GROWTH RATE OF COLIFORM ORGANISMS IN COTTAGE CHEESE AND RECONSTITUTED NONFAT DRY MILK1

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Abstract
Summary The objective of this study was to determine the population trends of representative strains of Escherichia coli and Aerobacter aerogenes in cottage cheese and reconstituted nonfat dry milk (skim milk). Inoculated milk samples were stored at 32, 39, 50 and 90 F for a maximum of 130, 76, 119, and 11 days, respectively. The inoculated cheese samples were stored at 39, 50 and 55 F for 80 days. In milk at 50 F the generation time of E. coli during the logarithmic growth phase varied from 516 to 642 minutes and at 90 F the generation time varied from 33 to 39 minutes, respectively. The corresponding generation times for A. aerogenes were 540 to 648 minutes and 29 to 33 minutes. Both coliform species decreased in population at 32 and 39 F and the decrease was greater at 39 F. In cottage cheese the population of both coliform species increased at 55 F and decreased at 39 F, but at 50 F the numbers of E. coli increased while those of A. aerogenes decreased. During the first five days at 55 F, the generation times of E. coli averaged 31.4 hours while A. aerogenes averaged 32.8 hours. At 50 F the generation time of E. coli averaged 48.2 hours.

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