryptone yeast-extract broth.

Samples of the mammary gland were obtained from heifers, lactating and dry cows by a freeze-clamping technique. Amounts of lactate, pyruvate, malate, ATP, ADP and AMP were determined in the sample extracts and calculated per g fresh weight and per mg DNA. Pyruvate, lactate and AMP amounts showed only minor changes at different stages of lactation, whereas quantities of malate, ATP and ADP were significantly higher in cows giving high milk yields. Pyruvate levels showed seasonal variations parallel to changes in milk composition. NAD: NADH ratios in mammary cytoplasm showed no significant differences between different groups of cattle, whereas ATP: AMP ratios in mammary gland samples were highest in lactating animals.

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.

Melting curves were determined by differential scanning calorimetry on 17 samples of milk fat obtained at approximately fortnightly intervals through one dairying season. Liquid-fat content at a number of temperatures was calculated from the melting curves.

Correlation between percentage liquid fat at 12 and at 22 °C and weight percentage of short-chain fatty acids (C4–C8) was low (r = 0·45 and 0·58 respectively) as was that with cis-unsaturated acids (r = 0·39 and 0·29). Correlation with the sum of short-chain and cis-unsaturated acids was higher (r = 0·71 and 0·68). With a multiple regression model including short-chain and cis-unsaturated acids the values of the multiple correlation coefficient (R) were 0·75 and 0·78.

The softening point showed a high correlation (r = 0·79) with weight percentage of long-chain saturated acids and also with the temperature at which 94 % of the fat was liquid (r = 0·75).

Possible explanations of these findings are discussed.

Low-angle X-ray diffraction techniques have been applied to the study of the structure of the fat-globule membrane of milk and of Cheddar cheese. Membrane was prepared by freeze-thawing, churning and ether extraction, the endproduct of each method being a pellet produced by centrifuging. High-angle and low-angle reflexions from membrane could be related to triglyceride spacings, and low-angle reflexions due to spacings at about 5·5 nm to phospholipid bilayers. Spacings due to triglycerides were observed in mature Cheddar together with spacings that agreed well with values for some of the amino acids.

A 23-week trial was conducted using 4 groups each of 6 ewes to study the effects on milk yield and composition of substituting non-protein nitrogen (NPN) for 75 % of the dietary nitrogen. The control group received a winter ration which included silage followed by a summer ration containing lucerne. The test groups received substitutes of urea, ammonium sulphate, or urea+(NH4)2SO4. The milk yield of the test groups was lower (P < 0·01) than that of the control group. Depression of the fat content of the milk reached statistical significance (P < 0·01) in the groups receiving (NH4)2SO4 and urea+(NH4)2SO4. In comparison with the control group, the protein content of the milk was greater in the group receiving urea and smaller in the other 2 test groups. The milks of the test groups had lower amounts of essential amino acids in the free state and higher amounts of non-essential amino acids than did the control milk. Milk-clotting time was increased in the test groups; the increase was greatest (P < 0·01) for the group receiving (NH4)2SO4 and least (P < 0·05) for that receiving urea.

Aliphatic monocarbonyls have been quantitatively estimated in casein and skim-milk powder during storage. Concurrently these products have been evaluated organoleptically. A close relationship was evident between monocarbonyl level and the flavour or age of the lactic casein. Furthermore, the greater flavour stability of rennet casein and skim-milk powder was coupled with reduced monocarbonyl content. Fractionation of monocarbonyls into classes and individual compounds has shown that the higher levels in lactic casein compared with other low-fat products can be accounted for by a high level of acetone. The absence of appreciable quantities of other odd-numbered methyl ketones suggests that either acetone is derived from a non-lipid constituent of acid casein or decarboxylation of acetoacetate is specifically catalysed at acid pH. All methyl ketones identified in these low-fat dairy products were present in subthreshold concentrations. Saturated and unsaturated aldehydes were present at similar levels in stored lactic casein, rennet casein and skim-milk powder samples irrespective of their degree of musty or stale flavour.

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.

More than 90% of the C2,4,6,8,10 fatty acids and C8,10, δ-lactones added at quantities between 0·5 and 40 ppm were recovered from volatile-free butter oil by cold-finger molecular distillation at 35°C, while approximately 65% of the C12 fatty acid and of the C12 δ-lactone was recovered. Cold-finger molecular distillation at 50°C increased the recovery of the C12 compounds to more than 90%. The acids and lactones were estimated quantitatively as the free compounds by gas chromatography on a mixture of Carbowax 20 M and isophthalic acid as the stationary phase.

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.

The dietary and individual animal factors influencing the utilization of digestible nitrogen and of metabolizable energy for milk production were studied and change in the optimal nitrogen:energy ratio during lactation was investigated using 35 different experimental diets and the individual results obtained from 119 Friesian cows whose daily milk production varied from 4 to 30kg.

The amount of N secreted in the milk and the daily milk production depended above all on the metabolizable energy intake and on the stage of lactation. They were related to the dietary N: energy ratio.

The percentages of digestible N and of metabolizable energy utilized for milk production were also related to the dietary digestible N: metabolizable energy ratio of the diet.

The optimal digestible N:metabolizable energy ratio ranged from about 2·2g/MJ during the first 3 months of lactation to about 1·7g/MJ in the sixth and seventh months and 1·3g/MJ from the tenth month. When expressed in g N intake per MJ energy intake, this optimal ratio ranged from 1·6 after calving to 1·1 at the end of lactation.

A simple in-line milk sampler is described for determining the bacterial contamination from the udders of cows milked in recorder machines. When sterilized, and used with a sterile cluster, it collects a milk sample bacteriologically representative of the cow's total yield and it can be used with minimum disruption of the milking routine.

The incorporation of 14C from [1-14C] and [6-14C]glucose and [2-14C]acetate into CO2 and fatty acids by rat liver slices was measured at intervals during pregnancy, lactation and involution.

During late pregnancy, the rates of oxidation of the C-1 and C-6 atoms of glucose were respectively 65 and 40 % higher than those for unmated animals. These increases were maintained during lactation, but the highest values were observed 3 days after weaning. Pregnancy and lactation had little effect on the oxidation of [2-14C]acetate.

The incorporation of14C from all 3 labelled substrates into fatty acids was increased by a factor of 3–4 during late pregnancy. There were further increases during lactation, and 3 days after weaning the values were as much as 10 times as high as those for unmated animals.

The incorporation of both [14C]glucose and [14C]acetate into cholesterol was increased by a factor of 6–7 during lactation.

The activities of the enzymes glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, ATP citrate lyase and acetyl-CoA carboxylase were also increased during lactation and involution.

The similarity between the changes summarized above and those brought about by changes in the pattern of food intake is discussed, and the idea that fatty acids synthesized from non-lipid precursors in the liver may make some contribution to the formation of milk fat is also considered.

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 incorporation of 14C from [1-14C] and [6-14C]glucose into CO2, fatty acids and glycerol by rat parametrial adipose tissue was measured at intervals during pregnancy, lactation and involution. There were small increases in the incorporation of both the C-1 and C-6 atoms into CO2 and glycerol on the seventh and sixteenth days of pregnancy. Fatty acid synthesis from glucose was negligible on the twentieth day of pregnancy and during lactation. Three days after weaning the rate of oxidation of the C-1 atom was doubled and the incorporation of both carbon atoms into fatty acids was increased at least 20-fold. There were no changes in the activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase during pregnancy and lactation, but 3 days after weaning the activities of both enzymes were doubled. These results are discussed in relation to other evidence that the total body fat is increased during pregnancy, and to the idea that the fat depots are mobilized during lactation.