High viability of dried probiotics is of great importance for immediate recovery of activity in fermented foods and for health-promoting effects of nutraceuticals. The conventional process for the production of dried probiotics is freeze-drying. However, loss of viability occurs during the drying and storage of the dried powder. It is believed that achieving the "glassy state" is necessary for survival, and the glassy state should be retained during freezing, drying, and storage of cells. Insight into the role of glassy state has been largely adopted from studies conducted with proteins and foods. However, studies on the role of glassy state particularly with probiotic cells are on the increase, and both common and explicit findings have been reported. Current understanding of the role of the glassy state on viability of probiotics is not only valuable for the production of fermented foods and nutraceuticals but also for the development of nonfermented functional foods that use the dried powder as an adjunct. Therefore, the aim of this review is to bring together recent findings on the role of glassy state on survival of probiotics during each step of production and storage. The prevailing state of knowledge and recent finding are discussed. The major gaps of knowledge have been identified and the perspective of ongoing and future research is addressed.

The proceeding change of water content in a freeze-drying process, as well as the physical state of cell-sugar suspensions with Lactobacillus paracasei ssp. paracasei, was monitored by means of an in situ weighing system. Due to the pressure and temperature variations in the drying chamber, the weighing system was found to be influenced. This deviation of the weighing system was quantified by means of drying simulations with drying-inert Teflon dummies. This quantified deviation could be used for correction of the following freeze-drying processes. The applicability of this correction method was verified by drying model sugar solutions with known dry matter. Taking the quantified drift of the balance into account, the measured water contents for the model solution were proved to deviate less than 5% from the actual value. Finally, this verified correction method was applied to freeze drying of cell-sugar suspensions. As a result, the drying course of the cell-sugar samples could be depicted in the appropriate state diagram. Thus, the weighing system could be used to monitor the state transitions, which the bacterial suspension experienced during freeze drying, and qualified for specific investigations on the glassy state during freeze drying.

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