TRANSPORT AND INTRACELLULAR ACCUMULATION OF ACETALDEHYDE IN SACCHAROMYCES-CEREVISIAE

The rate of acetaldehyde efflux from yeast cells and its intracellular concentration were studied in the light of recent suggestions that ac etaldehyde inhibition may be an important factor in yeast ethanol ferm entations. When the medium surrounding cells containing ethanol and ac etaldehyde was suddenly diluted, the rate of eff lux of acetaldehyde w as slow relative to the rate of ethanol efflux, suggesting that acetal dehyde, unlike ethanol, may accumulate intracellularly. Intracellular acetaldehyde concentrations were measured during high cell density fer mentations, using direct injection gas chromatography to avoid the nee d to concentrate or disrupt the cells. Intracellular acetaldehyde conc entrations substantially exceeded the extracellular concentrations thr oughout fermentation and were generally much higher than the acetaldeh yde concentrations normally recorded in the culture broth in ethanol f ermentations. The technique used was sensitive to the time taken to co ol and freeze the samples. Measured intracellular acetaldehyde concent rations fell rapidly as the time taken to freeze the suspensions was e xtended beyond 2 s. The results add weight to recent claims that aceta ldehyde toxicity is responsible for some of the effects previously asc ribed to ethanol in alcohol fermentations, especially Zymomonas fermen tations. Further work is required to confirm the importance of acetald ehyde toxicity under other culture conditions.