The Research Of Conformation And Functional Properties Of Soy Protein-saccharide Conjugates

Soy protein exhibits broad sources and low price. Recently, it was greatly cared because of its high nutritional value and unique features of (on) health care. However, its functional properties were dissatisfied and limited in the food industry. In this paper, soy protein isolate (SPI), lactose and gum Acacia (GA) were used as the starting materials, and SPI-saccharide grafts were synthesized by ultrasonic treatment, pulsed electric field (PEF). The conditions, mechanism of the reaction, conformation and functional properties of conjugates were studied. The aim was to find the functional properties of the products by studying the SPI-saccharide graft modification, and gain more effective way, which gave people academic and practical guides for industry.The wet-drying conditions of SPI-saccharide graft reaction were studied. The factors which affected the reaction, such as temperature, pH, heating time, the concentration of SPI and weight ratio of saccharide to SPI were analyzed. According to the degree of graft (DG) and browning, suitable conditions were chosen. The conformation and functional properties of SPI-saccharide conjugates were also studied. Comparing with the only SPI, solubility, thermal stability, emulsifying properties of the resultant conjugates were significantly (P<0.05) improved. When SPI was grafted with saccharide, the content of lysine and arginine,surface hydrophobicity of conjugates decreased. In addition, conjugate was proved by FTIR and HPLC that the SPI molecule was grafted with the saccharide molecule by the covalent bond. According to the analysis of SDS-PAGE, the bands of the grafted subunits decreased, but the glycosyl bands were seen more clearly, with the degree of graft (DG) increasing. According to the analysis CD, the content ofα-helical conformation decreased but that of random structure increased when SPI was grafted with saccharide. Moreover, it was observed with SEM that the structure of SPI was different with SPI-saccharide conjugate, whose structure was dispersed.Ultrasonic treatment could improve the rate and DG of the SPI-saccharide graft reaction greatly. The factors (ultrasonic power, ultrasonic time and the content of protein) that affect the reaction were analyzed and the conditions were optimized by surface response design. The most optimum conditions for SPI-lactose graft reaction were as follows: concentration of SPI 11 mg/mL, ultrasonic time 24 min, ultrasonic power 250 W. Moreover, the most optimum conditions for SPI-GA graft reaction were as follows: concentration of SPI 7 mg/mL, ultrasonic time 36 min, ultrasonic power 250 W. Ultrasonic treatment made part of peptide bonds of SPI unfold or break, that led to the increase of free amino groups in the reaction system. Meanwhile, spatiall structure of SPI was changed and steric hindrance was decreased. The conformation and functional properties of SPI-saccharide conjugates obtained by ultrasonic improvement were also studied. Comparing with conjugates obtained by wet-heating, solubility, thermal stability, emulsifying properties of conjugates obtained by ultrasonic treatment were better.At the base of classic wet-heating, SPI-saccharide graft reaction could be completed by PEF in several hundreds microsecond. The rate of SPI-saccharide graft reaction was increased greatly. The factors that affect the reaction were analyzed and the conditions were optimized by surface response design. The most optimum conditions for SPI-lactose graft reaction were as follows: concentration of SPI 14 mg/mL, PEF treatment time 1000μs, PEF strength 30 kV/cm. Moreover, the most optimum conditions for SPI-GA graft reaction were as follows: concentration of SPI 11 mg/mL, PEF treatment time 1000μs, PEF strength 30 kV/cm. PEF treatment made part of peptide bonds of SPI unfold or break. Free amino groups in the system were increased. Meanwhile, spatiall structure of SPI changed and steric hindrance was decreased at some extent. The conformation and functional properties of SPI-saccharide conjugates gained by PEF treatment were also changed. Comparing with conjugates obtained by wet-heating, solubility, thermal stability, emulsifying properties of the resultant conjugate was improved at some extent. However, that was worse than conjugates obtained by ultrasound.