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William M. Randle

Twenty bulbs from each of 10 onion (AIlium cepa L.) cultivars and one mass population were harvested from two locations and evaluated for three traits associated with flavor quality. Variance components for soluble solids content (SSC), pyruvic acid concentration (PAC), and percent S were calculated, and sampling schemes required to detect specific differences among treatment means were determined. In general, a five-bulb sample and four replications were sufficient to detect desired differences for SSC and PAC, whereas percent S required a larger sample size and more replications.

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William M. Randle

Sulfur fertility influences flavor in onions. To determine how sulfur is utilized by onions over the course of growth and development, five short-day onion (Allium cepa L.) cultivars were grown at high (4.0 mequiv.liter-1) and low (0.1 mequiv. liter-1) sulfur fertility regimes under greenhouse conditions. Plants were measured for foliar sulfur content at 4 week intervals during the course of the growing season and for bulb sulfur and pyruvic acid development at plant maturity. Patterns of sulfur uptake and utilization were similar for all five cultivars but cultivars differed in their magnitude of sulfur accumulation. Foliar sulfur accumulation correlated poorly with bulb sulfur and pyruvic acid development at all sampling dates.

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William M. Randle

World-wide, onions are the most important member of the vegetable Alliums. Members of this group are primarily consumed because of their unique flavors and aromas. Allium aroma is dominated by organosulfur compounds arising from the enzymatic decomposition of S-alk(en)yl-L-cysteine S oxide (ACSO) flavor precursors following tissue disruption. Primary products from the decomposition of the four ACSOs are sulfenic acids, including the lachrymator, pyruvate, and ammonia. The sulfenic acids, however, are short-lived and disassociate rapidly into thiosulfinates, which, in turn, are unstable and randomly rearrange or dissociate over time. The thiosulfinates each have unique sensory qualities and are responsible for the flavor notes of fresh cut Alliums, while of the degradation compounds can contribute to off-fl avors and bitterness. ACSO concentration affects ultimate flavor and aroma intensity, while ACSO composition determines among species flavor differences. Controlling sulfur uptake and sulfur metabolism that terminates in ACSO synthesis is one method of controlling ultimate flavor and aroma intensity. Cultivar difference in the ability to absorb and metabolize sulfur have been identified. Sulfur availability, plant growing temperatures, and irrigation intensity also influence sulfur absorption and metabolism, and can be manipulated. Differences in alliinase concentration and the efficiency at which alliinase decompose the ACSO substrates also affect aroma generation. Difficulties, however, exist in controlling alliinase activity. Alliinase has been cloned and anti-sense constructs have been made, but an efficient vectoring system has yet to be developed for the Alliums.

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William M. Randle

Field-grown `Granex 33' onions were subjected to four preplant calcium (Ca) treatments and evaluated for bulb quality and shelf-life over two seasons. Mature, cured bulbs were analyzed at harvest and after 1, 2, and 3 months of 4C storage. As preplant calcium increased, percentage of seed stems decreased, yield and soluble solids concentration increased, and then decreased, bulb firmness increased. Bulb pungency was unaffected by Ca fertility, except at the highest treatment. Percent bulb rot during storage first decreased with increasing Ca fertility, but then increased at the highest Ca treatment.

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William M. Randle

An interactive computer-based model has been developed to simulate the effects of precision planting onion on quality and yield. Variables used by the model are seed germination, plant survival, planter efficiency, onion growth potential, maximum onion size, sizing potential and inside-outside bed effects. Data bases obtained from 3 onion cultivars were used in the development of the model. The model shows when germination and plant survival are high, single seed drops by the planter results in high yield and large bulbs. At lower germination and survival values, however, a compromise is needed between maximizing yield and obtaining large bulbs.

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William M. Randle

Sixty onion (Allium cepa L.) entries were evaluated for nonstructural water-soluble carbohydrates (NWSC) under high (4.0 meq·liter-1) and low (0.1 meq·liter-1) S nutrition. Significant differences were detected among entries for sucrose, fructose, glucose, total fructans, total NWSC, and soluble solids content (SSC). Sucrose, total fructans, and SSC were highly correlated with total NWSC although deviant NWSC concentration was detected. Since a significant S × entry interaction was found for all NWSC concentrations tested. selection should be conducted at S concentrations indicative of targeted production areas.

