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Effects of pH and salt on yields, trypsin inhibitor content, and mineral levels of soybean protein isolates and wheys

Honig, D.H., Rackis, J.J., Wolf, W.J.
Journal of agricultural and food chemistry 1987 v.35 no.6 pp. 967-971
soy protein, protein isolates, pH, sodium chloride, trypsin inhibitors, phytic acid, mineral content
Protein isolates were prepared from water extracts of defatted soybean flour or flakes by acid precipitation at pH's from 3.5 to 5.2 with or without added 0.1 N NaCl. The resulting wheys and precipitates were analyzed for trypsin inhibitor (TI) activity (TIA), phytic acid, and minerals. TIA varied from 70 mg of trypsin inhibited/g of protein in an isolate precipitated at pH 4.5 from an extract with no added NaCl to 8 mg/g at pH 5.2 with added NaCl. Protein isolate yields varied from 85% of extract proteins at pH 4.5 with no added salt to 59% at pH 5.2 with added salt. The TIA was concentrated to 450 mg/g of freeze-dried permeate by ultrafiltration and dialysis. Retention of the TIA was 50% or more by an ultrafiltration membrane with a 100-kDa cutoff. Since TI's have a molecular weight of 20K or less, their retention indicates they were aggregated to a higher molecular weight. The phytic acid level was highest and zinc level lowest in the pH 3.5 protein precipitate compared to levels in the pH 4.5 and 5.2 precipitates. Levels of Ca, Mg, K, Na, Fe, Mn, and Cu in isolates, wheys, and a TI concentrate were also determined. Extent of removal of minerals and phytic acid from an acid-soluble TI concentrate at pH 2.4 was in the order K > Mn > Zn > Mg > phytic acid > P > Ca > Fe > Cu.