Micro-organism inactivation during drying of small droplets or thin-layer slabs – A critical review of existing kinetics models and an appraisal of the drying rate dependent model

In this article, we have reviewed the commonly used inactivation kinetics models in order to explore a fundamental basis for modelling the inactivation mechanism during drying of small droplets or thin-layer slabs of bioactive liquid-based materials. In the literature, there are only a few studies dealing systematically with the theoretical and/or experimental aspects of the inactivation kinetics during drying of small objects. In most studies, the inactivation kinetics model for enzymes and micro-organisms was formulated by incorporating the average temperature and water content of the material. The experimental validation of these models has been limited. In general, the inactivation kinetics models published in the literature were validated using ‘heating-only’ experiments (no evaporation involved) and large deviations were reported between the predicted and measured survival rates. It is uncertain if drying parameters such as the average moisture content and average temperature are most appropriate for correlating the inactivation kinetics. Recently, the drying-rate and temperature-rate dependent models were proposed and tested against the micro-organisms survival during drying experiments in order to develop a more accurate approach. The rate-dependent models seem to be more useful compared to the traditional models with the average temperature and water content as two prime parameters. In this paper, the authors have presented an assessment considering the spatial distribution of the inactivation kinetics emphasizing the difference between the surface moisture content and the centre moisture content of the material. This appraisal provides a fundamental qualification of the rate-dependent models, which have been obtained empirically.