| Chemical, structural, and functional characteristics of muscle (myosin and myofibrils), whey (beta-lactoglobulin and WPI), and soy (7S globulin and SPI) proteins treated with a free-radical-generating system (FeCl 3/H2O2/ascorbate) were studied to elucidate the impact of protein oxidation on the quality of muscle foods. Oxidized individual and mixed proteins in 0.6 M NaCl, pH 6.0 solutions were analyzed for protein carbonyl, free amine, and sulfhydryl/disulfide contents, peptide scission/polymerization, thermal stability, gelation properties, and in vitro digestibility. Muscle proteins were highly susceptible to oxidation. Oxidized myosin and myofibrils exhibited marked increases (≥10-fold) in carbonyls, small increases in amines, and SH → SS conversion. Oxidation caused fragmentation and polymerization (mainly via S-S bonds) of myosin and disappearance of several other myofibril components including actin, troponin T, and tropomyosin. Hydroxyl radicals destabilized myosin and actin and reduced elasticity of myosin (by >50%) and myofibril (by >40%) gels. Digestibility of myosin either increased or decreased depending on the reducing conditions of the digestion media.;Amino acid side chains of whey and soy proteins were also modified upon oxidation as indicated by 5-, 3-, 3-, and 2-fold increases in protein carbonyls of beta-lactoglobulin, 7S globulin, WPI, and SPI, respectively, as well as by changes in free amines and sulfhydryl-disulfide groups. However, thermal stability of these proteins was not significantly altered. Oxidation increased elasticity of both 7S globulin and SPI gels but not that of whey protein gels.;In mixed muscle/whey protein systems, oxidation altered the interaction of myosin and beta-lactoglobulin. but not that of myofibrils and WPI. In mixed muscle/soy protein systems, oxidation promoted myosin-7S globulin and myofibril-SPI interactions, covalently and non-covalently, resulting in 15--90% enhancements in the elasticity of the composite gels when compared to the individual oxidized proteins. The presence of the non-muscle proteins did not cause major shifts in muscle protein thermal stability.;The results suggest that, in processed muscle foods where oxidation occurs, functionality of muscle proteins can be influenced by the presence of non-muscle proteins. In particular, under oxidative conditions, soy proteins added as a formulation ingredient may enhance the functionality of myofibrillar proteins in the meat products. |
