QUANTIFYING ONION FLAVOR COMPOUNDS RESPONDING TO SULFUR FERTILITY - SULFUR INCREASES LEVELS OF ALK(EN)YL CYSTEINE SULFOXIDES AND BIOSYNTHETIC INTERMEDIATES

Three onion (Allium cepa L.) cultivars were grown to maturity at five S fertility levels and analyzed for Salk(en)yl-l-cysteine sulfoxide (A CSO) flavor precursors, gamma-glutamyl peptide (gamma-GP) intermediate s, bulb S, pyruvic acid, and soluble solids content. ACSO concentratio n and composition changed with S fertility, and the response was culti var dependent. At S treatments that induced S deficiency symptoms duri ng active bulbing, (+)S-methyl-L-cysteine sulfoxide was the dominant f lavor precursor, and the flavor pathway was a strong sink for availabl e S, As S fertility increased to luxuriant levels, trans(+)-S-(1-prope nyl)-L-cysteine sulfoxide (PRENCSO) became the dominant ACSO. (+)S-pro pyl-L-cysteine sulfoxide was found in low concentration relative to to tal ACSO at all S fertility treatments. With low S fertility, S rapidl y was metabolized and low gamma-GP concentrations were detected. As S fertility increased, gamma-GP increased, especially gamma-L-glutamyl-S -(1-propenyl)-1-cysteine sulfoxide, the penultimate compound leading t o ACSO synthesis. Nearly 95% of the total bulb S could be accounted fo r in the measured S compounds at low S fertility. However, at the high est S treatment, only 40% of the total bulb S could be attributed to t he ACSO and gamma-GP, indicating that other S compounds were significa nt S reservoirs in onions. Concentrations of enzymatically produced py ruvic acid (EPY) were most closely related to PRENCSO concentrations. Understanding the dynamics of flavor accumulation in onion and other v egetable Alliums will become increasing important as the food and phyt omedicinal industries move toward greater product standardization and characterization.