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Use of Streptococcus Lactis Lac- mutants for accelerating Cheddar cheese ripening; 1. Isolation, growth and properties of a C2 lac- variant

Authors: P.A. Grieve, B.A. Lockie and J.R. Dulley

Abstract:

Variants of Streptococcus lactis C2 which had spontaneously lost the ability to ferment lactose (lac-) were isolated from a litmus milk culture. These lac- isolates were also prt- having lost their surface-bound proteinase activity. Examination of one of the lac- prt- isolates (lac- 1) for stability of the lac- characteristics showed that reversion to the lac+ phenotype occurred after extensive sub-culturing of the mutant in broth medium containing lactose. Growth rate experiments on the C2 lac-1 mutant in skim milk showed that the mutant was capable of attaining relatively high cell populations in milk without producing acid from lactose. The optimum growth conditions for batch culture of the C2 lac-1 mutant isolate were determined and this culture was used in the production of a frozen concentrate. This lac- mutant concentrate retained complete viability on thawing and provided a means by which high levels of C2 lac- starter could be incorporated into cheese milk without affecting acid production during cheese manufacture.

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