Evaluation of effect of ultrasound and transglutaminase enzyme treatments on yield,, physicochemical properties and microstructure of soy cheese

Soy cheese (Tofu) is a quasi-dairy product that is prepared by coagulants through coagulation of soymilk. The present study was conducted to investigate the effect of transglutaminase (TG) and ultrasound enzymatic treatments on the yield, physicochemical characteristics and microstructure of tofu. After soaking and grinding soybeans with water (1: 6 ratio), the mixture was sonicated (frequency of 37 KHz with power of 520 W and temperature of 40 °C) at three time levels (0, 15 and 30 minutes). TG treatment of soymilk was performed at three levels (0, 0.35 and 0.7 Unit/g soymilk protein). After Tofu preparation with calcium sulfate as coagulant, the effect of these treatments on the studied parameters were investigated during two months storage (0, 30 and 60 days) under refrigeration conditions. The results showed that with increasing enzyme concentration, moisture content and production yield of tofu meaningfully increased while protein and fat contents slightly changed (p>0.05). Increasing the ultrasound time also significantly increased moisture content and yield of cheese making while considerably reduced the protein and fat content of cheese analogues. Moreover, as the storage time had no effect on yield, pH and chemical components of samples (p>0.05). Evaluation of the color of the product with a colorimeter showed that with increasing enzyme concentration, the amount of lightness (L*) increased and redness (a*) decreased significantly while yellowness (b*) insignificantly changed. Also, with increasing the ultrasound period and storage time, the amount of redness (a*) and yellowness (b*) parameters increased and the lightness (L*) decreased significantly. Microstructure of soy cheeses showed that TG treatment contrarily caused a compact structure in protein matrix of tofu samples. The results of this study showed that by using 15 minute ultrasound time and 0.7 transglutaminase concentration (Unit/g soymilk protein), a tofu with the acceptable quality and noticeable yield could be produced.