Effect of devitrification on the survival and resistance of dried Saccharomyces cerevisiae yeast

Yeasts are anhydrobiotes that accumulate large amounts of trehalose, which is involved in the vitrification of the cytoplasm during drastic desiccation. The effect of devitrification, which can be induced by the transient exposure of desiccated yeasts to increased humidity or elevated temperature, on the survival of yeast has been studied. A glass transition temperature (T-g)/water activity (a(w)) diagram of yeast was constructed based on differential scanning calorimetry analysis. The survival rate of yeasts that were equilibrated at different relative humidities (RHs) and temperature values over their T-g range was measured. The results revealed a long period of cell preservation at an intermediate RH (55%), with 100% survival observed after 3 months, a loss of 1.24 log colony-forming units/g recorded after 1 year at 25 degrees C and full preservation of viability at 75 degrees C for 60 min and at 100 degrees C and 12% RH for up to 10 min. These findings led us to conclude that dried yeast can resist low or intermediate RH values and elevated temperatures in the devitrified state. Considering the thermal and humidity fluctuations occurring in the yeast environments, we hypothesized that the supercooled state, which occurs immediately above the T-g after rehydration or heating, is a protective state that is involved in the persistence of yeasts at intermediate humidity levels.