Effects Of Other Plant Protein Addition On Heat-induced Gel Properties Of Soy Protein

In recent years,the heat-induced gelation of soy protein isolate(SPI)has been a hot research area for the structural and functional modification of soy protein with the rise of plantbased foods,such as plant-based meat.Protein-protein interaction has been a research hotspot and is considered an effective way to improve the performance of protein gels,but there are few studies on SPI.Based on the mechanism and influencing factors of SPI heat-induced gel and the research progress of protein-protein interaction,the following hypothesis was proposed:by combining proteins with different properties,hydrophobic interactions and disulfide bond interactions between proteins could be changed by improving protein solubility,increasing protein unfolding level,changing aggregate size,and changing surface charge,so as to improve the heat-induced gel properties of proteins.Therefore,various proteins could be used to interact with SPI to improve the SPI heat-induced gel property based on protein-protein interactions.Based on the improvement effect,the interaction conditions were optimized to enhance the heat-induced gelation ability of SPI further.The main research contents and results are as follows:First,the plant proteins that can enhance the heat-induced gelation of SPI through proteinprotein interactions were screened.A series of proteins with different physicochemical properties were prepared,including soy protein,black bean protein,mung bean protein,pea protein,potato protein,pumpkin seed protein,and camellia seed protein.Their physicochemical properties(e.g.,amino acid composition,subunit composition,solubility,hydrophobicity)and gel properties were determined.Six proteins with significant differences in composition and properties were mixed with SPI to explore whether the SPI gel properties could be improved through protein-protein interaction.The results showed that the pumpkin seed protein itself had good gel properties.Camellia seed protein had low molecular weight,which could not form a heat-induced gel.Its hydrophilic amino acid content was much higher than that of the other six proteins,which was about 1.8 times that of SPI.The cysteine content of camellia seed protein was about 2.5 times higher than SPI.We mixed those six proteins with SPI separately in different ratios and measured their gel properties.The results showed that only pumpkin seed protein and camellia seed protein could effectively enhance the strength of SPI heat-induced gel.When soy protein and camellia seed protein were mixed in a ratio of 7∶3,the gel strength was the highest,about 1.8 times that of SPI gel.These results of the mixed proteins and their gels indicated that the mechanism of the two proteins enhancing the SPI gel could be different.Based on the improvement effect of camellia seed protein and SPI,we selected camellia seed protein and SPI with different ratios to clarify the mechanism of protein interaction enhancement of SPI gel.The results showed that when the ratio of soy protein and camellia seed protein was 10∶2,the gelling temperature of the mixed gel decreased from 82.34°C to75.73°C.The storage modulus value reached the maximum value of 73 KPa,five times that of pure SPI gel;the gel strength reached the maximum at 411 g,twice that of pure SPI gel.The results of electrophoresis and surface-free sulfhydryl groups showed that disulfide bond interactions occurred between the two proteins and formed complexes between the subunits.The surface hydrophobicity of the mixed protein decreased,and the solubility increased.The particle size distribution showed that the interaction would lead to the disappearance of the particle size peak of camellia seed protein.The above results indicated that the enhancement of SPI gel strength is mainly based on the interaction between the two proteins,forming a complex involving disulfide bonds,which could effectively improve the solubility of the mixed protein and the particle size of aggregates.To further enhance the gel properties of SPI and camellia seed protein mixture(10:2),the influence of p H-shifting treatment,ultrasound treatment,and the addition of salt ions on the mixed gel properties were studied to strengthen the interaction further and improve the gel properties.The results showed that appropriate p H-shifting treatment improved the mixed gel properties.The p H-shifting treatment improved the solubility and surface hydrophobicity of the mixed protein,further promoted the formation of aggregates connected by disulfide bonds,and increased the aggregate size of the mixed protein.Ultrasound treatment could reduce the energy storage modulus and gel strength of the mixed protein.Ultrasound treatment could improve the solubility and surface hydrophobicity of the protein but had little influence on the number of disulfide bonds and greatly reduced the particle size.The negative effect of ultrasound on gel strength was mainly due to the sharp reduction of the particle size of the mixed protein.Appropriate salt ion strength could significantly improve the mixed gel properties.When the Na Cl concentration was 200 mmol/L,the gel strength increased to the maximum,2.4 times that of the unsalted gel,and the water holding capacity decreased from 99.83% to 96.07%.The improvement of gel strength was mainly caused by Na Cl promoting the formation of aggregates,improving surface hydrophobicity,and strengthening the disulfide bond.The decrease in water holding capacity was related to the decrease in solubility.Finally,from the above experimental results,it could be seen that ultrasound could reduce the size of protein aggregates,and could not improve protein solubility and surface hydrophobicity,while salt ions could significantly increase the aggregate size and surface hydrophobicity,strengthen disulfide bond,but reduced solubility.Therefore,it was assumed that ultrasound and salt ions could effectively control the protein aggregate size and surface charge quantity.They could also enhance disulfide bond interaction and improve surface hydrophobicity at the same time,thereby achieving the effect of improving the thermal induction gel of SPI.The mixed proteins were treated by ultrasound and salt ions to verify the above hypothesis,while pure SPI was used as a control.The results showed that the combination of ultrasound and salt ions could significantly improve the gel strength,reduce the gelling temperature and improve the water holding capacity of the gel.The combination of 200mmol/L Na Cl with 30 min ultrasound treatment could increase the heat-induced gel strength of both proteins by more than 3 times.Salt ion and ultrasound improved the heat-induced gel properties of the mixed protein and SPI by regulating protein surface charge,aggregate size,and type and intensity of interaction forces in the gel.Appropriate ζ potentials,small and uniform protein particles,enhanced hydrophobic interactions and disulfide bond interactions improved protein heat-induced gel properties.In conclusion,enhancing the interaction between proteins was an effective way to improve the heat-induced gel properties of SPI.Different treatment methods had different improving effects on the mixed gel.The p H-shifting treatment and salt ions could significantly improve the gel properties.Ultrasound treatment could not improve the mixed protein and pure SPI gel properties,but it could combine with salt ions to substantially improve heat-induced gel properties.