B. Reiter

The effect of the microbial flora on the flavour and free fatty acid composition of cheddar cheese

Comparisons were made of the flavour, free fatty acids and bacterial flora of commercial cheese made at different factories and experimental cheese made under aseptic conditions: (i) with δ-gluconic acid lactone instead of starter, (ii) with starter only, (iii) with starter and added floras derived from the curd of the commercial cheeses (reference flora cheeses). Comparison of the bacterial flora of commercial and reference flora cheeses showed that replication of organisms was better with some reference floras than with others. In all the cheeses the lactobacilli increased in numbers during maturation, whilst other groups of organisms died out. The amount of acetic acid present was influenced by the starter and by the lactobacilli. Single-strain starters produced some acetic acid, most of which was lost in the whey; commercial starters produced considerably more, due to the presence in them of Streptococcus diacetilactis . Later in maturation lactobacilli increased the acetic acid content, a greater increase being observed with homo-than with heterofermentative strains. The initial levels of butyric and higher fatty acids in the milk varied with source of the milk and with the season, summer milk having higher levels than winter milk. During cheese-making a slight increase of these acids occurred in every cheese made with starter and a further small increase occurred during ripening. However, there was no increase in the content of these acids in the cheese made with δ-gluconic acid lactone, indicating that lactic acid bacteria were weakly hydrolysing the milk fat. Flavour trials showed that Cheddar flavour was present not only in the reference flora and commercial cheese, but also in the cheese made with starter only. Different starters produced different intensities of flavour; one strain produced an intense fruity off-flavour. Cheeses made with δ-gluconic acid lactone were devoid of cheese flavour.

The release of intracellular dipeptidase from starter streptococci during Cheddar cheese ripening

Intracellular dipeptidase of Streptococcus cremoris NCDO 924 has been investigated as a marker enzyme for detecting the release of starter enzymes into Cheddar cheese during maturation. Dipeptidases were produced extra- and intracellularly by Str. cremoris in broth cultures, but the intracellular enzyme was distinguishable by its relatively high activity against L-alanylglycine at low temperatures. The presence of dipeptidase activity of intracellular origin was demonstrated in extracts of Cheddar cheese freed from whole starter cells, opaque material of high molecular weight and free amino acids by a combination of centrifugation and gel filtration on Sephadex G200. The enzyme was not detected in cheeses made without starter (δ-gluconic acid lactone cheeses). In fresh curd with a high viable count of starter the dipeptidase activity was low and mostly extracellular. As the count declined the proportion of intracellular enzyme activity increased, reaching a maximum when 90% of the starter cells had died. The total dipeptidase subsequently decreased until the level remained 30% higher than that of the original curd, and changed little on further ripening up to 120 d. There remained a significant proportion of extracellular dipeptidase activity throughout the maturation period studied.

Neutral volatiles in cheddar cheese made aseptically with and without starter culture

Analyses of the neutral volatile components from 3 Cheddar cheese are presented. Two were made aseptically in an aseptic vat, with and without starter culture; the third was made with starter culture in an open vat. Gas–liquid chromatography and mass spectrometry detected the same volatiles in starterless cheese having little or no Cheddar flavour as in cheese made with starter and having a characteristic Cheddar flavour. Methyl disulphide and dimethyl sulphide were the only compounds consistently detected in higher concentrations in the cheese made with starter than in the cheese made without starter. However, using a total trapping technique, it was found that the combination of components recovered from the effluents of the chromatographic columns did not have the cheese-like aroma of the distillate vapours that were injected. Reasons for this are discussed.

Esterases and Other Soluble Proteins of Some Lactic Acid Bacteria

SUMMARY: Electrophoresis patterns of soluble protein of 34 strains and esterases of 113 strains of lactic acid bacteria were determined. Similar protein patterns were obtained for the three species of lactic acid streptococci; with the lactobacilli most species gave constant species-specific patterns, but Lactobacillus acidophilus and L. delbrueckii strains differed markedly among themselves. Esterase patterns of lactic streptococci were generally species specific. Among the lactobacilli the thermobacteria had weak esterase activity which was only species specific for L. lactis, L. leichmanii and L. salivarius; in the streptobacteria, L. casei had a very consistent esterase pattern, whereas L. plantarum had very different patterns within the species; the unclassified strains were different from each other; in the betabacteria activity was weak and no consistent pattern of bands occurred. Leuconostocs grouped in patterns corresponding to their physiological groups. Esterases of a streptococcus and a lactobacillus examined were classified as ali esterases. When ten strains of lactic acid bacteria were tested for substrate specificity, nine of them had a higher activity against α-naphthyl acetate than against the butyrate and caprylate. A rapid test for esterase activity of whole organisms is described.

A method of maintaining a reference flora of constant bacteriological composition

A method is described by which the microbial flora of a raw milk sample was reconstituted on different occasions. After segregation on selective media into groups consisting of lactic acid bacteria, Gram-negative rods including coli-aerogenes, staphylococci, streptococci, and microbacteria, and propagation by replica plate techniques on optimal media, suspensions of these groups of organisms were then added in approximately the original proportions to 40 gal batches of heated milk of low bacterial count. When this was repeated on 18 different occasions over a period of 11 weeks, the milk flora was found to be almost identical with the original one.