Water sorption and transport in dry crispy bread crust

Water sorption and dynamical properties of bread crust have been studied using gravimetric sorption experiments. Water uptake and loss were followed while relative humidity (RH) was stepwise in- 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. Sorption curves of isotherm experiments could be best described by the Fickian diffusion model for low RH and by the exponential model for large RH. Isotherm equilibrium values could be best described by the GAB equation. Transport rates depend on moisture content and show a maximum around RH=70%. Adsorption and desorption curves from oscillatory sorption experiments could be described best by the exponential model. From comparison of experimental sorption curves and the power-law model for short times it followed for all bread crust that the diffusional coefficient n is close to one. Normally this is associated to so-called case II diffusion and water transport that is limited by relaxation of the solid material. However, additional observations suggest that this may not be a valid explanation and that a kinetic barrier for water transfer to the solid matrix may explain the observed exponential behavior