Rheological properties of mixed muscle/nonmuscle protein emulsions treated with transglutaminase at two ionic strengths

This study was conducted to determine the effect of transglutaminase on emulsifying properties and rheology of emulsion gels of mixed myofibrillar (MPI) and whey (WPI) or soy (SPI) protein isolates in aqueous solutions with (μ ∼ 0.6) or without (μ ∼ 0) 0.6 M NaCl at pH 6.5. Mixed MPI/WPI or MPI/SPI had a greater (P < 0.05) emulsifying activity but a reduced emulsion storage stability when compared with MPI alone at both ionic strength conditions. The enzyme treatment had a minor effect on protein emulsifying activity, but tended to decrease emulsion stability because of promoting flocculation. Mixed protein emulsions in salt-free solution formed a gel upon heating to about 50 °C; however, the gel elasticity was less than that of the MPI control emulsion. The addition of 0.6 M NaCl lowered the onset gelling temperature, altered the rheological pattern of the gel, but minimally affected the final gel strength; while the transglutaminase treatment resulted in a higher gel elasticity for all emulsion samples. The results suggest that transglutaminase influenced the rheological properties of mixed muscle/ nonmuscle protein emulsion gels mainly through affecting the interaction or cross-linking of oil droplets and not the physical characteristics of the fat globule membrane per se.