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.

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.

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.

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.

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.

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.

Samples of the mammary gland were obtained by the freeze-clamping technique from heifers, lactating and dry cows. Levels of acetyl CoA, citrate, isocitrate and α-oxoglutarate were determined in extracts of the samples and calculated per g fresh gland and per mg DNA. All mammary metabolite levels were significantly higher in lactating cows than in heifers or dry cows. The accumulation in the lactating gland increased in the sequence acetyl CoA–α-oxoglutarate–isocitrate–citrate. NADP/NADPH ratios in cytoplasm, as calculated from the equilibrium of isocitrate dehydrogenase, were lowest in lactating glands. The activities of some enzymes were also determined in the cytoplasm and calculated per g fresh gland, per mg protein and per mg DNA. Mammary citrate synthase activity did not vary significantly between different groups of cattle, whereas mammary isocitrate dehydrogenase activity was higher in lactating cows than in heifers or dry cows. No activity of ATP citrate lyase was detected.

A method for detection of proteolysis in milk was evaluated. Amino acids and peptides soluble in trichloroacetic acid were estimated by the Lowry–Folin procedure with expression of results in terms of colour yield equivalent to that of tyrosine (the ‘tyrosine value’ (TV)). Some variables and alternatives in the method of estimation were investigated.

TVs for milk freshly drawn from individual cows were extremely varied (0·31–0·92 mg/ml). TVs for samples of refrigerated milk from daily supply farms were in the range 0·40–0·58 mg/ml; no relationships between TV and bacterial counts were evident with these samples. With bulk refrigerated raw milk supplied to Brisbane for liquid consumption from country depots and local farms equipped with bulk vats, significant positive relationships were found between TV and total bacterial count (TBC) for supplies from some sources, but in no instance was a significant relationship found between TV and psychrotroph or proteolytic psychrotroph count. Significant positive relationships were found between TV of bulk milk supplies from some sources and atmospheric temperature, and between TBC of the supplies and atmospheric temperature. The significance of these various relationships for bulk milks is discussed.

Natural variation in TV imposed limitations on reliability of the method to provide an index of proteolysis; in general the results indicated that its application was restricted to bulk milk cold-stored for at least 3 days.

A study is reported of the effect of preincubation of raw and laboratory pasteurized bulk milk samples on the tyrosine value (TV) of milk and its relationship with bacterial populations. For raw samples, mean TVs before and after preincubation for 20±2 h at both 25 and 30 °C differed (P < 0·001). After preincubation at 30 °C, the TV of 47 % of raw samples exceeded that considered to represent the upper limit of variation in the TV of a normal bulk milk supply. At this temperature, however, there was no indication that proteolytic psychrotrophs were a major contributor to increased TV. With preincubation of raw samples at 25 °C, an increase to above the upper limit of variation occurred in only 20 % of samples; however, proteolytic psychrotrophs appeared more active at this temperature. In similar samples preincubated after laboratory pasteurization, large increases occurred in TV after 20 ±2 h at 30 °C, while at 25 °C the increases were much smaller and of the order found for preincubated raw samples. The results showed, however, that nonpsychrotrophic bacteria were also the principal source of proteinase giving rise to increased TVs in laboratory pasteurized samples.

The setting rate of butter during the first 5 h after manufacture was found to be increased by a higher maximum temperature during working, e.g. 15 as compared with 12·5 °C, or by a temporary rise in temperature to 18 °C for 30 min after working. A temporary lowering of temperature to 7 °C for 30 min, however, had no effect. The amount of working did not affect setting rate except where more prolonged working resulted in a higher final temperature in the butter.

When temperature conditions during and after working were standardized, it was found that temperature of pasteurization in either a Vacreator or plate pasteurizer or the presence of clumped fat in the cream had no effect on setting rate. The setting rate, however, was increased by Vacreator treatment as compared with plate pasteurization of cream or by low-pressure homogenization of cream after plate pasteurization.

Microscopic examination of butter samples showed much fewer intact fat globules in butter from Vacreator-treated or homogenized cream than in that from platepasteurized cream.

It is suggested that Vacreator treatment or homogenization of cream affect the setting rate of butter by causing changes in the fat-globule membrane which, in turn, influence the type of crystallization in the fat globules and the stability of the globules.

A relationship between the level of dissolved O2 in a product and the head-space volume of a sealed container after filling was derived. The validity of the relationship was confirmed by studies using both water and milk which had been subjected to ultra heat treatment (UHT), and stored in glass bottles. The relation held well for water, but some deviation was noted using milk. The relationship allowed selection of the processing conditions necessary for the manufacture of products with a desired dissolved O2 level.

As part of this study a device was developed which permits continuous monitoring of dissolved O2 in sterile liquids being pumped between a container and an O2 sensor. Only very minor problems were encountered with the device through biological contamination or entry of atmospheric O2.

Comparison of the carbohydrates of κ-casein with those of the glycopeptides released by the action of rennin on whole milk showed that there are consistent differences between them, which are due to the presence of carbohydrates in para-κ-casein. d-Mannose, d-galactose, 2-acetamido-2-deoxy-d-galactose and N-acetyl neuraminic acid were invariably present in κ-casein. N-acetyl neuraminic acid was located almost entirely on the glycopeptides whereas the greater proportion of d-mannose was attached to the para-κ-casein. d-Galactose and 2-acetamido-2-deoxy-d-galactose were detected in both para-κ-casein and the glycopeptides. In the glycopeptides there are usually about 2 molecules of d-galactose to 1 molecule of 2-acetamido-2-deoxy-d-galactose. Carbohydrates are attached to only a proportion of para-κ-casein molecules. Carbohydrates could not be detected in αs- and β-casein.

Average N-acetyl neuraminic-acid (NANA) content of milk from cow, sheep, goat, mare, donkey and sow (about 10 samples from each species) was 17·7, 10·1, 6·4, 6·1, 12·0 and 64·0mg/100 ml, of which 14·2, 8·4, 3·2, 3·1, 5·0 and 53·7mg/100 ml respectively was in the casein fraction. The NANA content of casein showed marked species variation ranging from an average of 2·18 % for the sow to 0·09 % for the goat.

About 28 % of the casein NANA of mare's milk was liberated in the fraction soluble in 12 % TCA by the action of rennin at pH 6·6 compared with 75 and around 100 % for donkey's and sow's milk respectively; about half the total NANA in the casein of cow's, sheep's and goat's milk was released under the same conditions.

A milk-clotting proteolytic enzyme was isolated and purified from the culture filtrate of Bacillus cereus strain x29 by fractionation with acetone or ammonium sulphate and subsequent column chromatography employing DEAE cellulose and DEAE Sephadex. The purified enzyme was found to be homogeneous by acrylamide gel electrophoresis from pH 3·5 to 8·6, with, a molecular weight of about 50000. The single absorption maximum of the native enzyme was at 277 nm and the value of at 280 nm was 7·79. Purification resulted in a 9-fold enhancement of activity with 24 % yield. The optimum activity of the enzyme was at pH 8·0 at 40 °C with casein as the substrate. The enzyme was found to be most stable at pH 6·0 and was stable to freezing and freeze-drying. Heavy metal ions were found to inactivate the enzyme, but no metal ion activation was found. Enzyme activity was inhibited irreversibly by EDTA and reversibly by 1,10-phenanthroline. The enzyme has been identified as a Zn-containing neutral protease.