Development of a quantitative model for solute diffusion in biopolymers based on free-volume theory and rheological properties

A quantitative method based on free volume theory was developed to study solute diffusion in biopolymers as a function of water activity (aw). This model is based on the characterization of free volume changes in a polymeric material through physical properties of polymeric matrix. Soy flour was used as a model food system.; Rheological properties of soy flour were characterized using mechanical spectrometry and differential scanning calorimetry (DSC). The WLF constants (C1 and C2) and glass transition temperatures (Tg) of soy flour were determined. Time temperature superposition principle was used to determine C1 and C2 of soy flour samples equilibrated to aw of 0.43, 0.57, 0.68 and 0.75. The WLF constants were found to be aw dependent and different from the universal constants. In the aw range from 0.43 to 0.75 C1 varied between 17 and 27 while C2 ranged from 116 to 164°C. Empirical relationships were developed that related the WLF constant to aw. Glass transition temperature of soy flour matrix was determined as a function of moisture. DSC was used for samples at moisture contents lower than 7.1%, while mechanical spectrometry was used for samples at moisture contents higher than 7.1%. Tg varied between -54.3 and 139.5°C in the moisture content range from 30.1 to 0%.; The plasticization effects of six individual flavor compounds namely carvone, hexanal, hexyl alcohol, limonene, octanoic acid and octanol as well as blueberry, strawberry and vanilla flavor formulations were studied using DSC on completely dried soy flour matrix. Hexanal and limonene were good plasticizer for soy flour. Octanol and hexyl alcohol did not show plasticization effects. Flavor formulations also displayed Tg depression.; Denaturation behavior of soy matrix was also investigated in the presence of flavors. The thermal denaturation temperature of 11S soy globulin was not affected from flavors. However, 7S was more sensitive to flavors. Hexanal and limonene reduced the thermal stability of 7S globulins. Commercial flavors had a dramatic influence on the denaturation behavior. Thermal stability and cooperativity of protein denaturation changed to a great extend in the presence of these flavors.; Moisture and flavor diffusion in soy flour was studied to validate the model. A dynamic sorption apparatus (Isohumectometer) was used to determine diffusion coefficients of water and flavors as a function of moisture content. Our results showed the importance of Tg on flavor/moisture diffusion in soy flour. There was a change in diffusion mechanism at the Tg of soy flour matrix. Parameters of modified free volume parameters were determined from the experimental diffusion coefficients. Proposed model described the diffusion behavior of solutes successfully. The model was able to capture the change in diffusion mechanism at Tg for all solutes studied.