Browsing by Author "Prachaub Kwanyuen, Committee Co-Chair"

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Investigation of Molecular Forces Involved in Gelation of Commercially Prepared Soy Protein Isolates
(2002-09-26) McKlem, Lacey Kay; Jonathan Allen, Committee Member; Christopher R. Daubert, Committee Member; Prachaub Kwanyuen, Committee Co-Chair; Tyre C. Lanier, Committee Co-Chair
Gelling behavior of soy protein isolates (SPI) commercially prepared from two soybean cultivars, Prolina and Brim, was studied. SPI (prepared at 12% protein w/w) were predenatured by commercial processing and gelled immediately upon hydration, prior to heat treatment. This research first investigated the effects of temperature, holding time, and reheating on thermal reversibility and reformability of these commercial SPI gels. Secondly, chemical reagents were added to determine the molecular forces contributing to their gelation. Small strain rheology was used to determine the effect of temperature (40 – 90 °C), holding time at elevated temperature (0 or 30 min), and reheating on thermal reversibility of Prolina and Brim gels. Final gel rigidity (measured by storage modulus G') increased with increasing endpoint temperature, and holding at each endpoint temperature further increased the final gel rigidity. During heating, all gels exhibited a decrease in G', which was largely reversible upon cooling. Prolina and Brim gels exhibited increases in G' during holding at temperatures ranging from 50 – 90 °C. Reheating did not enhance or diminish the gel rigidity. Vane fracture testing was utilized to investigate temperature effects and reformability of cooked gels. Fracture stress of heated and cooled gels was significantly higher than initial gels, while fracture deformation was significantly lower. Cooked gels were rechopped and gels reformed at 10 °C, again strengthening upon subsequent heating and cooling. Again, additional thermal treatment did not enhance or diminish gel properties. To investigate contribution of molecular forces, the chemical reagents urea (weakens hydrogen bonding and hydrophobic interactions) and dithiothreitol (DTT) (reduces disulfide bonds) were added to Prolina and Brim SPI and resulting gelling behavior was monitored during small strain rheology. Control (deionized water) and urea-added samples gelled initially and G' decreased upon heating to 80 °C, whereas DTT-added samples did not gel initially. Gel rigidity of both control and DTT-added samples increased during holding at 80 °C. Final gel rigidity of urea- and DTT-added samples was lower than control samples; the sum of the final G' values for the urea- and DTT-added samples neared the final G' value of the control sample for both Prolina and Brim gels. Urea had a more detrimental effect on gel rigidity of Brim gels, whereas DTT had a similar effect on Prolina and Brim. Prolina and Brim gels (unheated and heated) were solubilized in various chemical reagents. Addition of urea + DTT completely solubilized all samples, while solubilization in either reagent alone was greater than solubilization with the control treatment. More protein was solubilized from gels that exhibited lower gel rigidity. Results from this investigation indicate that hydrogen bonding, hydrophobic interactions, and disulfide bonding all play roles in commercial SPI gelation. Disulfide bonding is important in the initial gel network as indicated by a high degree of solubilization in DTT and formation of gel prior to heating by pastes containing urea. Disulfide bonds are not essential for gel formation, but their presence adds rigidity to the gel network. Hydrogen bonding and hydrophobic interactions, however, are the primary forces contributing to gelation of these commercially prepared SPI. This is evidenced by the extensive reformability of the gels as well as the similar effects of heating and holding on Prolina and Brim gels. More specifically, increases in G' during holding and cooling were attributed to hydrophobic interactions and hydrogen bonding, respectively.