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Factors Affecting the Activity and Heat Resistance of Swiss Cheese Starter Cultures. I. Influence of Time and Temperature of Incubation¹

Department of Agricultural Bacteriology, University of Wisconsin

ABSTRACT

1. When L. helveticus was carried for numerous transfers at different temperatures by methods similar to those commonly employed in handling starter cultures of this organism, those cultures carried at 37° and 40° were more active following heat treatment than were cultures carried at 30°, 35° and 42° C.
   
2. The 37° cultures of L. helveticus were more heat resistant than the 40° C. cultures after the first 12 hour transfer at the respective temperatures, but after numerous, successive 12 hour transfers at the respective temperatures, the 40° was more resistant than the 37° culture. This increased resistance of the 40° culture was not lost by one transfer at a lower temperature.
   
3. Cultures of Str. thermophilus carried for numerous successive transfers at different temperatures by the same methods used for L. helveticus showed greater heat resistance when grown at 30°, 35° and 37° than at 40° or 42° C. The 37° cultures were more active than were the 40° C. cultures of Str. thermophilus after both one and numerous successive 12 hour transfers at the respective temperatures.
   
4. Heat treatment in the usual manner, followed by incubation of the respective cultures at temperatures near their maximum and comparison of their subsequent rates of growth and acid production, emphasized the differences in activity following heat shocking of cultures of L. helveticus and Str. thermophilus.
   
5. Cultures of L. helveticus incubated at 37° and 40° C. for 12 to 16 hours were far more heat resistant than cultures incubated at 37° and 40° C. for seven and eight hours.
   
6. There was no marked difference in activity following heat treatment of cultures of Str. thermophilus incubated at 37° C. for periods varying from six to 16 hours, the range of incubation periods usually employed in growing starter cultures of this organism.
   

¹ This work has been aided by a grant from the Wisconsin Alumni Research Foundation.