New approach to model rehydration of dry food particulates utilizing principles of liquid transport in porous media

Rehydration of foods containing particulates is a common unit operation, and numerous publications have extensively covered the topic. However, the rehydration mechanisms were only partially investigated, focusing mainly on a comparison with its drying counterparts. A number of relevant studies focused on various empirical approaches highlighting several factors that influence the rehydration process. Liquid uptake is typically modeled by applying either an empirical and/or a quasi-mechanistic approach. Weibull and Fick's 2nd law of diffusion are widely utilized. However, growing body of data indicates that this generalization may not be justified, or in some cases even is misleading. Few researchers recently embarked on a new approach, which is motivated by the recognition that rehydration of dry food particulates could not be explained and/or modeled solely by a Fickian mechanism. This realization is a new insight, which should lead to more advanced and accurate physical models. Mechanisms, such as water imbibition, capillarity and flow in porous media, were suggested and are considered relevant for describing the ingress of water into the dried food particulates. Basic theory of flow in porous media already developed and widely applied in several domains (e.g. soil science, petroleum and chemical engineering) is readily available and should be utilized in order to elucidate the mechanism(s) governing the rate limited uptake of water and other liquids in foods. Simultaneously, the paramount role of physical properties of the food particulates (e.g. pore-size distribution, heterogeneity, tortuosity, contact angle) and the embedding liquid (e.g. density, viscosity, temperature) should be considered in order to overcome what seems to date a poor integration in the modeling of the process. The overall objective of this review is to assess the present understanding of the rehydration process, highlight new approaches and offer recommendations on future research required to facilitate a better understanding required for deriving the mechanisms playing key role in the transport of a liquid into porous media such as dried foods. This new and integrated approach is paramount to overcome the immense complexity that hampered previous studies, and to open new avenues for optimization of food rehydration.