The preparation of soy products with different levels of native phytate for zinc bioavailability studies

Soy products with low, intermediate and normal phytate levels were prepared in the pilot plant for subsequent rat-feeding experiments to evaluate zinc bioavailability. The low level (0.29%) phytate product was made by precipitation of the protein curd at pH 5.5, whereas the normal level (1.05%) phytate product was produced by a similar process except that the phytate previously isolated from the whey fraction was added back to the original curd as native phytate. The intermediate level (0.73%) phytate product was also produced by acid precipitation, but at pH 4.5. The pH 5.5 precipitation process yielded a large quantity of whey in which the ratio of water content to phytate was over 1,000 parts to 1. However, ca. 75% of the water was subsequently removed by reverse osmosis (RO), which increased the concentration of phytate in the whey fraction and facilitated its isolation. Protein was first removed from the whey by precipitation with trichloracetic acid, then phytate was precipitated in the supernatant with ferric chloride. Another series of experiments was run to find optimal conditions to convert ferric phytate to the more soluble sodium phytate form, using a minimal amount of sodium hydroxide so that the phytate could be recycled back to the curd without causing a large increase in sodium content of the product. There were only minor differences in the protein, lipid and mineral contents of the three products.