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William M. Randle and Rachel Snyder

Mild onion consumption is increasing in the U.S. The ability to produce mild onions depends on selecting proper cultivars and growing them in an appropriate environment. A major decision in producing onions with mild flavor is determining when to stop applying sulfate to the crop. While adequate sulfur is necessary for good early onion growth, high levels of sulfur increase bulb pungency. A study was conducted where sulfate was eliminated from the fertility program at biweekly intervals during onion growth and development. Mature bulbs were then analyzed for flavor precursors and their biosynthetic intermediates, and pungency. Pungency linearly increased from 3.7 to 5.1 μmols pyruvic acid from the earliest cut-off date to the latest cut-off date, respectively. While total milligrams of flavor precursors did not significantly change in response to sulfate elimination, the methyl cysteine sulfoxide: 1-propenyl cysteine sulfoxide ratio did. Methyl cysteine sulfoxide concentration decreased in a quadratic manner while 1-propenyl cysteine sulfoxide linearly increased as sulfate fertility was extended in the growing season. Changes in individual precursors will significantly affect flavor perception as well as flavor intensity.

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Pai-Tsang Chang and William M. Randle

Onion is classified as a salt-sensitive crop, though it is found in production on saline soils around the world. While onion flavor intensity has been studied in response to various growing conditions, little is known about its response to salt stress. To understand if NaCl affects growth, flavor development, and mineral content in onion, `Granex 33' plants were grown to maturity with six different concentrations of NaCl ranging from 0 (control) to 125 mm in nutrient solutions. NaCl affected onion fresh weight and altered onion flavor intensity and quality. Plants did not survive the 125 mm NaCl treatments and are not included in the results. As bulb Na+ and Cl- content increased in response to increasing NaCl concentrations, leaf and bulb fresh weight of mature plants decreased. Total bulb S content also decreased with increasing NaCl solution concentrations, while bulb SO4 2- content increasing linearly, indicating that less S was entering the S metabolic stream. Though bulb soluble solids content was not influenced by NaCl concentrations, pungency increased, but only at the highest NaCl concentration. Total flavor precursors and methyl cysteine sulfoxide content increased in response to NaCl, but only at the 100 mm treatment. 1-Propenyl cysteine sulfoxide was generally unresponsive to the salt treatment. Propyl cysteine sulfoxide content decreased then increased in responses to increasing NaCl levels, but was found as a minor flavor precursor. Peptide intermediates measured in the pathway leading to 1-propenyl cysteine sulfoxide and propyl cysteine sulfoxide decreased linearly with increasing NaCl exposure. While NaCl affected onion flavor in this study, severe reductions in growth would prevent onion production under similar saline conditions. For practical purposes, the effects of NaCl on flavor are, therefore, minimal.

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David E. Kopsell and William M. Randle

Pungency and bulb quality changes during storage were evaluated using onion (Allium cepa L.) cultivars representing different storage abilities, pungency, and soluble solids content. Bulbs were harvested from greenhouse-grown plants, cured, and stored for 3 or 6 months at 5 ± 3 °C, 80% ± 5% relative humidity (0.8 to 1.1 kPa vapor pressure deficit). Prior to storage, and after each month of storage, bulbs were evaluated for pungency by measuring enzymatically formed pyruvic acid (EPY), soluble solids content (SSC), percent loss in mass (%ML), and loss of dormancy. Pungency differed among cultivars prior to and during storage. Among short-day (SD) cultivars, EPY either decreased or increased linearly with increased storage duration. Among intermediate (ID)- and long-day (LD) cultivars, EPY decreased linearly or quadratically during storage. Short-day cultivar SSC increased, then decreased quadratically during storage, while ID and LD cultivar SSC decreased linearly over time. Percent loss in mass increased linearly during storage among all cultivars, although SD cultivars exhibited greater %ML than did ID or LD cultivars.

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David E. Kopsell and William M. Randle

Short-day (SD) and long-day (LD) cultivars of onion (Allium cepa L.) representing various storage and flavor characteristics were greenhouse-grown to maturity. Bulbs were harvested and cured, then stored at 4C and evaluated monthly for pyruvic acid concentration (EPY), soluble solids content (SCC), and weight loss (WL). The EPY of `Dehydrator #3' (SD) decreased linearly with storage while EPY of `Granex 33' (SD) increased linearly. The EPY of `Zenith' (LD) had a quadratic response, decreasing then increasing during storage, while EPY of `Sweet Sandwich' (LD) increased then decreased quadratically during storage. Cultivar SSC generally decreased, while WL increased during storage.