Quantifying Onion Flavor Compounds Responding to Sulfur Fertility-Sulfur Increases Levels of Alk(en)yl Cysteine Sulfoxides and Biosynthetic Intermediates

Abstract

<jats:p>Three onion <jats:italic>(Allium cepa</jats:italic> 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 <jats:italic>Alliums</jats:italic> will become increasing important as the food and phytomedicinal industries move toward greater product standardization and characterization.</jats:p>

Ammonium nitrate fertility levels influence flavour development in hydroponically grown ‘Granex 33’ onion

“…In onion, the flavour precursors are a strong sink for available S and much of the organic-S in the bulb may be used in their synthesis. 4 Though N did significantly influence SO 4 2− -S levels in bulbs (P ≤ 0.012), no meaningful trend could be fitted. Levels of SO 4 2− -S ranged from 1.0 to 1.5 g kg −1 dry-weight.…”

“…4 Though N did significantly influence SO 4 2− -S levels in bulbs (P ≤ 0.012), no meaningful trend could be fitted. Levels of SO 4 2− -S ranged from 1.0 to 1.5 g kg −1 dry-weight. The amount of organically bound S in the bulbs was significantly affected by N level (P ≤ 0.001), and, like total bulb S, organically bound S responded quadratically to N levels (P ≤ 0.001; Fig 3).…”

“…Three, 5 mm wedges were cut longitudinally from each bulb. One wedge was used to determine total bulb S and sulphate (SO 4 2− ), a second to measure ACSOs and related γ -glutamyl peptides (γ GPs), and a third to measure total pyruvic acid (TPY) and soluble solids content (SSC).…”

“…SO 4 2− concentrations were determined using anion analysis and high-performance liquid chromatography (HPLC). SO 4 2− was extracted from 0.25 g of ground tissue in 50ml of HPLC-grade water in 125ml Erlenmeyer flasks. Flasks were shaken on an orbital shaker for 30 min at 150 rpm.…”

“…These values are similar to those reported for 'Granex 33' onions grown with high N and similar S fertility levels. 15 Because SO 4 2− is generally stored in the vacuole and is unable to contribute immediately to the production of flavour compounds in onion, it is important to determine the amount of total S that is contributed by SO 4 2− . By knowing total S and SO 4 2− -S, the amount of S that is bound organically (organic-S) in the bulb can be estimated.…”