Effect of porosity on rehydration of dry food particulates

Rehydration of food particulates is a complex phenomenon affected by numerous factors that typically include pre-drying treatments, mode of dehydration, structure, composition and medium viscosity. Freeze drying (FD), air drying (AD) and their combinations, were utilized to produce an array of porosities, ranging from very high to very low values for FD and AD carrots, respectively. Bulk porosity correlated significantly with open, but not with closed, porosity. Bulk and open porosities decreased with AD time. Scanning electron micrographs of the FD samples verified their organized and more open structure in comparison with the AD carrots. Rehydration ratio increased with bulk and open porosity, and was not affected by the closed porosity. The effective moisture diffusivity, derived from fitting the normalized Weibull distribution, increased with bulk and open porosity and was about two orders of magnitude higher for the FD, than for the AD, carrots. The Weibull shape parameter, (beta), was inversely related to porosity. Its values indicated that water uptake of only the AD carrots followed a Fickian diffusion. A critical porosity value above which water mechanism changed from a Fickian diffusion to imbibition into a porous medium is suggested.