The Contribution of the Inherent Restricted Mobility of Glassy Sugar Matrices to the Overall Stability of Freeze-Dried Bacteria Determined by Low-Resolution Solid-State ¹H-NMR

The aim of this work was to evaluate whether, in case of freeze-dried bacteria, the protective effect of a surrounding glassy matrix can be ascribed to its inherent restricted molecular mobility. Lactobacillus paracasei ssp. paracasei together with three different protectants (lactose, trehalose and dextran) was freeze-dried and stored at different temperatures and water activities (a _w). The spin-spin relaxation time T ₂ was determined by means of low resolution ¹H-NMR spectroscopy and described in relation to the storage conditions and the glass transition temperature T _g. Compared to the disaccharides, dextran generally showed lower absolute T ₂ values and a weaker dependence on storage a _w and temperature. For lactose and trehalose, the plasticising effect of water and temperature was significantly stronger. Their relaxation time T ₂ was shown to be only dependent on {increment}T, the temperature distance to T _g. Furthermore, both disaccharides showed an increase in T ₂ already 20-40 °C below T _g. Thus, T _g in reference to T ₂ does not act as an absolute threshold. This fact could explain the finding of several publications that an absolute stability for freeze-dried bacteria can only be achieved 30-50 °C below T _g. Further comparison between T ₂ and the corresponding inactivation rates revealed that the relevance of mobility for the stability of lyophilisates is strongly temperature and system dependent. At low storage temperatures, a potential rate-limiting effect due to the lack of detrimental chemical and physical reactions is invisible. However, with increasing storage temperature, the restricted mobility is shown to become a rate limiting bottleneck. Thus, the harsher the environmental condition, the more relevant is the protective effect of a surrounding glassy matrix.