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Factors Affecting the Activity and Heat Resistance of Swiss Cheese Starter Cultures. IV. Effect of Variations in Time and Temperature of Incubation and of Storage on Heat Resistance of Cultures^*

Department of Agricultural Bacteriology, University of Wisconsin

ABSTRACT

1. Successive transfers every 12 and 24 hours at 37° and 40° C. resulted in more heat resistant cultures of L. helveticus at 40° than at 37° when mother cultures were grown in freshly reconstituted skimmilk. In a medium with added accessory substances, such as Neopeptone or malt extract, 12 to 24 hours at 37° and 12 hours at 40° gave best results.
   
2. Consecutive transfers of Str. thermophilus at a constant temperature led to most active cultures when incubation in the fortified medium varied from 12 to 24 hours at 37°; but at 40° highest resistance to heat was shown by 12-hour cultures carried in the fortified medium.
   
3. Storage at 12° was least harmful to L. helveticus following incubation at 37°. Greatest heat resistance was observed when 12-hour cultures at 37° were held at 12° no longer than 60 hours, and 24-hour cultures at 37° could be stored for 48 hours at 12°. Improvement of the skimmilk medium with Neopeptone increased slightly the heat resistance of stored mother cultures.
   
4. Str. thermophilus grown in the fortified medium at 37° for 10 to 12 hours and then stored 84 hours at 12° or 36 hours at 4°, and 24-hour cultures at 37° held 24 hours at 12° were equivalent in heat resistance to cultures transferred every 12 hours at 37°. Storage of cultures at 12° was better than at either 4° or 20° C.
   
5. After incubation at 40° for 8 to 15 hours, L. helveticus could be stored at 20°, 12°, or 4° C. until cultures were 48 hours old. Storage at 12° appeared to be least harmful to mother cultures.
   
6. When grown at 40° C. Str. thermophilus gave heat resistant cultures only when Neopeptone was added to the skimmilk medium. Incubation at 40° for 6 to 7 hours followed by storage at either 12° or 4° C. until cultures were 48 hours old resulted in cultures as active after heating as those transferred serially at 37°.
   
7. Mixed cultures of the film yeast, Candida krusei, and L. helveticus exhibited greater heat resistance than pure cultures of this bacterium after incubation at 25° C. for 72 to 96 hours, and also when incubation at 37° C. for 12 hours was followed by storage at 20° C. for 36 hours. Frequent transfer of associated cultures at 37° reduced the influence of the "myco-derm"; these cultures were equal in heat resistance to pure cultures of the bacteria carried under similar conditions.
   
8. Str. thermophilus, Mc, grown with "mycoderm" was more active after heating than pure cultures of the bacteria carried under similar conditions. Numerous successive 12-hour transfers of the associated culture at 37° did not decrease the influence of the film yeast.
   
9. In old cultures bacteria nearest the "mycoderm" film grew and fermented better following heat treatment than bacteria in areas farthest removed from the pellicle formed by the film yeast.
   

^* The Wisconsin Alumni Research Foundation provided a grant for this work. It is published with the permission of the Director of the Wisconsin Agricultural Experiment Station.