Whole cells of Streptococcus lactis C10, when incubated with an energy source, converted fumarate to succinate and malate to lactate. Cell-free extracts of Str. lactis C10 contained fumarate reductase, but no aspartase, adenylosuccinate synthetase and lyase or argininosuccinate synthetase and lyase activity could be detected. Cells grown in the presence of [14C]bicarbonate produced labelled succinate during the synthesis of purine bases. However, the amount of succinate produced by this pathway only accounted for approximately one-sixth of the succinate produced by the cells.

Activation of the antibacterial lactoperoxidase system in milk, i.e. increasing the thiocyanate concentration to 0·25 mM and adding an equimolar amount of H2O2, results in a substantial reduction of the bacterial flora and prevents the multiplication of psychrotrophic bacteria for up to 5 d. This treatment has no effect on the physico-chemical properties of milk and does not lead to the accumulation of resistant bacteria. The practical application of the lactoperoxidase system in prolonging the storage period of raw milk at low temperatures is discussed.

The fixation of [14C]bicarbonate into aspartate by Streptococcus lactis C10 was achieved by the combined reactions of pyruvate carboxylase (E.C. 6.4.1.1) and glutamate-oxaloacetate transaminase (E.C. 2.6.1.1). The pyruvate carboxylase from Str. lactis C10, which was most active at pH 8·0, was activated by the divalent metal ions Mn2+, Mg2+ and Co2+, and inhibited by sulphydryl reagents. The enzyme was inhibited non-competitively by aspartic acid and competitively by oxaloacetate.

Discovery of an endopeptidase by gel chromatography and separation of 3 exopeptidases (a dipeptidase, an aminopeptidase and a specific carboxypeptidase) from Lactobacillus casei NCDO 151 by affinity chromatography is described. The 3 exopeptidases were strongly inhibited by the metal chelators EDTA and 1,10-phenanthroline but were reactivated with Co2+ and Mn2+. The pH optima for aminopeptidase, dipeptidase and carboxypeptidase activities were 6·5, 7·6 and 7·2, respectively. Maximum activity was obtained at 45 °C for the aminopeptidase, at 30 °C for the dipeptidase and at 40 °C for the carboxypeptidase.

The substrate specificities of the 3 enzymes were also studied. The properties of these 3 enzymes are compared with those of other bacteria.

A microcalorimetric method has been developed for the identification of 9 representative strains of lactic acid bacteria used in the dairy industry. A chemically defined medium containing several carbohydrates gave reproducible and characteristic growth patterns. The technique is proposed as a rapid alternative method for the identification of bacteria selected for commercial use.

Cheddar cheese was produced free of non-starter bacteria, acidified with starter or glucono-δ-lactone and containing active coagulant (chymosin or pepsin) or inactivated coagulant (pepsin). The level and type of proteolysis in the experimental cheeses was monitored by protein solubility at pH 4·6 and in 12 % TCA, polyacrylamide gel and high voltage paper electrophoresis, gel filtration and paper chromatography. The results show that the coagulant was primarily responsible for the formation of large peptides while small peptides and free amino acids were produced principally by the starter, possibly from coagulant-produced peptides.

Cheddar cheeses were pressed from a batch of curd each month over a period of 1 year to study the production of headspace volatiles and the development of flavour in ripening cheese. At 2-monthly intervals, samples taken from selected sites in the cheese were presented to a trained panel for assessment of quality and intensity of Cheddar flavour. Using a headspace technique samples of vapours from bore holes in different parts of the cheese were analysed for H2S, methanethiol, acetone, butanone, methanol, ethanol and 2-pentanone. Most of the compounds increased in concentration during the ripening period, but ethanol and butanone varied in concentration in an unsystematic way. Possible precursors of the headspace volatiles are discussed.

The effect of added gastric lipases on the ripening and flavour development in Ras cheese was studied. Ras cheese was made from pasteurized milk with the addition of Italase, Capalase K or Capalase KL at the rate of 12 g/100 kg milk before renneting and analysed at different intervals for fat, moisture, salt, pH, soluble tyrosine and tryptophan and Shilovich ripening index. The organoleptic properties of the cheeses were also assessed. The addition of lipases was found to accelerate the development of flavour, particularly when Capalase K was used. These lipases had no effect on the gross composition of cheese during ripening.

The development of free volatile fatty acids (FVFA) in Ras cheese treated with lipases was investigated. Ras cheese was made with the addition of Italase, Capalase K and Capalase KL and then analysed periodically for the FVFA by quantitative gas-liquid chromatography. Lipase treatment markedly increased the total FVFA content of the cheese during storage, particularly when Capalase K was used. The FVFA of Ras cheese were found to consist of propionic, butyric, caproic, caprylic and traces of acetic acid. The relative concentration of FVFA varied with storage and lipase treatment.