| Soy protein isolate(SPI)is prepared from defatted soybean meal,and it is frequently used as an emulsifier during food processing.SPI has a compact structure in its native state and large aggregates are easily formed during its preparation process.These characteristics result in poor solubility,which limits the full expansion and adsorption of SPI protein molecules at the emulsion interface and leads to poor surface activity.Therefore,the emulsifying performance of SPI cannot satisfy the requirements of high-emulsifying protein products in the food industry.In this paper,the effects of combined high hydrostatic pressure and pH shifting treatment on inducing protein structure unfolding and exposing the glycosylation site were evaluated.Changes in the structure and emulsifying properties of protein samples and their glycosylation products were determined to explore the mechanism of how the emulsifying performance were improved.Finally,emulsions stabilized by glycosylated product or unmodified SPI were obtained,and effect of different oil fraction on the rheological properties and stability of the emulsions were analyzed.The main conclusions are as follows:(1)The effects of high hydrostatic pressure combined with acid or alkaline pH shifting treatments on the structure and emulsifying properties were investigated.Results showed that the solubility of SPI increased 1.71 times and 2.94 times after being treated by high hydrostatic pressure(400 MPa)combined with acid or alkaline pH shifting,and these treatments induced the formation of smaller-size aggregates and increased the content of surface charge of SPI.SDS-PAGE profiles indicated that the combined treatments increased the content of soluble protein,and the disulfide linkages in protein molecule were weakened by a combined alkaline pH shifting and pressurization process.The contents of free SH group and surface hydrophobicity showed that the protein molecular conformation was partially unfolded,indicating that the internal sulfhydryl groups and hydrophobic groups migrated from the inside of protein particles to the surface.The ultraviolet spectroscopy and circular dichroism analysis showed that compared with a single pressurization or pH shifting treatment,the combined treatment induced a looser tertiary structure and enhanced the transition of secondary structure from?-sheet to?-helices and random coils.Compared to the untreated SPI,both the emulsifying activity and emulsifying stability of SPI subjected to the combined treatment were markedly improved.The droplet size,zeta potential and fluorescence images of the emulsion showed that pressurization at 400 MPa at pH 11 yielded an SPI that favors the formation of the smallest emulsion droplets,and the surface of this emulsion sample carried the highest amount of charge.(2)In this experiment,high pressure treatment and alkaline pH-shifting treatment were applied individually and in combination on soybean protein isolate(SPI),and then SPI was glycated with maltodextrin.The structural and emulsifying properties of the conjugates under the different pretreatment were characterized.Results showed that after the pressurization(400 MPa),alkaline pH shifting,and the combination process,the degree of grafting of the three kind of glycosylated products reached 13.10%,7.00%,and 16.79%,respectively.The combined treatment increased the browning values of the glycosylation products,enhanced the transition of secondary structure from an ordered to a disordered state,facilitated the exposure of hydrophobic groups initially buried inside,and loosed the tertiary structure of protein molecule.It was found that pressurization at400 MPa combined with alkaline pH shifting treatment has the most significant effect on promoting the glycosylation of SPI.Meanwhile,the emulsifying ability and emulsion stability reached the maximum value of 31.38 m~2/g and 36.88 min,which were 1.89 and1.60 times as that of control group.(3)Two kinds of emulsion were prepared with the high-speed shear homogenization method,using unmodified SPI or 400Sh-M as the emulsifying agent.The increase in oil fraction(from 55%to 75%)increased the droplet size of the emulsions,and the droplet size of the emulsion stabilized by 400Sh-M was smaller than that of the emulsion stabilized by SPI when a same oil fraction was appilied.The rheological properties showed that a more obvious viscoelastic characteristic in the emulsion stabilized by400Sh-M was detected,compared to the emulsion stabilized by SPI.This indicated that protein glycosylation could improve the viscoelastic properties of the emulsion and enhance the stability of the emulsion network structure.The amount of adsorbed proteins at the interface indicated that the adsorption efficiency of the glycosylated protein is higher at the oil-water interface.In addition,the test of storing and thermal stability showed that the emulsion stabilized by 400Sh-M exhibits stronger anti-creaming effect and thermal stability. |
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