Effect of combined application of transglutaminase enzyme and ultrasonication process on physicochemical and rheological properties of drinking yogurt

In this thesis, drinking yogurt samples were produced by using milk samples either treated with ultrasonication (150W, 5 min), transglutaminase enzyme added at the ratio of 0.01%, 0.02% and 0.03% without ultrasonication treatment or treated by both applications. Some physicochemical, rheological, sensory and microbiological properties of drinking yogurt samples were determined during 10 days storage at + 4°C, and the effects of the applications on drinking yogurt samples were determined. Individual or combined applications of ultrasonication and transglutaminase treatments during drinking yogurt production did not have a significant effect on dry matter, fat and protein contents and CIELAB color parameters (L*, a* and b*) of drinking yogurt samples. Ultrasonication of milk and treatment with transglutaminase enzyme did not influence the starter culture growth during storage. The total lactobacilli and total streptococci counts of drinking yogurt samples did not fall below 7 log cfu/mL during storage. Treatment with ultrasonication+transglutaminase enzyme did not lead to a change in the pH value of drinking yogurt samples (p<0.05) but decreased the acidity values (%) of the samples compared to the control sample (p<0.05). In comparison to the control group, all applications (ultrasonication treatment, transglutaminase enzyme addition and their combination) decreased serum separation values significantly in drinking yogurt samples and also increased their apparent viscosities (p<0.05). According to the results, the addition of 0.01% transglutaminase enzyme and ultrasonication may be more effective to decrease the serum separation of drinking yogurt samples and to increase their apparent viscosity at the same time. Similarly, the results of sensory analysis showed that the combined application of ultrasonication treatment and 0.01% transglutaminase enzyme addition, instead of their individual use, resulted in higher sensory flavor scores. In conclusion, the combined use of transglutaminase enzyme and ultrasonication treatment may have a significant contribution to the reduction of the use of this enzyme in dairy industry.