The role of glass transition temperature (Tg) and storage temperature (Ts) in explaining the survival behavior of dried Bacillus cereus

Abstract

Recent studies have indicated a correlation between glass-transition and desiccation tolerance in bacteria contaminating dry food. To validate and extend the applicability of this relationship, we focused on Bacillus cereus with underexplored glass-transition mechanisms. This study aims to investigate the vitrification characteristics of B. cereus and assess the potential of glass-transition temperature (T_g) in indicating bacterial survival behavior in desiccated environments. We measured the mechanical T_g and metabolic changes of dried B. cereus, prepared by different drying methods (air-drying and freeze-drying) and stored at various water activity (a_w) levels (0.43, 0.62, 0.75, and 0.87). The T_g of air-dried B. cereus cells was higher than that of freeze-dried cells, and unlike air-dried cells, freeze-dried cells did not show a decrease in T_g due to the increase in a_w. Changes in air-drying temperature and humidity had little effect on the T_g. Metabolite profiles varied with the drying method, indicating the potential for vitrification in B. cereus cells. In addition, we examined the survival of dried B. cereus cells under different storage temperatures (T_s; 4 °C, 25 °C, and 42 °C) and a_w levels (0.43 and 0.87). Freeze-dried cells were inactivated faster than air-dried cells across all T_s and a_w levels. Air-drying temperature and humidity had minimal impact on survival behavior. Furthermore, the difference between T_g and T_s (T_g − T_s) was considered an indicator of survival for dried bacteria. When the temperature differential (T_g − T_s) reached 18.27 °C, a clear distinction was observed between surviving and inactivated cells. Therefore, (T_g − T_s) can serve as an alternative parameter for predicting the desiccation tolerance of dried bacterial cells.