Intrinsic sulfite content of isolated soy proteins

Abstract

AbstractCommercial isolated soy proteins (ISP) contain 22 and 31 ppm sulfite as measured by the optimized Monier‐Williams method (Sulfites in Foods, Official Methods of Analysis, 16th edn., AOAC, Washington, DC, 1995, Official Method 990.28). A method was developed to cryogenically trap and quantify the sulfur dioxide produced by this method using GC‐MS. The same commercial ISP samples were found to contain 17 and 26 ppm sulfite, respectively, with GC‐MS. ISP prepared in the laboratory contained 33 ppm sulfur dioxide, and defatted soybean flakes contained only a trace. Adding dithiothreitol after beginning the boiling step of the Monier‐Williams assay had no significant effect on the sulfite content of a commercial ISP, whereas adding dithiothreitol prior to bringing the sample to a boil reduced the sulfite content from 17 to about 1 ppm.