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W. B. YOON¹, S. Gunasekaran¹, and J. W. Park². (1) Food and Bioprocess Engineering Laboratory, Biological Systems Engineering Department, University of Wisconsin - Madison, 460 Henry Mall, Madison, WI 53706, (2) Seafood Laboratory & Dept. of Food Sci. and Tech., Oregon State University, 2001 Marine Dr., #253, Astoria, OR 97013

The sol-gel transition is common during processing of many food and biomaterials. Classic thermodynamic functions for phase transition, such as first- and second-order phase transition, are not applicable to describe the critical behavior of sol-gel transition. The percolation model has been used to explain the universal power law relation at critical moments of gelation. The power law exponent reflects the nature of mechanisms involved in a universal manner. In case of synthetic polymers, critical behaviors during gelation have been defined and studied based on percolation theories. However critical behaviors of biopolymers have not been well understood.

The objectives of this study were to: 1) investigate the critical behavior of biopolymer systems, and 2) determine the critical exponents of both physical and chemical gelation of biopolymers.

Xanthan and carob mixture (X/C) and fish protein (surimi) were used to study critical behaviors of physical and chemical gelation, respectively. For X/C mixture, the total concentration of polymer varied from 0.01 to 1.5% (g/100 mL), and, for surimi, the total moisture content was adjusted from 95 to 80%. Changes in rheological properties (G', storage modulus) were measured during gelation. A dimensionless concentration (C_r, ratio of total polymer concentration to critical concentration) was introduced to describe the modulus and the concentration relationship in the vicinity of sol-gel transition.

The critical concentration of 1:1 X/C mixture and surimi were determined to be 0.01% and 2%, respectively, and power law relations between C_r and G' were observed in both systems. The critical elasticity exponent of X/C and surimi were 2.25 and 1.62, respectively.

The classic elasticity exponent determined from the percolation theory is 1.95. The differences between X/C and surimi are possibly due to the structural variables, such as the stiffness of xanthan backbone and the rod-like shape of muscle proteins, respectively.