CRYOPROTECTION OF YEAST BY ALCOHOLS DURING RAPID FREEZING

We have investigated the cryoprotective effect of alcohols on Saccharo myces cerevisiae and other yeast under various freezing conditions. Fo r S. cerevisiae, at a cooling rate of 3 degrees C min(-1) methanol and ethanol acted as cryosensitizers. However, at a cooling rate of 200 d egrees C min(-1), both methanol and ethanol proved superior to all oth er cryoprotectants tested, including glycerol, dimethyl sulfoxide, lac tose, trehalose, polyethylene glycol, and polyvinyl pyrrolidone. Propa n-2-ol also demonstrated a small but measurable effect although propan -1-ol and butan-1-ol demonstrated no cryoprotective effect. A minimum cooling rate of 25 degrees C min(-1) to elicit the cryoprotective effe ct of ethanol was necessary; below this rate it acted as a cryosensiti zer. At cooling rates up to 650 degrees C min(-1) substantial cryoprot ective effect was still evident. Although the effect of ethanol was va riable for other yeast genera tested, ethanol acted positively for all strains of S. cerevisiae. We hypothesize that the cryoprotective effe ct of alcohols during rapid cooling is a result of their ability to in duce increased membrane permeability, allowing rapid water equilibrati on during extracellular freezing and avoidance of intracellular ice cr ystal formation. (C) 1994 Academic Press, Inc.