Onion Pungency Research Articles

A Simplified Pyruvic Acid Analysis Suitable for Onion Breeding Programs

Onion pungency is estimated by analyzing enzymatically produced pyruvic acid (Schwimmer and Weston, 1961). This acid is produced by alliinase hydrolysis of a group of flavor-precursor S-alk(en)yl-L-cystein sulfoxides in onion tissues when they are mechanically chopped or macerated. Many sulfurvolatiles and ammonia also are produced during this reaction. There is a high correlation between enzymatically produced pyruvic acid and pungency perception (Schwimmer and Weston, 1961; Wall and Corgan, 1992). Thus, pyruvic acid concentration is suggested as a mildness selection criterion in onion breeding (Wall and Corgan, 1992). Large-scale screening of onion bulbs for pungency requires a fast and cost-efficient method. The original procedure by Schwimmer and Weston (1961) is time-consuming because of extraction difficulties and juice dilution. A rapid method for collecting the onion extract using a press significantly reduced analysis time (Randle and Bussard, 1993). In the onion breeding program at Texas A&M Univ., we routinely screen ≈1000 onion bulbs per year for mildness. Weighing onion tissues and blending with a fixed volume of water is slow, while filtration and additional dilution steps are cumbersome and require much glassware. We simplified the original method of Schwimmer and Weston (1961) to shorten sample preparation and analysis time by 1) blending onion tissue without the addition of water to eliminate the weighing step needed to obtain 5-fold diluted juice, 2) filtering the blended puree into disposable Styrofoam cups, and 3) analyzing pyruvic acid using undiluted juice. This paper details the method and compares its accuracy with the original method using diluted juice. Three or five onion bulbs from three cultivars (total 18 bulbs), out of storage or fresh harvest, were used in this study. The onion bulbs were cut longitudinally into two pieces. One-half was immediately chopped into pieces ≈2 cm square and homogenized without water for 1 min in an Osterizer (Oster, Milwaukee) blender at speed “mix.” The other half was wrapped in a plastic bag, then cooked in a

Antiplatelet Activity is Positively Correlated with Pungency and Solids in Onion (Allium cepa L.)

Onion (Allium cepa L.) and other vegetable Alliums have long been recognized for the antiplatelet properties. Consumers may benefit from the medicinal value of onions because they are commonly eaten raw in salads and the antiplatelet factor is destroyed by heat. Recent work indicates antiplatelet activity in Allium sp. may be due to the presence of native organosulfur compounds. The concentration of organosulfur compounds correlates positively with pungency, varies with onion cultivar, and is influenced by environmental factors. Bulb dry matter content, or solids, is positively correlated with pungency. Because antiplatelet activity may also be based on the activity of organosulfur compounds, it is possible these three factors are significantly correlated. The objective of this investigation was to examine the relationship among pungency, solids, and antiplatelet activity in four diverse onion genotypes. Replicated trials consisting of two mild and two pungent genotypes were conducted at four locations in 1994. Onion bulbs were harvested and analyzed for all three traits. Results from this investigation indicate significant positive correlations between antiplatelet activity, pungency, and solids in onion.

Pungency and Sugar Changes in lLong-day Onions Due to Different Storage Times and Temperatures

Twenty-four genotypes of `Walla Walla' sweet onion (Allium cepa L.) grown in two locations were evaluated for several characteristics associated with bulb flavor and storage losses. The range of pyruvic acid content in bulbs stored (at 5C and 65–75% RH) for 0, 2, and 4 months were 3.4–7.54, 3.48–18.81, and 3.92–12.61 (μmol·g–1), respectively, among different genotypes. Bulb quality of several genotypes decreased during storage, as indicated by lower total sugar concentration (fructose, glucose, and sucrose) and greater pungency. At 5C after 4 months of storage, the range of marketable bulbs (percent by weight) was 31% to 89% among genotypes; however, at 15C, only two genotypes survived with 60% marketable bulbs. Pungency and sweetness changed independently during storage. Pyruvic acid was not correlated (r = 0.038) with the percentage of marketable bulbs remaining after 4 months of storage. In comparison with the short-day sweet onions (`Vidalia' and `Texas Grano 1015Y), `Walla Walla' sweet onions showed two-fold higher sugar: pungency ratio among genotypes.

