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

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, Tg, 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< Tg) and nonglassy (T> Tg) 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 Tg and T. Inactivation below Tg was low; however, Tg could not be regarded as an absolute threshold of bacteria stability during storage. When the cells were stored in the nonglassy state (T> Tg), 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 – Tg).

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