Root cultures of Allium cepa L. (onion) were fed potential precursors in the sulfate eduction pathway to the (+)-S-alk(en)yl-L-cysteine sulfoxides (RCSOs) which are flavor precursors in Allium sp. Sulfate, valine, and methacrylic acid did not increase RCSO production, indicating that these steps were not rate limiting. Cysteine, glutathione, and methionine could increase both the methyl- and propenylcysteine sulfoxides. The ratio of the propenyl to methyl form could be dramatically altered (over a range of 8.5) depending on the amount and type of precursor used. Additionally, novel flavor components, not normally found in A. cepa L., were made in substantial amounts by feeding the corresponding thiol (propyl, 2-propenyl (allyl), and ethyl). These results demonstrate that customized flavor profiles can be produced from Allium root cultures and provide an example of the use of plant cell cultures to produce novel metabolites.