Effect of dynamic high-pressure microfluidization on functional properties of soy protein isolate-soybean soluble polysaccharides system

Abstract: Soy protein isolate (SPI) is an important highly purified commercial soy protein product, and has been widely used in food manufacturing owing to its good nutrition and abundant availability. However, application of SPI is still restricted due to its poor solubility and interfacial properties. Dynamic high-pressure microfluidization (DHPM) is an emerging physical technology widely used to modify the functional properties and structure of protein. It is an available method using the combined forces of high-velocity impact, high-frequency vibration, instantaneous pressure drop, powerful shear, cavitation force and ultra-high pressures. In recent years, DHPM has been successfully used for improving the functional properties of whey protein, rice protein, SPI, as well as used to increase the enzymatic hydrolysis of soy isolated protein and the emulsifying properties of hydrolysates, and improve the extraction yield of polyphenols and polysaccharides from plant materials. Our previous research indicated that DHPM treatment at 20-160 MPa could significantly change the solubility, foaming and emulsifying properties, and decrease the average particle size of SPI. Moreover, DHPM could promote the glycation reaction between bovine serum albumin and glucose. In addition, it was found that high pressure homogenization could improve the foam overrun of soy protein isolate - hydroxypropyl methyl celluloses dispersed systems and potentially create new functional aggregates. Soybean soluble polysaccharides (SSPS) could bind with protein via electrostatic and hydrophobic interactions, and stabilize the protein-polysaccharide complexes in aqueous solution. Therefore, for improving functional properties of SPI, the influence of DHPM in various pressures on the functional properties of SPI in the presence of SSPS, namely SPI-SSPS system, was investigated in this work. The SPI-SSPS was processed at 0, 60, 100, 140 and 180 MPa of DHPM treatment, respectively. The foaming and emulsifying properties, as well as solubility, particle size distribution and surface hydrophobicity of SPI-SSPS were detected to evaluate the influence. The results indicated that DHPM treatment improved the solubility and foaming properties of SPI, and the presence of SSPS greatly strengthened these effects (P<0.05). Samples subjected to 100 and 60 MPa treatments showed the best foaming ability and foaming stability, which were 1.2 and 2.4 folds of that from untreated sample, respectively. DHPM treatment at 140 MPa yielded the highest solubility, which was 1.8 folds of that from control. However, DHPM processing significantly decreased emulsifying properties, particle size distribution and surface hydrophobicity of SPI-SSPS (P<0.05). With DHPM pressure increasing, average particle size and surface hydrophobicity of SPI-SSPS gradually declined. Moreover, the smallest average particle size and the lowest fluorescence intensity were obtained at 180 MPa treatment. In some way, the reduced surface hydrophobicity could be used to explain the increased foaming properties and the decreased emulsifying properties. To show good foaming, the protein must be capable of migrating at the air-water interface, unfolding and rearranging at the interface. With surface hydrophobicity decreasing, more hydrophilic groups were unmasked, causing that the protein was kept better touching with water molecule and developed to strong film at the air-water interface. For emulsifying properties, surface hydrophobicity was able to influence the ability of the protein to be adsorbed to the oil side of the oil-water interface. Lower hydrophobicity of the protein led to weaker adsorption with consequent worse emulsion capacities. However, in this work, the surface hydrophobicity was not highly correlated with the emulsifying or forming properties as suggested by the different changing trends of them under the varying pressure of DHPM. It indicated that surface hydrophobicity may not be the only factor that causes the changes in the emulsifying or forming properties. Above findings suggest that DHPM processing combined with SSPS is a potential alternative to modify certain functional properties including solubility and foaming properties of SPI to extend its application in food industry. The results in this study can give the reference for the functional modification of SPI. However, further research is needed to elucidate the detailed mechanism of DHPM processing on the interfacial properties of SPI-SSPS, as well as the interaction between SPI and SSPS.