Temperature Influences Flavor Intensity and Quality in 'Granex 33' Onion

The effects of temperature and developmental age on flavor intensity and quality were tested by growing 'Granex 33' onions (Allium cepa L.) at 16.5, 22.1, 26.7, and 32.2 (±0.4) o C for 50 days and to maturity. Plants were harvested and evaluated for growth characteristics. Bulbs were then analyzed for sulfur (S) assimilation and flavor development parameters. Total bulb S increased linearly with temperature regardless of bulb age. Bulb sulfate changed little over temperatures, indicating that organically bound S increased with temperature. Total pyruvic acid content (pungency), total S-alkenyl cysteine sulfoxide (ACSO) content and individual ACSOs increased linearly in response to temperature when measured at the two developmental stages. Though trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide was the predominant ACSO at most temperatures, (+)-S-methyl-L-cysteine sulfoxide accumulation was greatest among the individual ACSOs in mature bulbs grown at 32.2 o C. Additionally, (+)-S-propyl cysteine sulfoxide was present in the least amount at all treatment levels and developmental stages. Gamma glutamyl propenyl cysteine sulfoxide and 2-carboxypropyl glutathione peptides in the flavor biosynthetic pathway also increased linearly with temperature. When ACSOs were assessed in onion macerate as a measure of alliinase activity, levels of degraded ACSOs increased linearly with growing temperature. The relative percentage of most ACSOs hydrolyzed, however, did not change in response to growing temperature. This suggested that the activity of alliinase was proportional to the amounts of flavor precursors synthesized. Growing temperature, therefore, should be considered when evaluating and interpreting yearly and regional variability in onion flavor. Observations made on commercial sweet onion production suggest that yearly temperature variations were possibly linked to changes in onion flavor quality (personal observation). Growing temperature was reported to affect total-S accumulation, pungency and the production of total S-volatiles (Platenius, 1941; Randle et al., 1993; Yamaguchi et al., 1975). Pungency and total-S volatiles, though useful in describing flavor intensity, give no insight into the overall flavor quality, determined by changes in individual ACSOs, or in how S is metabolized in onion. To gain knowledge of the flavor pathway and flavor quality, the ACSOs and their intermediates should be investigated. Furthermore, results obtained by Platenius (1941) concerning onion flavor were confounded with the develop- mental age of the bulb. In those studies, plants were grown for a set period of time at different temperatures. Plant development is influenced by growing temperature and developmental age can influence onion flavor intensity (Hamilton et al., 1998; Platenius and Knott, 1936). The purpose of this research was to determine the effect of growing temperature on flavor intensity and quality in onion by measuring the composition of the ACSOs and their related peptides in onion. To determine if alliinase activity was affected by growing temperature, decomposed ACSOs were assessed in onion bulb macerates. And finally, to determine if the effect of temperature on flavor was independent of the developmental age, two experiments were conducted. First, plants were grown for a specific period of time. As such, plant development varied with temperature, but the time of exposure to the different temperatures was the same. In the second experiment, plants were grown to maturity where plant development was similar, but the exposure time to the different temperatures varied.