30C-28 |
Effects of reducing agents on gelling behavior of commercially processed soy isolates during heating and cooling |
L. K. MCKLEM, Department of Food Science, North Carolina State University, 231 Schaub Hall, Raleigh, NC 27695-7624, T. C. Lanier, and P. Kwanyuen, USDA-ARS, Crop Science Department, North Carolina State University, Raleigh, NC 27695-7631. Commercially prepared soy isolates are intentionally denatured in order to improve their functionality for various food applications. Their gelling properties have been less well studied than those of isolates prepared so as to minimize protein denaturation. Our objective was to determine, by observing the effects of added reducing agents, the relative roles of hydrogen, hydrophobic, and disulfide bonding on rheological changes of pastes prepared from commercial isolates during heating and cooling. Commercially prepared soy isolates used in this study were partially denatured as determined by differential scanning calorimetry. Pastes were prepared at 12% protein (w/w) in deionized water, 6M urea, or 10mM dithiothreitol (DTT). G* (complex modulus) was continuously evaluated during heating/holding/cooling cycles on an ATS StressTech rheometer. All samples were initially cooled from 25C to 10C at 2.5C/min, then heated at 2.5C/min to 80C, held for 30 minutes at 80C, and cooled to 10C at 2.5C/min. Frequency sweeps prior to heating indicated that pastes prepared in deionized water or urea had already formed gels, whereas samples containing DTT had not. Samples that gelled initially exhibited a decrease in G* (but did not melt) as the temperature was increased to 80C. G* increased significantly during holding at 80C only for pastes prepared in deionized water. During subsequent cooling to 10C, all pastes exhibited an increase in G*. At this point, pastes prepared in deionized water and DTT had increased to G* values greater than those measured prior to heat treatment (at 10C). Frequency sweeps confirmed that the DTT-added sample was now also gelled. These results suggest that disulfide bonding is important in the initial gel formation of these pre-denatured isolates, but that heating to 80C and holding induces bonding of a different nature that strengthens gels made without reductants, and gels a DTT-added sample.
Session 30C, Food Chemistry: Proteins
|