Thermal destruction of dried vegetative yeast cells and dried bacterial spores in a convective hot air flow: strong influence of initial water activity

Thermal treatment of Bacillus subtilis spores and Saccharomyces cerevisiae cells dried on glass beads was performed at various initial water activities (in the range 0.10-0.90). Experiments were carried out at 150 degrees C, 200 degrees C and 250 degrees C for 5-120 s. Significant destruction of up to 10(7) vegetative cells and up to 10(5) spores g(-1) was achieved, depending upon treatment conditions. This study demonstrated that the initial water activity (a(w)) value of a sample is very important in the destruction or survival of microorganisms treated with hot air stresses. As described previously, the heat resistance of spores and vegetative cells was strongly enhanced by low initial a(w) values until an optimal a(w) value between 0.30 and 0.50, with maximal viability at 0.35 for both S. cerevisiae and B. subtilis. However, our results highlighted for the first time that very low initial a(w) values (close to 0.10) greatly improved the destruction of spores and vegetative cells. Factors and possible mechanisms involved in the death of vegetative cells and spores are discussed.