Changes in trans-S-1-Propenyl-l-cysteine Sulfoxide and Related Sulfur-Containing Amino Acids during Onion Storage

trans-S-1-Propenyl-l-cysteine sulfoxide (PRENCSO) is the most abundant flavor precursor in onions. The changes in PRENCSO, its biosynthetic precursors, and cycloalliin contents in bulbs stored at 0 °C [i.e., recommended by the Food and Agriculture Organization of the United Nations (FAO)] are investigated. Ion-exchange high-performance liquid chromatography analysis revealed larger amounts of PRENCSO and cycloalliin, which were stoichiometrically balanced by the decrease in the combined amounts of the three biosynthetic precursors. The molar balance of the PRENCSO biosynthesis process during onion storage was explained well by the changes of these five compounds. High-throughput integrated liquid chromatography-tandem mass spectrometry analysis was employed to analyze multiple samples at 5 and 20 °C. The molar concentration data, converted using linear regression equations, showed a similar molar balance for the PRENCSO biosynthesis process comprising these five compounds during storage at all temperatures tested. Furthermore, the content of cycloalliin significantly increased at the expense of that of PRENCSO during storage at 20 °C.