Dehydration and thermal inactivation of Lactobacillus plantarum WCFS1: Comparing single droplet drying to spray and freeze drying

We demonstrated that viability loss during single droplet drying can be explained by the sum of dehydration and thermal inactivation. For Lactobacillus plantarum WCFS1, dehydration inactivation predominantly occurred at drying temperatures below 45 °C and only depended on the moisture content. Above 45 °C the inactivation was due to a combination of dehydration and thermal inactivation, which depended on the moisture content, temperature, and drying time. A Weibull model was successfully applied to describe the thermal and dehydration inactivation and enabled the prediction of residual viability of L. plantarum WCFS1 after single droplet drying. Subsequently, the model was evaluated to predict the viability loss during laboratory scale spray drying, showing a remarkable agreement if assumed that only thermal inactivation occurred. This indicated that very high drying rates in laboratory scale spray drying could induce instant fixation of the cell suspensions in a vitrified matrix and thereby preventing dehydration inactivation. Finally, the influence of drying rate on remaining viability was evaluated by comparing single droplet drying, freeze drying and laboratory scale spray drying of the same bacterial suspension. It was shown that slow drying leads to large dehydration inactivation, which diminished in fast drying processes such as laboratory scale spray drying where thermal inactivation appears to be the predominant mechanism of inactivation