The effect of season on milk yield, milk composition, oestrus manifestation, oestrus cycle characteristics and conception rate was studied in high-yielding Israeli-Friesian cows. ‘Summer’ cows were allotted to control (SC) and forced ventilation (SV) groups. ‘Winter’ cows (W) were kept in one group. During July-September mean rectal temperatures at 16.00 h in primiparous and multiparous cows were 39·6 and 39·8°C, respectively in cows of the SC group, and 39·4 and 39·3°C, respectively in cows of the SV group. Mean rectal temperatures at 16.00 h of the animals in the W group during December-March were 38·9°C in both primiparous and multiparous cows. The mean 122-d milk yields in primiparous cows were 3508, 3251 and 3571 kg/cow in the SC, SV and W groups respectively (not significant (NS)). In the multiparous cows the respective yields were 4183, 4416 and 4538 kg/cow (P < 0·05). Differences in milk composition were small and statistically NS.

Observations on oestrus manifestation, performed 4 times daily, indicated that standing oestrus lasted 9·2 and 10·6 h in primiparous and multiparous cows respectively (P < 0·05); differences between groups were statistically not significant. Oestrous cycle lengths of primiparous and multiparous cows were 21·1 and 22·5 d respectively (P < 0·05) between ovulations and 23·0 and 28·05 d respectively (P < 0·01) between standing heats. Cows were inseminated following standing heat only. Conception rates (on all services within experimental periods) in primiparous cows were 50, 35 and 72% in the SC, SV and W groups respectively (NS). In the multiparous cows, the respective rates were 22, 52 and 80% (P < 0·05).

It is concluded that in the present experiment hyperthermy decreased milk production to a very small extent, in spite of very high milk yields, but fertility was severely affected. It is also suggested that primiparous cows are less affected by heat stress than multiparous cows.

Twelve adult West African dwarf (Fouta djallon) does, about 2 years old and weighing from 25 to 28 kg were kept for lactation studies lasting two 19-week periods. During these periods the does were hand-milked twice daily and the daily samples were bulked for each animal for subsequent analysis.

The results showed that the colostrum was much richer in its content (g/kg) of Na 1·44 ± 0·17, Κ 3·38 ± 0·22 and Cl 4·83 ± 0·29 than the mature milk which contained (g/kg) Na 0·65 ± 0·09, Κ 1·57 ± 0·19 and Cl 2·46 ± 0·58. The corresponding values obtained for Ca (0·65 ± 0·02) and P (0·36 ± 0·10) in the colostrum were, however, lower than 2·01 ± 0·98 and 1·18± 0·28 g/kg obtained for Ca and P respectively in the mature milk. The composition of these elements in the colostrum approached that of the normal goat's milk on the sixth d after parturition. The results showed a rise in Ca, P, Na and Cl levels with stage of lactation and a fall in Κ content of the milk with advancing lactation with the trends being highly significant (P < 0·01).

A high correlation was found between soluble calcium and citrate concentrations during a study of seasonal changes in the composition of the commercial milk supply to South-west Scotland. A simple physicochemical model of the ion equilibria in the aqueous phase of milk suggests that the observed correlation is causal. This hypothesis was corroborated by an analysis of data on individual cow milks and by experiments involving the addition of citrate to milk at constant pH.

A simplified moving boundary electrophoresis technique has been developed for the measurement of the electrophoretic mobility of casein micelles. The zeta potentials of casein micelles from different skim-milk samples were calculated using Henry's equation and shown to decrease with decrease in pH between pH 6.9 and 5.3 and to increase with increase in temperature between 10 and 45 °C. Neither severe heat treatment (up to 135 °C for 51 min) nor centrifugal fractionation of micelles into different micelle size ranges had any significant effect on zeta potential. The ionic composition of the serum phase has been shown to be extremely important in determining the electrophoretic mobility. Casein micelles electrophoresed through milk ultrafiltrate consistently gave a lower mobilities than the same micelles centrifuged through milk centrifugate. The results are discussed in relation to present theories of casein micelle structure; these theories do not accommodate all of the observations.

