Experimental study and optimization of freeze-drying cycles of a model Casei type probiotic bacteria

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The objective of this paper was to present experimental results concerning the optimization of freeze-drying cycles of a model type casei probiotic bacteria with water based formulations using lactose and polymer (PVP) as cryo/lyo protectants. First, we investigated the influence of different formulations and of different freezing protocols (freezing rates and pre-freezing treatment) on the viability of the model probiotic strain after the freezing step. Secondly, the heat transfer coefficient was characterized by an overall heat transfer coefficient determined with pure water. In our very soft operating conditions - total gas pressure from 10 to 30 Pa and shelf temperature from −20 °C to 10 °C – the sublimation process was mainly controlled by the heat transfer from the vial surroundings to the sublimation front that take place by conduction, by contact at the vial bottom and by radiation from the self and from the chamber walls. Next, as concern the single freeze-drying process approach, with the preselected formulation - composition and freezing rate - we studied the influence of the sublimation operating parameters, namely the total gas pressure inside the sublimation chamber and the shelf temperature on the bacteria survival ratios and on the sublimation times. In agreement with previous literature data, we observed that the sublimation rate increased with the increasing total gas pressure and the shelf temperature. It is worth to note that these experimental data have shown that sublimation times could be reduced by about 40% without reducing the bacteria viability by realizing the sublimation at temperatures slightly higher (about 4 °C) than the classical limit temperatures encountered during the freeze-drying processes of thermosensible biological systems like vaccines i.e., the vitreous transition temperatures of the formulations.