Fickean and non-fickean water desorption during vacuum drying of L. bulgaricus

Abstract

The recovery of Lactobacillus bulgaricus was studied in correlation to the kinetics of cell drying. When bacteria were dehydrated at 30°C, either in the presence or the absence of sucrose, the drying kinetics corresponds to a Fickean diffusion in correspondence with a short lag time. In contrast, when the bacteria were dehydrated at 70°C in the absence of sugar, the kinetics corresponds to an anomalous diffusion, and the lag time is four to five times higher than that at 30°C. However, when drying at 70°C was carried out in the presence of sucrose, drying kinetics turned into a Fickean process parallel to a substantial decrease in the lag time. The pattern of water desorption was correlated with the critical water activity. When the drying kinetics corresponds to a Fickean diffusion, the lag time started to increase at 0.7 water activity, but when the cells were dried at 70°C, the damage started at 0.5 water activity. This observation indicates that the drying rate affects the pattern of water desorption, and it can change the value of critical water activity. These results put into relevance that the cell recovery is due to the drying history and that the recovery increase produced by sucrose can be related to the maintenance of kinetic barriers for water desorption.