The influence of drying conditions upon reactivation of baker's yeast

For the production of dried starter cultures the conditions for thermal drying have to be controlled exactly to reach maximum viability and enzymes activity after rehydration. In the course of the drying process some parameters i.e. gas bulk temperature and moisture content of the microorganisms influence significantly the viability of the cells. A mathematical model using shrinking core analogy is developed to describe the drying process of microorganism pellets in a fluidized-bed drier. This model is combined with the deactivation kinetics of enzymes. Drying experiments were carried out in laboratory and technical scale fluid driers of 0.15 respectively 1.0 m diameter using baker's yeast. Enzyme deactivation kinetics were determined in separate experiments for alcohol dehydrogenase (ADH), glucose-6-phosphate dehydrogenase (G-6-PDH) and 6-phosphogluconate dehydrogenase (6-PGDH). Viability wasmeasured as CO2-production in anaerobic fermentation and O2-demand in aerobic conditions. The experimental results of drying process and enzyme deactivation are compared to calculations according to the proposed drying model showing very good agreement.