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Rheological and ultrastructural properties of wheat gluten Cumming, Daniel Brian

Abstract

A three part study is presented in which the ultrastructural and dynamic shear properties of modified and unmodified rehydrated commercial, vital wheat gluten are investigated. Both scanning and transmission electron microscopy were employed to observe the ultrastructural interactions of starch, protein and lipid fractions of rehydrated gluten and flour doughs. Dynamic shear properties of rehydrated gluten were studied with a cone and plate rheometer in oscillatory shear mode, and the effect of adjusting the levels and nature of the protein, starch and lipid phases was also observed using both electron microscopy and dynamic shear testing. Gross stressing of gluten and flour specimens resulted in noticeable alteration to their ultrastructural appearance, manifested as a directional orientation of the lipoidal inclusions in gluten and the formation of a definite fiber or sheeted structure in flour dough. Removal of free lipid seemed to affect protein-protein as well as starch-protein interaction as evidenced by the microscopic appearance. Gluten proved to be a ideologically complicated material requiring multiple regression analysis to adequately describe the relationships of dynamic shear storage and loss moduli, dynamic viscosity and dynamic shear loss tangent with factors such as oscillatory frequency and cycle, specimen moisture content and age, and amplitude of strain input. Moisture content of the specimens and oscillatory cycling had significant influence over the storage and loss moduli and apparently were responsible for the failure of simple relationships to account for variation in the rheological parameters for pooled or even replicated samples. It was found that over the range of oscillatory frequencies employed (0.075 - 0.949 sec⁻¹) the storage modulus or elastic component increased with increasing frequency. However, the loss modulus or viscous component was found to have a greater rate of increase and consequently the loss/storage ratio or loss tangent showed a shift of the viscoelastic response toward the viscous end of the behavorial continuum. Adding of lipid (wheat germ oil and unemulsified, hydrogenated rapeseed shortening), and pearled wheat starch and petroleum ether extraction of free lipid had no statistically significant effect on the nature of the viscoelastic response. The level of response was affected markedly, ranging from an intercept value for dynamic viscosity (vs. oscillatory frequency) of 8330 poise for 5% shortening to 31,050 poise for 30% starch. Increasing level of lipid from 0% free lipid to 5% added wheat germ oil caused a general decrease in response level. However, increasing lipid level beyond about 3% did not result in any significant change in response. Lipid seemed attracted to starch granule surfaces and began to coalesce as evidenced by ultrastructural appearance as the level of added lipid increased. The rheological behavior of gluten in both modified and unmodified form agreed well with a recent molecular model for wheat gluten viscoelasticity based on hydrogen bonding.

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