In this study, a rapid and sensitive method to determine the flavor precursors was established on thebasis of modified Kubec's gas chromatography (GC) method. For the new method, the interferenceof halogen was removed by adding borohydride that replaced halide as a reducing reagent, and theanalysis was completed within 25 min for the determination of
S-alk(en)yl-
L-cysteine sulfoxides(ACSOs). In comparison with the results of high-performance liquid chromatography (HPLC), themodified GC method is much better in the separation and detection of ACSOs in yellow onion bulbsthan the original method. A small amount of 2-propenyl-
L-cysteine sulfoxides (2-PeCSO) was detectedwith the modified GC method, and propyl-
L-cysteine sulfoxides (PrCSO) were not detected. The resultsof GC determination indicated that 2-PeCSO probably was the hydrolysate of a byproduct or anisomer of
-glumtyl-1-propenyl-
L-cysteine sulfoxides in biosynthesis process.