Residual sulfur metabolites in isolated soy proteins: Sulfite to cysteine

Mean methanethiol headspace concentrations above aqueous slurries of isolated soy proteins (ISP) increased 17- to 36-fold over the controls with the addition of L-cysteine. Corresponding hydrogen sulfide levels were also greatly increased. Dithiothreitol, sodium sulfite, and glutathione increased headspace methanethiol from aqueous ISPs 23- to 44-fold, 8- to 9-fold, and 5-fold, respectively, but did not elevate hydrogen sulfide. These observations, along with the effects from the addition of dithiothreitol/O-acetyi-serine, the addition of a pyridoxial phosphate inhibitor and the intrinsic sulfite content of ISP samples (22 to 31 ppm), indicate that methanethiol from soy proteins is formed by way of components of a sulflte-to-cysteine pathway.