Effects of Na+ on the cold gelation between a low-methoxyl pectin extracted from Premna microphylla turcz and soy protein isolate

In this paper, the cold gelation between an ammonium oxalate extraction pectin (OEP) extracted from Premna microphylla turcz and soy protein isolates (SPI) as well as the effect of sodium ion (Na+) were studied by texture test, rheological analysis, microstructure observation, thermodynamics study and gelling forces analysis. The results showed that OEP could cold gelate with SPI at room temperature. Besides, there was an optimal complex ratio (OEP 0.3% w/w, SPI 2% w/w) for the two biopolymers to form a gel with the best gel strength and the highest solid-like character. Addition of low content of Na+ (10 mM and 20 mM) could improve the gel hardness, water holding capacity (WHC), thermal stability and crystallinity of the composite gels. However, higher content of Na+ (30 mM and 40 mM) would reduce the gel hardness, WHC and thermal stability, and lead to the formation of new amorphous structure with poor crystallinity. Gelling forces analysis revealed that hydrogen bonds and hydrophobic interaction were the main interaction forces in the composite gels. In addition, low content of Na+ could promote the interaction between OEP and SPI, while higher content of Na+ would weaken the gelling forces of the two components by electrostatic shielding. The results of this study can provide a theoretical basis for the utilization of OEP, SPI and Na+ to develop gels with various texture and viscoelasticity.