Publication no. C-2004-0129-03R |  VIEW ARTICLE

Effect of Moisture Content on Viscoelastic Properties of Hydrated Gliadin.

S. E. Martling (1), S. J. Mulvaney (1,2), and C. Cohen (3). (1) Department of Food Science, Stocking Hall, Cornell University, Ithaca, NY 14853. (2) Corresponding author. E-mail: <sjm7@cornell.edu> (3) School of Chemical and Biomolecular Engineering, Olin Hall, Cornell University, Ithaca, NY 14853. Cereal Chem. 81(2):207-219. Accepted September 7, 2003. Copyright 2004 American Association of Cereal Chemists, Inc.

The effect of moisture content on the linear viscoelastic properties of gliadin hydrated to 30 and 40% moisture content [gliadin(30%) and gliadin(40%), respectively] was determined. These two moisture contents bracketed the equilibrium moisture content of gliadin, which was 37.6%. Time-temperature-superposition was used to develop master curves of the elastic modulus (G(prime)), viscous modulus (G(double prime)), dynamic viscosity (eta(prime)), and tan delta (G(double prime)/G(prime)) from isothermal frequency sweep data obtained at 25-80°C. Smooth master curves were obtained for all of the viscoelastic functions for both gliadins. G(prime) and G(double prime) showed a power law dependency on frequency (with G(double prime) > G(prime)) for frequencies <0.1 rad/sec for gliadin(30%) and <1 rad/sec for gliadin(40%). The low-frequency-limiting slopes on log-log coordinates for G(prime) and G(double prime) were 0.700 and 0.646 for gliadin(30%), respectively. Corresponding values were 0.658 and 0.614 for gliadin(40%). G(prime) crossed over G(double prime) at a frequency of approximately 0.3 rad/sec for gliadin(30%), while G(prime) and G(double prime) for gliadin(40%) only became congruent at higher frequencies. Both gliadin samples showed appreciable frequency dependence of eta(prime) over the entire frequency range, while eta(prime) was greater for gliadin(30%) than for gliadin(40%) at all frequencies, but especially at the lowest frequencies. Tan delta increased gradually from a value of approximately 1 at 0.1 rad/sec to approximately 2 at the lowest frequency of 0.0002 rad/sec for both gliadins, but tan delta decreased rapidly for gliadin(30%) for frequencies >0.1 rad/sec. Thus, the main difference between gliadin(30%) and gliadin(40%) was that elastic effects (G(prime) > G(double prime) and decreased tan delta were more prominent for gliadin(30%) at the higher frequencies. In addition, the frequency dependence of G(prime), G(double prime), eta(prime), and tan delta for the two gliadin samples was compared directly with two samples of poly(dimethylsiloxane) (PDMS) a linear silicone-based entangled polymer with molecular weights (MW) of 140,000 and 385,000. The substantial differences in the magnitude and overall patterns of the frequency dependence of the viscoelastic functions between the gliadin and PDMS samples was attributed to the dominant effect that noncovalent secondary associations apparently have on the linear viscoelasticity of the gliadins. The energy of activation for flow (determined from the temperature dependence of the shift factors) for the gliadin samples for the range 25-45°C was higher than is typical for entangled linear polymer melts. The activation energy decreased for temperatures greater than approximately 60°C for gliadin(30%) and approximately 50°C for gliadin(40%). Thus, hydrated gliadin cannot be considered to be a simple viscoelastic liquid.