Cross-linking Of Casein Or Soy Protein Isolates With Gelatin By Transglutaminase And Some Functional Properties Of The Product Prepared

Casein and soy proteins are the most important food proteins, and gelatin is the denatured product of collagen. They are widely used in food industry as main components, or as food infredients. To find more applications of casein or soy protein isolates in food industry, new modification technologies are needed to be developed. For example, transglutaminase (TGase, EC 2.3.2.13) induced-cross-linking was widely used.In this work, TGase was applied to cross-link casein or soy protein isolates (SPI) in the presence of gelatin to prepare composite cross-linked proteins. 4-Hydroxyproline content in the modified product was measured and used as an index to select suitable TGase addition, reaction time and temperature by single factor experiments. The modified products were prepared in the optimized cross-linking conditions. SDS-PAGE analysis demonstrated that some protein polymers existed in the modified product. Some related functional properties of the modified products were evaluated and compared to those of original proteins and cross-linked proteins which were prepared by TGase in the absence of gelatin. The functional properties evaluated included solubility, enzymatic digestibility in vitro, emulsifying properties, rheological properties and water holding or oil binding capacities. The aim of the present work was to investigate the impacts of gelatin on these important properties of the modified products, which is necessary for developing new food protein constituents. The results obtained from the present study are listed as follows.1. When total proteins were fixed at 50 g/L, the ratio of casein to gelatin was 4:1 (v/v) and original pH was 7.5, the suitable reaction conditions for the cross-linking of casein and gelatin were TGase addition 20 U·g-1 proteins, reaction time 4 h and temperature 45°C. With these selected conditions, a modified product with 4-hydroxyproline content about 41 mg·g-1 proteins was prepared.2. As total proteins, the ratio of soy protein isolates to gelatin and original pH were fixed at 50 g/L, 4:1 (v/v) and 7.5, respectively, the suitable reaction conditions for the cross-linking of SPI and gelatin were TGase addition 20 U·g-1 proteins, reaction time 2 h and the temperature 40°C. With these selected conditions, a modified product with 4-hydroxyproline content about 36 mg·g-1 proteins was prepared.3. SDS-PAGE analysis using a SDS-Tris-glycine buffer system with 4 % stacking gels and 12 % resolving gels was carried for the composite prepared. The band profiles indicate that there exist some new peptides with higher molecular weights than 97.4 kDa in the composite prepared. Besides this result, the measurement of the 4-hydroxyproline content in the composite also indicates that the TGase-induced cross-linking between casein (or SPI) and gelatin happened.4. The composite prepared exhibited lower solubility at all pH and lower digestibility in vitro. Compared to original casein or SPI, casein-gelatin composite was less susceptive enzymatic hydrolysis, for less peptides about 32% (in pepsin hydrolysis) or 7% (in pepsin-trypsin hydrolysis) were released from it. Less peptides about 26% or 11% were released from SPI-gelatin composite during its pepsin hydrolysis or pepsin-trypsin hydrolysis, respectively.5. When the protein concentration was fixed at 0.5%, the emulsifying activity of the composite prepared was evaluated. Compared to the original protein, the emulsifying activity of casein-gelatin composite was decreased by 73.7%, while that of SPI-gelatin composite was decreased by 88.9%. The emulsion stability of casein-gelatin composite and SPI-gelatin composite exhibited different results. Compared to the original proteins, the emulsion stability of casein-gelatin composite gave a decrease of 52.7%, but SPI-gelatin composite showed an increase of 422.5%.6. The rheological properties and the water holding capacity of the composite prepared were improved significantly. The apparent viscosity as well as the storage modulus G′and viscous modulus G′′of their dispersions were alao improved. The water holding capacity of casein-gelatin composite was much higher than original casein with a 11-folds increase. The water holding capacity of SPI-gelatin composite had an increase about 15.4% than original SPI. The oil binding capacity of the composite didn't improved. Casein-gelatin composite has the same oil binding capacity than original casein. SPI-gelatin composite displayed a lower oil binding capacity than original soy protein isolates, which was decreased 40.3%.