Physicochemical and structural characteristics of sodium caseinate biopolymers induced by microbial transglutaminase

Some physicochemical properties and structural characteristics of microbial transglutaminase (MTGase)-induced biopolymers of sodium caseinate (SC) were investigated. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size-exclusion-high-performance liquid chromatography analyses showed that all components of SC were easily polymerized or transformed by MTGase to form high-molecular weight biopolymers, and the susceptibility order of individual components was kappa-Casein (C) > alpha-C > beta-C. The emulsifying properties of biopolymers depended on the incubation time with MTGase. The emulsifying activity index of biopolymers persistently increased with the MTGase (0-12 h) incubation time. The emulsion stability also increased with the incubation time (< 4 h), then declined a little with longer incubation (4-12 h). The differential scanning calorimetry analysis showed that the thermal properties of the biopolymers obtained after a 12-h incubation were different from that of native SC or biopolymers obtained after a shorter incubation time (< 4 h), suggesting that the former has higher thermal stability. In addition, the ultraviolet (UV) spectra showed that the UV absorbance (at 275 nm) of MTGase-induced biopolymers of SC decreased with an increasing incubation time with MTGase, and the maximal emission wavelength (lambda(max)) slightly shifted to the blue side. The fluorescence spectra showed that the lambda(max)was related with incubation time with MTGase, slightly shifting to the blue side after 4 h with no further changes; its relative fluorescence intensity also increased. These results suggest a relationship between the functionalities and structural characteristics of the MTGase-induced biopolymers of SC.