Impact of Water Activity,Temperature,and Physical State on the Storage Stability of Lactobacillus paracasei ssp.paracasei Freeze-Dried in a Lactose Matrix

摘要

The aim of this study was to determine whether the combined effect of water activity and temperature on inactivation rates of freeze-dried microorganisms in a lactose matrix could be explained in terms of the glass transition theory.The stabilized glass transition temperature,T_g,of the freeze-dried products was determined by differential scanning calorimetry at two different temperatures,T (20 and 37 °C),and different water activities (0.07-0.48).This information served as a basis for defining conditions of T and water activity,which led to storage of the bacteria in the glassy (T < T_g) and nonglassy (T > T_g) states.The rates of inactivation of the dry microorganisms subjected to different storage conditions were determined by plate counts and could be described by first-order kinetics.Rates were analyzed as a function of water activity,storage temperature,and the difference between T_g and T.Inactivation below T_g was low;however,T_g could not be regarded as an absolute threshold of bacteria stability during storage.When the cells were stored in the nonglassy state (T > T_g),inactivation proceeded faster,however,not as rapid as suggested by the temperature dependence of the viscosity above the glass transition temperature.Furthermore,the first-order rate constant,k,was dependent on the storage temperature per se rather than on the temperature difference between the glass transition temperature and the storage temperature (T - T_g).