Physical-enzymatic modification methods improving emulsifying properties of soybean protein isolate under acidic conditions

Abstract: Soybean protein isolated (SPI) is a kind of important raw material and widely used in food processing industry due to the excellent nutrition and functionality. Among all the properties towards SPI, emulsifying property is one of the most important functions. If adding SPI to foods, it can maintain emulsified state of system and improve the taste. However, because of the compact globular structure stabilized mainly by hydrogen bonds and disulfide bonds, native soy proteins have lower molecular flexibility and a rather poor emulsifying capability compared to other protein emulsifiers such as milk protein. Emulsifying performance of ordinary SPI is degraded obviously in acidic conditions, which limits its application in a few kinds of food (such as acidic beverages, salad dressings). Therefore, an effective approach to improve emulsifying properties of SPI in acidic conditions has became a needed technology. In order to improve the emulsifying properties of SPI under acidic conditions, this paper mainly studied the effects of physical-enzymatic modification methods on the emulsifying properties of SPI. The comparison between different modification methods showed that the physical-enzymatic modification, also called ultrasound-enzymatic modification and extrusion-enzymatic modification, had the best emulsifying property under the acid condition of pH value of 4. The study used extrusion's characteristics, which produced the effect of shearing, extensional deformations, inertial effects and cavitations to destroy cell tissue and structure, and also used the ultrasonic's characteristics of rapidness and energy saving by producing the effect of cavitation, shearing and intense mixing in liquid with the dispersion effect to damage the cell wall. Micro-structure features of modified SPI at pH value of 4 were analyzed based on particle size distribution, circular dichroism spectroscopy, scanning electron microscope (SEM) and confocal laser scanning microscopy (CLSM), and the influence of different modification methods on protein structure was also analyzed. Results were as follows: the emulsifying activity index (EAI) and emulsifying stability index (ESI) of the SPI modified by ultrasound combined with phytase-acidic protease separately reached 0.53 m2/g and 17 min, which significantly increased by 196% and 25.9% compared to those of the original SPI condition (P<0.05). Likewise, the EAI and ESI of the SPI modified by extrusion combined with bromelain separately reached 0.46 m2/g and 17 min, which increased by 155% and 25.9% (P<0.05). Compared with the original SPI at pH value of 4, the physical-enzymatic treatment significantly improved the solubility of SPI (P<0.05) under the ultrasound combined with phytase-acidic protease modification and the extrusion combined with bromelain modification. The analysis of structure illuminated that compared with ordinary SPI, the SPI under physical-enzymatic modification methods could decrease average particle size. The CLSM showed that in emulsion, oil and protein solution were melting stably, which improved the spatial repulsion force between oil droplets. The number of free hydrosulfide thiols in tertiary structure of the SPI under physical-enzymatic modification was significantly increased (P<0.05), and the amount of disulfide bonds was significantly reduced (P<0.05). Under the observation of SEM, the microstructure of the SPI under physical-enzymatic modification was loose and crushing uniformly. Thus, the improvement of emulsifying property of the SPI was achieved by the deep change of protein structure. This research can provide references for the study on improving emulsifying properties of SPI under acidic conditions.