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Interrelationships of Microorganisms in Cream. II. Lactobacillus casei, Bacillus subtilis, and Streptococcus lactis¹

Department of Dairy Industry, Iowa State College, Ames

ABSTRACT

1. When sterile cream was inoculated with Lactobacillus casei and incubated at 10°, 20°, and 30° C., the maximum tryptophan was 9.4 p.p.m. and occurred in the 30° C. sample after 7 days incubation.
   
2. When Streptococcus lactis and L. casei were inoculated into sterile cream, S. lactis had no influence on L. casei population. L. casei slightly inhibited the maximum S. lactis population attained, and after 10 days at 30° C. the S. lactis count decreased to 20 per milliliter. The maximum tryptophan produced by the combination of S. lactis and L. casei was 29.6 p.p.m. and occurred in the 20° C. sample after 7 days incubation.
   
3. When Bacillus subtilis was inoculated into sterile cream, the organism population and amount of proteolysis increased as the temperature increased, with a maximum tryptophan value of 46 p.p.m. occurring in the 30° C. sample after incubation for 3 days.
   
4. When S. lactis and B. subtilis were inoculated into sterile cream, substantial proteolysis occurred at all temperatures. The B. subtilis population declined rapidly after the maximum S. lactis population was attained.
   
5. Bitter flavors suggestive of proteolysis were uniformly present in cream containing 18 to 25 or more p.p.m. of tryptophan. Tryptophan values of 10 p.p.m. or more were usually associated with cream scoring 90 (grade B) or lower, but all cream of this quality did not possess high tryptophan values.
   
6. None of the organisms or combinations used in this work produced W.I.A. values high enough to be significant from the quality standpoint.
   

¹ Journal Paper No. J-2938 of the Iowa Agr. Expt. Station, Ames. Project No. 1050.

² Present address: Dairy Department, Michigan State University, East Lansing.