Sulfite formation in isolated soy proteins

Defatted soybean flour contained 2820 parts per million (ppm) inorganic sulfate. The corresponding laboratory isolated soy proteins (ISP) contained 1364 ppm sulfate bound to a greater than 3000 molecular weight fraction. Defatted soy flour contained 0.98 nmoles of adenosine triphosphate (ATP) per 0.1 g. ATP levels decreased when the defatted flour was hydrated under a variety of conditions. Neither adenosine-5′-phosphosulfate or 3′- phosphoadenosine-5′-phosphosulfate could be detected in defatted soybean flour or in aqueous extracts of defatted flour. Defatted soy flour contained 1034 ppm reduced glutathione and 647 ppm reduced homogluatathione. These levels dropped to 62 and 13 ppm, respectively, immediately after the isoelectric precipitation step of ISP processing. No oxidized glutathione or homoglutathione were detected. Defatted flour contained 106 ppm cysteine-S-sulfonate, alkaline extracts of defatted flour contained 329 to 579 ppm and ISP contained 0 to 43 ppm. Neither sulfite nor methanethiol were detected until after the soluble components at pH 4.6 were separated from the precipitated soy proteins. These findings indicate that cysteine-S-sulfonate is present in defatted flour and increases during ISP processing. Also, the formation of sulfite during the final stages of ISP processing corresponds to elevated methanethiol levels.