42-9 |
Evaluation of the WLF constants of extruded soy flour and their dependence on water activity and degree of cross-linking |
B. K. ASHOKAN and J. L. Kokini. Department of Food Science, Rutgers University, 65 Dudley Rd, New Brunswick, NJ 08901 Williams, Landel and Ferry (WLF) equation is essential in many calculations in rheology including estimation of viscosities in the rubbery state, prediction of diffusion coefficients and dielectric properties in the glassy and rubbery states. The WLF constants as a function of water content or water activity are not available in food literature. The WLF equation is a useful tool in predicting the temperature induced changes in foods that are exposed to specific food processing and storage conditions and hence knowledge of the WLF constants is essential. The objective of the present study is to determine the WLF constants of a food polymer, extruded soy flour. The variation of the WLF constants with the degree of cross-linking in the polymer, which is a function of temperature, was studied. The influence of the water activity of the polymeric matrix on the WLF constants was also investigated. Roasted, defatted soy flour was extruded at 115, 130 and 145°C. The extruded product was then stored at various water activities. The degree of cross-linking was semi-quantitatively estimated by the preferential solubility of the extrudate in various solvents. WLF constants were determined using time-temperature superposition principles and Ferry's reduction method. The WLF constants (C1, C2) of soy flour extruded at 130 and 115°C (aw=0.973) were (21.901, 87.82°C) and (26.048, 208.66°C) respectively. The constants at other cooking temperatures and water activities have also been determined. The variation of the constants was correlated with the degree of cross-linking. The WLF constants are known to be material properties rather than 'universal values'. The present study will allow a quantitative determination of the effect of water activity and degree of cross-linking on the WLF constants of extruded soy flour. This can lead to a better understanding of the variability of the WLF constants in food polymers.
Session 42, Food Engineering: Rheology and texture
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