Towards a maximal cell survival in convective thermal drying processes

Freeze-drying is a traditional approach of producing desiccated microorganisms. Industrial mass-production of desiccated microorganisms, however, pursues a more economic drying process, which leads to an increasing research interest in the thermal drying processes of microorganisms. These thermal drying processes include spray drying, fluidized bed drying, vacuum drying, air-convective drying, etc. Cells being thermally dried suffer from simultaneous heat and dehydration stresses. To maximize the cell survival and to study the mechanisms of cell inactivation during such processes, it is necessary to have understandings of both the chemical engineering principles involved and the biological properties of the microorganisms to be dried. This review attempts to give a balanced discussion on both aspects. Factors influencing cell viability during thermal drying are discussed in two groups, viz., intrinsic factors related to the microorganisms being dried and extrinsic factors related to process conditions. Finally, modeling of the inactivation kinetics is briefly reviewed. This article aims to bring together some common observations and findings for different organisms subjected to thermal drying, and discuss the mechanism underlying these observations. Efficient drying which is associated with fast removal of water content has economical benefits, but hurts cells. The desiccation of microorganism thus is a balance between these two considerations and needs to be strain-specifically optimized.