The water adsorption process by dough films at 20 and 40 ℃ was measured using a dynamic vapor sorption system under nine relative humidity (RH) levels. Water diffusivity was expressed as a function of water content and temperature according to the Vrentas & Duda free volume theory. The parameters of the free volume model were obtained by inversion of water content as a function of time. The results showed that the water diffusion coefficient determined based on the free volume theory could well simulate the water adsorption process by dough films at most water contents tested, but the predicted values were significantly lower than the actual ones at low water contents. Therefore, the Vrentas & Duda was improved by adding a free volume term as a function of water content. As the water content increased, the water diffusion coefficient slightly decreased initially, then increased, and finally increased rapidly. Finally, the mechanism of water diffusion at low water contents was discussed from the perspectives of β-transition and free volume changes.

Water sorption and its hysteresis are widespread phenomena that are important for food processing and food quality. In the present study, water sorption and desorption isotherms of wheat flour were measured at three temperatures using a dynamic vapor sorption (DVS) instrument, and modeled according to the Vrentas-Vrentas (VV) theory. Results showed that the degree of water sorption hysteresis decreased with an increase in temperature. In addition, overlapping between desorption and sorption isotherms was observed at high water activity. Based on this, an approach was proposed to determine the critical water content at which glass transition occurs. For both water sorption and desorption, the Flory-Huggins interaction parameter χ varied with the volume fraction of water, and a modified Fermi function was proposed using the Peleg model to describe this relationship. When desorption isotherms were modeled based on the sorption parameter χ, the VV model parameter k was expressed as a function of the volume fraction of water using a Fermi-type function.

This study was performed in order to investigate water clustering and its contribution to water diffusion in wheat flour. Based on the water adsorption and desorption isotherms of wheat flour at 20, 30 and 40 ℃ , the water clustering properties were explored using the Park isotherm model, the Zimm-Lundberg method and the Brown method. Consistently, the results of the three methods suggested that water clustering occurred at high water activity (a_w) levels, while the critical a_w for clustering obtained from the three methods were only slightly different, ranging from 0.70 to 0.75, from 0.73 to 0.79 and from 0.56 to 0.68, respectively, which varied with temperature and the water sorption process. The predicted mean cluster size at a_w of 0.95 was approximately 3.5, 4.5 and 6.5 from the three approaches, respectively. The number of water clusters kept almost unchanged at a_w not lower than 0.90. Water diffusivity sharply decreased when a_w was over 0.62–0.70. Furthermore, based on thermodynamic factor and self-diffusivity, it was found that water molecules were more likely to interact with themselves, leading to water clustering, which could be the major contributor to the sharp decrease of water diffusivity. When a_w was equal to or more than 0.90, the strong self-diffusion capacity brought about by high water content could counteract the negative effect of water clustering, resulting in an almost constant water diffusivity.