Functionality and Digestibility of a Highly Phosphorylated Soybean Protein

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Acid precipitated soybean protein (Soybean Protein Isolate, SPI) was phosphorylated with POCl3 under an alkaline condition. The amount of incorporated phosphorus was 14μg per mgof SPI. Whenthe molecular weight of the SPI was taken as 360, 000 daltons, the mol ratio of phosphorus to protein was about 140. Aminoacid analysis for the phosphorylated SPI suggested that the type of phosphate bound to the protein was ortho-phosphoric acid, and that the phosphorylated amino acid residues were lysine and histidine. The association of phosphate to SPI was stable for more than 2 weeks at pH>5.0. The functions ofphosphorylated SPI were examined at various pH levels. The high solubility of phosphorylated SPI at pH>3.0 did not decrease the emulsifying ability and gel forming ability, even in the acidic range when compared with the neutral range, whereas the SPI lost most solubility and functionality at pH<5.5. There was no difference between the digestibility of phosphorylated SPI and SPI. These results suggested that phosphorylated SPI is a very useful food material possessing good functional properties throughout a wide pH range.
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