Emulsifying Properties of Soy Protein Isolate at pH near the Isoelectric Point
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Abstract:
At pH near the isoelectric point (pI), soy protein exhibits reduced electric charge and enhanced hydrophobic interactions, causing aggregation that results in poor solubility. Therefore, the emulsifying properties of soy protein under these conditions have received little attention. In this study, a soy protein isolate (SPI) was used to prepare stable emulsions at pH 5.0, which was near the pI, and pH 7.0, which was far from the pI. Soy protein aggregate particles were used to stabilize the emulsion. The interfacial properties of SPI under such conditions and the storage stability of the resulting emulsions were investigated. The results indicated that the solubility of SPI was 4.70 ± 0.15% at pH 5.0, which was much lower than that at pH 7.0 (93.28 ± 1.89%). At a concentration of 0.5% SPI, higher surface tension was observed at pH 5.0 when compared with that at pH 7.0. Moreover, the surface protein adsorption capacity of the emulsion prepared at pH 5.0 was 87.04 ± 1.28%, which was significantly higher than that prepared at pH 7.0 (36.15 ± 1.48%). After two months of storage, the average size of the droplets in the emulsion prepared at pH 5.0 was 63.15 ± 0.30 μm, which was similar to that of the droplets in freshly prepared emulsion (62.36 ± 0.41 μm). In the case of the emulsion prepared at pH 7.0, the average size of the droplets increased from 45.78 ± 0.38 to 55.19 ± 1.86 μm after storage for two months. Thus, we conclude that SPI prepared at pH near its isoelectric point can be used as an efficient emulsifier.
