Effect Of Soybean β-conglycinin Subunit Composition On Its Emulsification Properties And Mechanism

Soy protein is a high-quality vegetable protein resource with many excellent functional properties,such as gelation,foaming and emulsification properties.Because of its unique amphiphilic structure,soy protein is an ideal macromolecular emulsifier,which is widely used in food processing and manufacturing.Soybeanβ-conglycinin is one of the main components of soy protein and plays an important role in its emulsification.β-Conglycinin consists of three subunits,α,α′andβ,and the relative variation of subunit content is crucial to its emulsification.α-subunit andα’-subunit have high homology,whileβ-subunit is different from the previous two in structure and property,and the effect ofβ-subunit on emulsification of soybean protein is less studied.Therefore,the study focuses on theβ-subunit as a starting point to investigate the effect of subunit composition on the emulsifying properties of soybeanβ-conglycinin.Firstly,the effect ofβ-subunit on the properties ofβ-conglycinin aqueous dispersion system was investigated.The aqueous dispersions were prepared at different ratios(9∶1,7∶3,5∶5,3∶7 and 1∶9)of isolated and purifiedβ-subunit and soybeanβ-conglycinin.It was found that the increase ofβ-subunit content significantly decreased the average particle size ofβ-conglycinin aqueous dispersions,butβ-subunit decreased the absolute value ofζ-potential as well as solubility ofβ-conglycinin to some extent.The increase ofβ-subunit content changed the conformation ofβ-conglycinin,resulting in more conversion ofβ-sheet andβ-turn toα-helix.These changes improved the emulsification activity and emulsion stability ofβ-conglycinin.On the basis,the effect of different soybeanβ-conglycinin subunits composition on the properties of the aqueous dispersion system compounded was investigated.The aqueous dispersions were prepared from the isolated and purifiedβ-subunit andαα′-subunit in different ratios(9∶1,7∶3,5∶5,3∶7 and 1∶9).It was found that when the ratio ofαα′-subunit toβ-subunit was 5∶5,the subunit aqueous dispersion had the smallest particle size and the highest solubility,which were 47.58 nm and 87.21%,and which made the subunit water dispersion system more stable.At the ratio,the subunit system also had the optimal emulsion stability and emulsification activity.This is mainly because the conformational of the subunit system has change and the cross-linking has occurred between subunits.Then,β-conglycinin emulsions with differentβ-subunit contents were prepared,and the properties of the emulsions were analyzed to reflect their emulsification properties.It was found that theβ-subunit could synergize withβ-conglycinin to enhance the physical stability of the emulsions.By dynamic rheological analysis,a gel network structure was formed inside the emulsion and the strength of the gel network structure increased with the increase ofβ-subunit content.The presence of more protein cross-linked structures in the emulsions could be observed by CLSM images,which promoted the structural stability of the emulsions.During the storage period of the emulsions,β-conglycinin emulsions with higherβ-subunit content experienced more severe fat uplift,indicating that theβ-subunit had a detrimental effect on the storage stability of theβ-conglycinin emulsions.Finally,emulsions composed of different soybeanβ-conglycinin subunits composition were prepared,and the effect ofβ-conglycinin subunits composition on the emulsions properties was investigated.It was found that the emulsion with subunit ratio of 5∶5 had the best overall stability.At this ratio,the emulsion has the smallest volume average particle size of 41.32μm.The adsorption capacity and concentration of interfacial protein also reached the maximum value,which were 23.43%and 11.79 mg/m~2,respectively.The emulsion also had the strongest gel network structure.The CLSM images also supported the conclusion.By analyzing the main driving forces of emulsion formation and combining the interfacial properties and structure of emulsions,the mechanism of the action of subunit in the process of emulsion formation was initially discussed,and the“three-stage”emulsification conclusion was proposed,which was mainly the formation of preliminary weak protein network structure in aqueous dispersion,the formation of interfacial layer of emulsions and the formation of gel network structure of emulsions.Through the above research,it can be concluded that the appropriate content(>50%)ofβ-subunit has a positive effect on the emulsifying properties of soybeanβ-conglycinin system and subunit system.This is mainly due to the fact that theβ-subunit makes theβ-conglycinin aqueous dispersion system and the subunit aqueous dispersion system more stable.Secondly,the interaction between theβ-subunit and theβ-conglycinin and the interaction between theβ-subunit and theαα′-subunit make the emulsion system have a good structural basis,thereby improving the stability of the emulsions.