Tryptophan sulfonate: A new chemical marker for accurate and efficient inspection of sulfur-treated food products

Sulfur treatment for the pesticidal and antibacterial processing of food products has been criticized since it impairs the quality of treated products. The inspection of sulfur-treated products is thus required to achieve the regulation of sulfur treatment. Sulfite assay is currently available for the inspection, but it bears the disadvantages of inaccurate results and complex experimental procedures. Here we report a new chemical marker, namely tryptophan sulfonate, that can be used for the accurate and efficient inspection of sulfur-treated foods. First, the marker was discovered in sulfur-fumigated ginger, yam, and ginseng by untargeted metabolomics. The marker identity was then elucidated using chromatographic separation, nuclear magnetic resonance analysis and chemical synthesis. Finally, to demonstrate its applicability in the inspection, a tryptophan sulfonate assay was developed to test 50 commercial food samples, and the results indicated that it performed better than the sulfite assay in terms of both accuracy and efficiency.