Effective Maxwell−Stefan diffusion model of near ambient air drying validated with experiments on Thomson seedless grapes

Abstract

With 8% of the world's grapes being dried into raisins, grape drying constitutes a sizable part of agricultural revenue. The drying process in the developing regions of the world struggles with a lack of reliable drying techniques and rising energy costs. In this work, we report a systematic study to elucidate the intrinsic drying kinetics of Thomson seedless grapes. The grapes are subjected to drying by warm air (40À55 C) rather than hot air (>60 C), which has the advantage of producing the sought-after green raisins. The air velocity is varied between 0.05 and 0.15 m/s to isolate the effect of external mass transfer resistance. A 1D mathematical model was developed that incorporates the effects of internal diffusion and external mass transfer resistance as well as the effects of temperature and relative humidity on the kinetics and driving forces. In addition, the model incorporates a novel thermodynamic model to describe the vapour pressure of water at the surface of the grape. The uniqueness of the model is that it combines thermodynamics and mass transfer in a single framework. Therefore, the parameters that describe the evaporation are the same parameters that describe the internal diffusion within the matrix, making the model robust. This robust, experiment-trained model can be reliably transferred to develop detailed computational protocols to model cost-effective grape drying processes in the field.