Residual Soybean Sulfur Metabolism in Isolated Soy Proteins: Sulfate to Cysteine

Soybeans are the second largest food crop in the U.S. with about 3 billion bushels produced in 2001. In 2000, only about 1.5 percent of the available protein from soybeans was used in human foods largely because of their characteristic flavor. The removal of methanethiol and dimethyl trisulfide from aqueous slurries of isolated soy protein and soymilk has been shown to significantly improve the odor of these products. A basic understanding of the mechanism, and reactants, involved in the formation of these odorants is necessary for the development of commercially practical solutions to the longstanding flavor problem associated with soy protein products. Our preliminary results strongly suggest that the formation of methanethiol in aqueous slurries of isolated soy proteins is an ongoing process involving all, or part, of a sulfur metabolism responsible for the conversion of sulfate-to-cysteine. Similar mechanisms have been reported in soybean leafs and seeds (developing and mature), as well as in other plants. This is a mechanism unlike any other previously reported in foods. We propose to monitor the levels of the most likely metabolites (reactants) including free amino acids (particularly cysteine), sulfates and sulfides (e.g., adensosine-5'phosphosulfate and hydrogen sulfide), and methyl donors (e.g., S-adenosylmethionine, methyl-tetrahydrofolate, choline/betaine and methylcobalamin). This will be done under a variety of conditions to determine the effect that treatments have on the pool of each reactant and determine how they relate to the formation of methanethiol. We will also track isotopic-labeled sulfur from L-cysteine with an emphasis on hydrogen sulfide and methanethiol. After the primary reactants have been identified and plausible mechanisms proposed, we will attempt to isolate the most likely enzymes (catalysts) affecting the formation of methanethiol.