Rehydration temperature is critical for metabolic competence and for membrane integrity in active dry yeast (ADY)

Respiration and fermentation were lower in active dry yeast (ADY) rehydrated at 0°C than in ADY rehydrated at 40°C. In agreement with other reports, it was found that membrane permeability increased during rehydration. In addition, ADY rehydrated at 0° did not reseal, even after hours of incubation at 40°C. Using 32P-nuclear magnetic resonance it was found that the cellular concentration of sugar phosphates, phosphate, pyrophosphate, NADH and ATP were lower in ADY rehydrated at 0°C. In addition, the phospholipid peak had a higher height to broadness ratio at 0°C than at 40°C, suggesting that membranes in the 0° sample were more disordered. The lower fermentation rate in ADY rehydrated at 0° could not be due solely to membrane permeation since addition of cofactors that leaked from these cells did not reactivate fermentation. In cell free extracts or in toluenized cells it was observed that some activities were modified after rehydration at 0°C. In the 40°C sample a lower activity of pyruvate decarboxylase and higher fructose-1,6-bisphosphatase and ATPase activities were detected. As a result, higher levels of ADP and pyruvate were found in the cell. Higher ADP levels could contribute to the higher fermentation rate of the cells rehydrated at 40°C. Enzyme modification might explain the low viability of ADY observed by a plating method, even in cells that were impermeable to a vital dye.