The aim of this study was to investigate the effect of transglutaminase (TG) addition on rheological properties, textural characteristics and microstructure of fermented milk products manufactured by different starters (probiotics and kombucha inoculum). Rheological analysis revealed that all manufactured fermented milk products had higher storage modulus than loss modulus and exhibited thixotropic and a typical shear thinning behavior. The addition of TG in milk increased approximately 10.5% hysteresis loop area, 39% firmness and 48% consistency in sample produced with probiotic starter and had more firm and stable gel structure than kombucha fermented milk products. The scanning electron microscopy micrographs showed that casein matrix of fermented milk products containing TG is continuous and uninterrupted except for void spaces occupied by milk serum and starter culture cell.
Transglutaminase (TG) is a transferase that forms inter- and intramolecular isopeptide bounds in and between many proteins by cross-linking of the amino acid residues of protein-bound glutamine and lysine. Among the dairy proteins, the casein fraction represents a favorable substrate for TG, mainly due to the highly accessible and flexible open chain structure. Heat treatment prior to cross-linking is necessary because the reactivity of TG is very low in unheated or pasteurized milk, despite the high reactivity of the casein. In the case of low-fat products, behavior of milk proteins during the gelation process is of particular importance. Cross-linking of preheated milk with reduced fat content results in improved gel firmness and reduced syneresis of acid-induced milk protein gels. Also, TG treatment at optimal technological parameters, pH value and temperature of milk, can solve these problems in kombucha fermented milk products, improving firmness, consistency and viscosity of gel.