Protective effect of sugars on storage stability of microwave freeze-dried and freeze-dried Lactobacillus paracasei F19

Aims: Microwave freeze drying (MWFD) in comparison with conventional freeze drying allows for intensification of the preservation process of lactic acid bacteria without imposing additional processing stress. Viability as a function of storage time of microwave freeze-dried Lactobacillus paracasei ssp. paracasei F19 was investigated in comparison to conventionally lyophilized bacteria of the same strain. Furthermore, the impact of the protectants, sorbitol, trehalose and maltodextrin, on shelf life was analysed. Methods and Results: The highest inactivation rates of 0·035 and 0·045 day⁻¹, respectively, were found for cultures without protectants. Thus, all additives were found to exhibit a protective effect during storage with inactivation rates between 0·015 and 0·040 day⁻¹. Although trehalose and maltodextrin samples were in the glassy state during storage, in contrast to samples containing sorbitol as protectant, the best protective effect could be found for sorbitol with the lowest inactivation rate of 0·015 day⁻¹. Conclusions: Due to its low molecular weight, it might protect cells owing to better adsorption to the cytoplasma membrane. Sorbitol additionally shows antioxidative properties. Storage behaviour of microwave freeze-dried cultures follows the typical behaviour of a product dried by conventional lyophilization. No significant influence of the drying technique on storage behaviour was detected. Significance and Impact of the Study: General findings concerning storage behaviour in freeze drying are likely to be applicable in MWFD with only slight adjustments.