Gelation properties of protein mixtures catalyzed by transglutaminase crosslinking
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Gelation properties of a salt extracted pea (Pisum sativum) protein isolate (PPIs) were evaluated with a goal of using this isolate as a meat extender. Microbial transglutaminase (MTG) was used to improve gelation of PPIs, muscle protein isolate (MPI) from chicken breast and the two combined. Gelation properties were evaluated using small amplitude oscillatory rheology and texture analysis. SDS-PAGE and differential scanning calorimetry were used to examine protein structure. Minimum gelation concentration for PPIs was 5%, lower than the 14% obtained for a commercial pea protein isolate (PPIc), possibly because the PPIc undergone denaturation whereas PPIs had not. Storage modulus (G') and loss modulus (G") increased with protein concentration and maximum gel strength for PPIs occurred at pH 4.0 in 0.3M NaCl. Higher or lower pH values affected protein charge and the potential for network formation. Higher salt concentrations resulted in increased denaturation temperatures, to a point where the proteins did not denature at the 95ºC temperature used for gel formation. When both heating and cooling rate were increased, gel strength decreased, though the cooling rate had a greater impact. Chaotropic salts enhanced gel strength, whereas non-chaotropic salts stabilized protein structure and decreased gel formation. Based on effects of guanidine hydrochloride, urea, propylene glycol, β-mercaptoethanol, dithiothreitol and N-ethylmaleimide, hydrophobic and electrostatic interaction and hydrogen bonds were involved in pea protein gel formation but disulfide bond contribution was minimal. Gels formed with MPI at concentrations as low as 0.5% and were strongest at 95ºC, higher than the ~ 65ºC normally used in meat processing. Good gels were formed at pH 6 with 0.6 to 1.2 M NaCl. Addition of MTG increased gel strength for PPIs, MPI, and a combination of the two. SDS-PAGE showed that bands in the 35~100kDa range became fainter with higher MTG levels but no new bands were found to provide direct evidence of interaction between muscle and pea proteins. Improved gel strength for the MPI/PPI mixture (3:1) containing MTG suggested that some crosslinking occurred. Higher heating temperatures and MTG addition led to the formation of MPI/PPI gel and demonstrated the potential for utilization of pea protein in muscle foods.