Cereal Chem. 73 (1):115-122 |
Effects of Cysteine on Free Radical Production and Protein Modification in Extruded Wheat Flour (1,2).
B. K. Koh (3,4), M. V. Karwe (3), and K. M. Schaich (3,5). (1) Contribution D-10125-1-95 of the New Jersey Agricultural Experiment Station. (2) Presented at the AACC 79th Annual Meeting, Nashville, TN, 1994. (3) Dept. of Food Science, Rutgers University, New Brunswick, NJ 08903-0231. (4) Current address: Dept. of Foods and Nutrition, Keimyung University, Daegue, Korea. (5) Corresponding author. Fax: 908/932-6776. Accepted September 18, 1995. Copyright 1996 by the American Association of Cereal Chemists, Inc.
Addition of cysteine as a free radical scavenger during extrusion markedly affected physical and chemical properties of wheat flour extrudates. Radial expansion at the die decreased in linear extrudates and longitudinal expansion through surfaces cut at the die increased, reflecting weakened dough strength. Cell size decreased, cell walls thinned, and cells became more evenly distributed and densely packed. Cysteine decreased electron paramagnetic resonance (EPR) free radical signal intensity but increased free radical density of the extrudates, preferentially quenching nitrogen-centered radicals. Nonprotein thiols decreased and disulfides increased in samples with added cysteine, whereas the opposite occurred with protein SH-SS groups. Protein solubility in 1.5% sodium dodecyl sulfate (SDS) without beta-mercaptoethanol decreased markedly with extrusion but increased with each increment of cysteine. Addition of cysteine slightly increased protein solubility in SDS plus beta-mercaptoethanol compared to that of unextruded flour. Polyacrylamide gel electrophoresis (PAGE) patterns of extracted proteins showed increasing amounts of high molecular weight material solubilized from previously insoluble aggregates. Glycoproteins, probably starch-protein complexes, were detected in the highest molecular weight material. PAGE patterns after reduction of disulfide bonds showed shifts in molecular weight distributions from high to low molecular weight products with added cysteine. All together, data indicates that cysteine acts predominantly by free radical quenching. Little or no thiol-disulfide interchange or sulfur radical addition was detected.