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Factors Affecting Activity and Heat Resistance of Swiss Cheese Starter Cultures. II. Influence of Culture Medium^*

Department of Agricultural Bacteriology, University of Wisconsin

ABSTRACT

1. L. helveticus, L. lactis and Lactobacillus sp. all developed more actively in freshly prepared than in old, sterile reconstituted skim milk. The age of the sterile reconstituted skim milk used as culture medium had no significant influence on growth of Str. thermophilus, Str. lactis and E. coli.
   
2. Substances such as peptone, tomato juice, yeast water and lactic filtrate stimulated growth, of L. helveticus in both old and freshly prepared sterile reconstituted skim milk.
   
3. The oxidation-reduction potential of steamed, freshly prepared, sterile reconstituted skim milk was significantly lower than that of steamed, old sterile reconstituted skim milk. Addition of a reducing substance, neutralized thioglycollic acid, to old sterile reconstituted skim milk stimulated development of certain lactobacilli but did not significantly stimulate development of Str. lactis, Str. thermophilus and E. coli.
   
4. A succession of transfers of L. helveticus in old sterile reconstituted skim milk resulted in a gradual decrease, while a succession of transfers in a milk more favorable for growth resulted in an increase in their activity and in their ability to develop at 44° C. following heating. When cultures of L. helveticus were carried at 37° and 40° C., respectively, for numerous successive transfers in a favorable medium, those grown at 37° were more active following heat treatment than were those grown at 40° C. However, when cultures were carried in the same manner in a poor medium, those grown at 40° were more active following heating than were the 37° C. cultures.
   
5. When stock or starter cultures of lactobacilli similar to those used in these investigations are grown in milk of the type used in these studies, their activity may be insured if they are carried in freshly prepared milk, in milk in which the oxidation-reduction potential has been adjusted to a level favorable for initiation of growth, or in milk to which growth stimulants have been added.
   

^* This work has been made possible by a grant of the Wisconsin Alumni Research Foundation.