Helen R. Chapman

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.

Effect of proteolytic raw milk psychrotrophs on Cheddar cheese-making with stored milk

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. 10 7 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.

Making Cheddar cheese on a small scale under controlled bacteriological conditions

A cover for a 40 gal. cheese vat has been specially designed to allow cheese to be made under conditions which prevent the entry of micro-organisms into the vat from the dairy environment. The technique employed is described. Using milk free from lactobacilli it has been possible to make Cheddar cheese in which no lactobacilli grew during the ripening period. An assessment of the role of these and other micro-organisms in cheese ripening should now be possible. The technique should also be of help in other cheese investigations where control of the microflora is desired.