Three onion (Allium cepa L.) cultivars were grown to maturity at five S fertility levels and analyzed for S-alk(en)yl-L-cysteine sulfoxide (ACSO) flavor precursors, γ-glutamyl peptide (γ-GP) intermediates, bulb S, pyruvic acid, and soluble solids content. ACSO concentration and composition changed with S fertility, and the response was cultivar dependent. At S treatments that induced S deficiency symptoms during active bulbing, (+)S-methyl-L-cysteine sulfoxide was the dominant flavor precursor, and the flavor pathway was a strong sink for available S. As S fertility increased to luxuriant levels, trans(+)-S-(1-propenyl)-L-cysteine sulfoxide (PRENCSO) became the dominant ACSO. (+)S-propyl-L-cysteine sulfoxide was found in low concentration relative to total ACSO at all S fertility treatments. With low S fertility, S rapidly was metabolized and low γ-GP concentrations were detected. As S fertility increased, γ-GP increased, especially γ-L-glutamyl-S-(1-propenyl)-L-cysteine sulfoxide, the penultimate compound leading to ACSO synthesis. Nearly 95% of the total bulb S could be accounted for in the measured S compounds at low S fertility. However, at the highest S treatment, only 40 % of the total bulb S could be attributed to the ACSO and γ-GP, indicating that other S compounds were significant S reservoirs in onions. Concentrations of enzymatically produced pyruvic acid (EPY) were most closely related to PRENCSO concentrations. Understanding the dynamics of flavor accumulation in onion and other vegetable Alliums will become increasing important as the food and phytomedicinal industries move toward greater product standardization and characterization.

Although it is realized that flavour is the result of a multifaceted interaction among many different compounds, this chapter reviews the sulfur compounds that give alliums their characteristic flavours and odours.

To test the effects of high nitrogen (N) fertilization levels on onion quality and bulb flavor, `Granex 33' onions (Allium cepa L.) were greenhouse grown in hydroponic solution culture with increasing N concentrations. Nitrogen was adjusted in the solutions with NH4NO3 and increased incrementally from 0.22 g·L-1 to 0.97 g·L-1 over five treatments. Plants were harvested at maturity and subjected to quality, flavor, and mineral analysis. As solution N increased, bulb fresh weight and bulb firmness decreased linearly. Gross flavor intensity, as measured by enzymatically developed pyruvic acid (EPY) increased linearly for N concentrations between 0.22 and 0.78 g·L-1, but EPY was reduced slightly in bulbs grown at the highest N level (0.97 g·L-1). Soluble solids content was unaffected by solution N concentration. Solution N had an affect on flavor quality. Methyl cysteine sulfoxide, which gives rise to cabbage (Brassica L. sp.) and fresh onion flavors upon eating, generally increased in concentration as solution N increased. 1-Propenyl cysteine sulfoxide, which imparts heat, mouth burn, pungency, and raw onion flavors increased between the two lowest N concentrations, and then decreased as solution N increased. Propyl cysteine sulfoxide, which imparts fresh onion and sulfur flavors upon eating, generally increased with increasing solution N concentration. Several minerals were also affected by solution N concentration. Total bulb N and NO3- increased linearly while B, Ca, and Mg decreased linearly. Total bulb S and K increased and then decreased quadratically in response to increasing solution N. Nitrogen fertility can have a pronounced affect on onion flavor and as a consequence, needs to be considered when growing onions for specific flavor quality and nutritional attributes.

Glucosinolates (GSs) are S-containing compounds found in Brassica species and whose degradation products may provide protection against cancer. Sulphoraphane, a product of 4-methylsulphinylbutyl GS degradation, is a particularly potent inhibitor of anticarcinogenic detoxi®cation enzymes. Selenium also has anticancer properties, and consumption of plants containing Se may be an effective way to increase dietary Se. Since plant uptake of Se and S is competitive, GS synthesis may be affected by Se fertilisation. The objective of this study was to determine how Se fertilisation of rapidcycling B oleracea would affect Se and GS concentrations. Plants were grown in hydroponic solutions containing 0.0, 1.0, 2.0, 3.0, 6.0, 7.2 or 9.0 mg l À1 Na 2 SeO 4 . Mineral and glucosinolate concentrations were measured in shoots harvested just before anthesis. Total GSs decreased from 5.84 mmol g À1 (0.0 mg l À1 Na 2 SeO 4 ) to 1.90 mmol g À1 (9.0 mg l À1

Sixty-two onion (Allium cepa L .) entries grown under high (4 .0 mequiv .liter -1 ) and low (0 .1 mequiv .liter -1 ) S fertility were evaluated for foliar and bulb S utilization, and pungency . Significant differences were detected among entries at both S fertility levels for foliar and bulb %S, and enzymatically developed pyruvate (pungency) in bulb tissue . Correlations between foliar %S and enzymatically developed pyruvate and between bulb %S and enzymatically developed pyruvate were poor suggesting that S is differentially partitioned into flavor and non-flavor compounds in the bulbs of onion germplasm . Significant S by entry interactions were found for all variables tested indicating that selection for S utilization and bulb pungency in onions be conducted at S fertility levels indicative of targeted production areas .