Water Sorption and Transport in Dry, Crispy Bread Crust

Abstract

Water - sorption and dynamic properties of bread crust have been studied in gravimetric sorption experiments. Water uptake and loss were measured while relative humidity (RH) was stepwise increased or decreased (isotherm experiment) or varied between two adjusted values (oscillatory experiment). Experimental results were compared with the Fickian diffusion model and empirical models like the exponential and power - law model. The sorption curves that resulted from the isotherm experiments were best described by the Fickian diffusion model for low RH and by the exponential model for high RH. Transport rates depended on moisture content and showed a maximum around RH = 70%. Adsorption and desorption curves from oscillatory experiments were best described by the exponential model. From comparison of the experimental sorption curves and the power - law model for short times it followed for all bread crust that the diffusion coeffi cient n is close to 1. Normally, this is associated with so - called case II diffusion and water transport that are limited by relaxation of the solid material. However, additional observations suggest that this may not be a valid explanation and that a gradual, instead of stepwise change in RH and/or a kinetic barrier for water transfer to the solid matrix may explain the observed exponential behavior. © 2010 Blackwell Publishing.