The Effect of Freezing on Yeasts.

Abstract

The effect of freezing on yeasts has not received much study, at least under conditions which give information concerning their longevity, rate of death, etc. Doemens 1 exposed beer yeast to a temperature of about—190° C. for six minutes. It still retained its vitality. When he suspended the yeast in water and exposed to liquid air for 5 minutes and 20 minutes, and then thawed in cold water, its power of development was totally destroyed. Macfadyen and Rowland 2 subjected microorganisms, among which were yeasts, to a temperature of —252° C. for 6 months, after which the yeasts were reported to have suffered no reduction in vitality. They gave good growth and possessed unaltered powers of fermentation. One would infer from this report that yeasts are resistant to freezing and that there was no reduction in numbers. Bokorny 3 kept a sample of brewers'rressed yeast at a temperature of —15° C. for 24 hours and then allowed it to warm up slowly to a temperature of 7° C.; fermentation and reproduction went on for a short time but soon stopped. In another experiment the yeast was very quickly raised to 10° C.; after 4 weeks in a suitable medium it exhibited slight fermentation. Microscopic examination at this stage showed that most of the cells were dead and only a few were budding. Bokorny thus showed that beer yeast was resistant to cold but not as resistant as other microorganisms.

In the experimental work reported here 8 species of yeasts were used; 4 species of common lsactcria were also included in order to act as controls since so milch umrk has heen clme on the effect of freezing bacteria.