The effect that changes in composition which occur in milks possessing high cell counts have on milk lipolysis has been investigated. High cell counts were produced either by intramammary infusion of Escherichia coli endotoxin or Streptococcus agalactiae or by addition of washed cells which were isolated from milk obtained from quarters infused with endotoxin. Free fatty acid levels in milk were measured in terms of acid degree value (ADV) either as initial ADV measured immediately after milking or ADV developed after a prescribed incubation period.

There was an increase in initial ADV after the infusion either of endotoxin or of Str. agalactiae relative to a control quarter. This increase appeared to be associated with changes in cell count, but in absolute terms the influence of cells on ADV became less as cell count increased. Neither type of infusion had any effect on lipoprotein lipase activity. The addition of washed cells to normal milk resulted in an increase in developed ADV, but the increment was not as large as that produced by the addition of 1% blood serum. When cream and skim-milk from endotoxin-treated quarters and control quarters were mixed in different combinations with and without additional cells, developed ADV was higher in those samples containing endotoxin cream and those with added cells. Milk from a quarter treated with endotoxin was diluted with its own skim-milk to produce different cell counts and ADV was determined after various time intervals at 4 and 37 °C. Lipolysis increased with increasing cell count, but a depression in lipolytic rate was noted after incubation for 6 h at 4 °C and 20 min at 37 °C.

The proportion of skim-milk lipoprotein lipase activity in milk serum was larger both in milks possessing high cell counts and in normal milk adjusted to between 5 and 20 mM-NaCl by addition of solid NaCl. These levels of NaCl inhibited lipolysis.

The possible direct and indirect effects of high cell count on milk lipolysis are discussed.

Individual fatty acids were added to liquid skim-milk and to reconstituted dried skim-milk in concentrations similar to those produced by natural lipolysis, and their effects on deposit formation from the milk on a heated surface were determined. The use of reconstituted dried skim-milk considerably improved the reproducibility of the results. Butyric, caproic and caprylic acids did not influence the amount of deposit formation. Capric acid had a marked effect in reducing deposit at low concentrations: the effect of ageing of milk in reducing deposit formation is thought to be due to capric acid development. Laurie, myristic and palmitic acids had a slight effect, increasing deposit formation, while stearic acid caused a considerable increase. Capric acid was found to adsorb to the casein micelles in milk, and it may therefore act by interfering with casein aggregation during the build-up of deposit.

The effects of solubilization, treatment with organic solvents and storage under alkaline conditions on membrane-associated peptidases of intact cells of Streptococcus cremoris HP were studied. Differences in the response of the peptidase activities towards these membrane perturbing treatments were observed. Pyrrolidonecarboxylylpeptidase (PCP) and an endopeptidase (P50) showed 50% irreversible inhibition at the same concentration of each solvent tested. An amino- and proline iminopeptidase activity and the endopeptidase P37 were in this respect much more sensitive to the action of the solvents. Within a homologous series of n-alkanols irreversible inhibition of PCP showed a dependence on the hydrophobicity of the solvent molecules. Only P37 activity was increased considerably upon solubilization of the enzyme. Similar levels of activation were found upon treatment of cells with 3% (v/v) n-butanol at 25 °C or storage at 30 °C at an alkaline pH. Optimal activity of P50 during n-butanol treatment was at 25 °C using a concentration of 5% (v/v), but no activation was observed upon solubilization. The results are discussed in terms of enzyme–lipid interaction and accessibility of the enzymes in situ. It is concluded that the enzymes apparently occupy different positions within the membrane although they may together constitute a functional peptide-hydrolysing unit.

Microbial thermograms have been used to investigate the growth of Streptococcus thermophilus and Lactobacillus bulgaricus in single and mixed culture in milk medium. Growth has been observed with different ratios of the strains in the inoculum and, at one ratio, the effect of the amount of inoculum. The effect of serial sub-culture both with and without pH control has been observed, and stimulation of Str. thermophilus by cell-free fitrates of L. bulgaricus has been examined. The development of the technique is discussed in terms of the application of microbial thermograms in selecting cultures for incorporation into mixtures and evaluating their performance.

