Influence of cross-linking between milk and soy proteins by transglutaminase on physicochemical properties of nonfat yoghurt

In order to investigate of cross-linking of milk and soy proteins by transglutaminase and its influence on physicochemical properties of nonfat yoghurt and 4 treatments including (control, Soy Protein Isolate (SPI) enriched, Enzyme treated and SPI enriched with enzyme treated) were prepared from reconstituted milk. Decreasing of protein bands intense in electrophoretic patterns of yoghurt samples SPI enriched with enzyme treated in contrast to samples of SPI enriched improved the cross-linking of soy and milk proteins by transglutaminase and formation of high molecular polymers. In the beginning of storage time, the pH values for enzyme treated sample was lower than control while during storage period, the influence of SPI on pH variations was significant (P<0.05).The sample with SPI enrichment increased acidity value but the effect of enzyme on acidity was not significant (P<0.05). At the first day of storage period, the highest content of WHC was obtained for enzyme treated samples but during storage time, the sample containing SPI showed more effect on WHC enhancement. The SPI enriched with enzyme had maximum WHC in whole storage time (P<0.05). The sample with SPI enrichment caused higher viscosity (P<0.05). The presence of SPI resulted in a sponge like structure in enriched samples; SPI enriched with enzyme treated samples caused a structure with a more homogeneous porosity and smaller particles..