Empirical Models of Sigmoid and Non-Sigmoid Hydration and Moisture Sorption Curves

There are several published empirical mathematical models to describe the water content vs. time relationships in dry or dried-food spontaneous hydration, or in deliberate rehydration, similar to or the same as those originally proposed for water vapor sorption kinetics. Most of these models come in one of two forms: for non-sigmoid and for sigmoid relationships. Some, notably the stretched exponential (“Weibullian”) with an adjusted shape parameter, can describe both. All the empirical hydration models are rarely, if ever, unique, and most of them can be used interchangeably for a given set of experimental data. It is proposed to add an expanded version of a particular popular hydration model of non-sigmoid curves so that it, too, can describe both kinds of hydration patterns. Either model would facilitate the mathematical description of systems or processes where a non-sigmoid hydration pattern turns into a sigmoid one, or vice versa. In principle, variants of these two model types can be used to describe water loss or drying curves, at least qualitatively.