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ABSTRACT:
Commercial 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.
Paper no. J10830 m JAOCS 81, 829-833 (September 2004).
KEY WORDS: GC-MS, methanethiol, Monier-Williams assay, soy protein, sulfites.
A major impediment to the expanded use of soy protein products in human foods is their characteristic flavor. Methanethiol is among the most potent odorants found in aqueous slurries of isolated soy proteins (ISP), concentrates, and soy milk. A proposed mechanism for the synthesis of methanethiol in soy proteins (1) involves residual components of a sulfur assimilation pathway (Fig. 1) responsible for cysteine synthesis during soybean germination (2,3). For this to be a viable mechanism of methanethiol synthesis in soy protein products, the amount of endogenous sulfite must be sufficient to act as a substrate.
A variety of methods are available to quantify sulfites in foods (4,5). The most widely used is the optimized MonierWilliams method (AOAC Official Method 990.28; Ref. 6). This method converts free and reproducible amounts of bound oxosulfur(IV) anions in foods to sulfur dioxide, which is distilled and oxidized to sulfuric acid in a hydrogen peroxide trap. The acid is then determined by titration. Several HPLC methods are useful for quantifying sulfites. Some analyze the sulfur distilled from the Monier-Williams procedure as sulfate ions (7,8), whereas others avoid the distillation step. Warner et al. (9) treated samples with formaldehyde to convert sulfites to hydroxymethylsulfonate. Other HPLC methods measure sulfite directly using electrochemical detection (10) or direct UV detection (11).
Madl (12) found that ISP prepared without added sulfite contained...