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The Ability of Escherichia Coli and Serratia Marcescens to Survive 62.8°C, for Thirty Minutes in Milk

Department of Bacteriology, University of Illinois, Urbana

ABSTRACT

Of the 23 cultures of Escherichia coli (Bacterium coli) studied only one was found repeatedly to withstand heating in sealed tubes for 30 minutes at 62.8°C, the temperature of experiment. This one strain was found to survive only 21 and 30 minutes in open containers.

Of the 8 strains of Serratia marcescens (Bacillus prodigiosus) tested none survived heating for more than nine minutes by any of the methods used.

Of the four sporous, lactose-fermenting forms two were found very resistant both in twenty-four hours and in seven-day old cultures; the other two strains were destroyed in three minutes in both cases. The strain which formed spore-like structures after a few days' incubation was no more resistant in a week-old culture than in a twenty-four hour culture, being killed within three minutes at 62.8°C. in either case.

On the whole, results by the three methods of treatment—in sealed tubes, in open flasks, and in litmus milk tubes—checked fairly closely. The actual time of survival was in most cases shorter in the open flasks than in the sealed tubes. No constant relation could be observed between the concentration of organisms and the time of survival.

Although neither the Escherichia coli (Bacterium coli) cultures nor the Serratia marcescens (Bacillus prodigiosus) strains used proved to be very good as indicators of efficiency of pasteurization by the holder process, for several reasons the Escherichia coli (Bacterium coli) would probably prove to be more accurate as such an indicator. It seems fair to assume, in view of the results obtained, that Escherichia coli, in concentrations in which it occurs in milk, would be destroyed by exposure to 62.8°C. for 30 minutes, yet there is always the possibility of encountering resistant strains or cultures containing some resistant cells.