The effect of proteolytic, psychrotrophic strains of Pseudomonas fluorescens, Ps. putida and Acinetobacter spp. on cheese-making with stored milk has been investigated. Ps. fluorescens and Ps. putida growing for 72 h in raw milk at 7·5 °C to levels of approx. 107 colony-forming units/ml caused a low degree of β- and к-casein breakdown detectable by gel electrophoresis, but this was insufficient to affect N losses in whey or cheese yields. Variations in cheese-making times with pasteurized milks were not attributable to the counts of psychrotrophs in the corresponding raw milks. The water-soluble and TCA-soluble N fractions of maturing cheeses were unaffected by psychrotroph counts in raw milks, but small differences in levels of casein fractions of cheeses made from milks stored for 72 h were detected by quantitative polyacrylamide gel electrophoresis. The incidence of casein breakdown in raw milk and subsequently in cheese were not necessarily related. None of the cheeses developed off flavour related to excessive protein breakdown but many became lipolytically rancid, despite the selection of strains with low lipolytic activity on a diagnostic medium. It is concluded that the numbers of psychrotrophic bacteria likely to occur in stored raw milk under commercial conditions are unlikely to cause significant changes in the yields or quality of Cheddar cheese through their proteolytic activity.

Lipid and aqueous extracts of bovine milk and 5 fractions prepared from bovine milk, including a membrane-rich fraction (MRF) isolated during butter oil production, were analysed for neutral lipid, phospholipid, protein and glycoprotein. The MRF showed an 8·5-fold enrichment in the phospholipid: protein ratio compared to whole milk. The percentage distribution of the 3 major phospholipid classes, phosphatidyl ethanolamine, phosphatidyl choline and sphingomyelin was 30, 36, 31 % respectively, the values of which differ slightly from those of whole milk. Fatty acid analysis of the major neutral and phospholipid classes indicated a predominance of palmitate, stearate and oleate. SDS polyacrylamide-gel electrophoresis in polyacrylamide gradients indicated that the percentage protein having a mol. wt in excess of 75000 increased by a factor of 2 in the MRF when compared to milk. Glycoprotein profiles of the MRF and its precursor cream differ significantly from the patterns evident in whole milk and skim-milk.

Multiple regression analysis has been used to relate contributions of individual flavour components to the flavour of Cheddar cheese. Equations have been derived that relate Cheddar flavour to the headspace concentrations of 2-pentanone, methanethiol and methanol. The concentrations of ethanol and H2S found in cheese headspace have been related to the flavour defects ‘fruity’ and ‘sulphide’ respectively

Possible mechanisms for the production of volatile sulphur compounds during the ripening of Cheddar cheese have been studied. Slurries of cheese made with and without starter organisms were treated with dithiothreitol (DTT) and the chemical composition of the headspace examined. Cheeses were also pressed from curd treated with DTT and glutathione (GSH). Reactions occurring between methionine, cystine and Na caseinate and reducing agents cysteine, DTT and GSH were also studied. In addition, the reactions of H2S with methionine and Na casein. ate were investigated.

Treatment of cheeses with DTT and GSH resulted in the production of H2S and methanethiol (CH3SH) together with the development of Cheddar flavour. Similar treatment of methionine and Na caseinate also yielded H2S and CH3SH whereas Cystine produced only H2S. H2S reacted with both Na caseinate and methionine to yield CH3SH.

These results are consistent with the occurrence of some of the chemical reactions studied during the ripening of Cheddar cheese.

A method is described for the chromatographic analysis of the headspace of hard cheese. Among the cheeses examined were 12 commercial cheeses and 12 Cheddars made in the NIRD experimental dairy. The 12 commercial cheeses were Parmesan, Caerphilly, Stilton, Leicester, Double Gloucester, Cheshire, Gouda, Wensleydale and 4 varieties of Cheddar. It was found that those varieties containing low concentrations of sulphur compounds had little aroma and derived most of their flavour from taste and texture. The concentration of 2-pentanone was found to be a good indicator of maturity of Cheddar cheese.