Pungency and Sugar Changes in lLong-day Onions Due to Different Storage Times and Temperatures

Twenty-four genotypes of `Walla Walla' sweet onion (Allium cepa L.) grown in two locations were evaluated for several characteristics associated with bulb flavor and storage losses. The range of pyruvic acid content in bulbs stored (at 5C and 65–75% RH) for 0, 2, and 4 months were 3.4–7.54, 3.48–18.81, and 3.92–12.61 (μmol·g–1), respectively, among different genotypes. Bulb quality of several genotypes decreased during storage, as indicated by lower total sugar concentration (fructose, glucose, and sucrose) and greater pungency. At 5C after 4 months of storage, the range of marketable bulbs (percent by weight) was 31% to 89% among genotypes; however, at 15C, only two genotypes survived with 60% marketable bulbs. Pungency and sweetness changed independently during storage. Pyruvic acid was not correlated (r = 0.038) with the percentage of marketable bulbs remaining after 4 months of storage. In comparison with the short-day sweet onions (`Vidalia' and `Texas Grano 1015Y), `Walla Walla' sweet onions showed two-fold higher sugar: pungency ratio among genotypes.

Heritability Estimates of the Pungency and Single-center Traits in Onion

Heritabilities of the pungency and single-center traits were estimated in onion breeding lines using selection response and half-sib family analyses. Pungency was determined by measuring enzymatically produced pyruvic acid in individual bulbs. After one generation of selection, pungency was lowered by 0.37 and 0.42 μmol pyruvic acid/gram fresh weight in the breeding lines 90-61-1 and 89-69-8, respectively, and realized heritabilities of 0.21 and 0.51 were estimated. Heritability estimates calculated through half-sib progeny analysis were 0.53, 0.48, and 0.25 for pungency in the breeding lines 90-61-1, 90-62, and 89-69-8, respectively. The number of single-centered onions was increased by 19% and 22% in the lines 90-62 and 89-69-8, respectively, after one generation of selection, and the realized heritability estimates were 0.37 and 0.34, respectively.

Antiplatelet Activity, Solids, and Pungency of Four Onion (Allium cepa L.) Genotypes during Cold Storage

Raw onion extract contains organosulfur compounds that prevent aggregation of platelets in human blood plasma and influence onion pungency. Organosulfur compounds are volatile and may change concentration during storage. A study was conducted to determine 1) whether antiplatelet activity of filtered onion extract decreases during time in cold (4C) storage; and 2) correlations among antiplatelet activity, pyruvic acid content, and percent solids during time in cold storage. Two low-pungency genotypes (8155 and Exhibition) and two high-pungency (W420 and W434) genotypes were grown in replicated plots in two Wisconsin and two Oregon locations in 1994. Bulbs were evaluated for antiplatelet activity, percent solids, and pyruvic acid content at 40-day intervals after onion harvest. Significant differences were found for pyruvic acid content, solids, and antiplatelet activity among dates of sampling, genotypes, and locations. Mean pyruvic acid concentrations ranged from 6.4 μm·ml–1 of extract for Exhibition, to 8.0 μm·ml–1 of extract for W420. Mean solids concentrations ranged from 5.8 g/100 g for Exhibition to 11.4 g/100 g for W434. Antiplatelet activity averaged over all genotypes increased over 120 days and was positively correlated with percent solids and pyruvic acid content.

Genetic analysis of pungency and soluble solids in long-storage onions

Pungency and soluble solids are important quality attributes of onion. An eight-parent diallel over two years indicated a preponderance of additive genetic variation for both attributes. Variation among years was significant for pungency but not solids. No reciprocal cross differences were noted for pungency or solids. Generatio means analysis for four crosses over two years had a good fit with a simple additive-dominance model to explain the inheritance of both traits in most cases. Both diallel and generation means analyses demonstrated some dominance for low pungency. Broad sense heritability estimates averaged 0.64 for pungency and 0.83 for soluble solids. Selection was effective for increasing and decreasing pungency in derivatives of five crosses. Pungency and soluble solids were correlated among parental inbreds and hybrids but not within F3's. The additive control and heritability suggest that onion pungency and soluble solids can be effectively and independently selected.

Pungency and Sugars of Short-day Onions as Affected by Sulfur Nutrition

Sixteen short-day onion (Allium cepa L.) cultivars with high (4.0 meq·liter-1) and low S (0.1 meq·liter-1) fertility were evaluated for several characteristics associated with bulb flavor. Sulfur levels interacted with cultivars in influencing bulb pungency and concentrations of S and individual sugars, except for fructose. Enzymatically formed pyruvic acid correlated poorly with bulb S concentration, which suggests differential partitioning of S into flavor and nonflavor compounds among cultivars. Bulb percent dry weight correlated negatively with bulb S concentration. Since poor correlations were found between enzymatically formed pyruvic acid and water-soluble carbohydrates, we postulate that pungency and sweetness function independently in bulbs of fresh-market short-day onion cultivars.

Streamlining Onion Pungency Analyses

Breeding onions for desired pungency requires reliable methods for discriminating between individuals in a segregating population. Organoleptic evaluation has been used traditionally in onion breeding programs; however, it lacks precision and repeatability, especially when evaluating highly pungent onions. Schwimmer and Weston’s method (1961) has been proven to be effective in quantifying onion pungency (Lancaster and Boland, 1990; Whitaker, 1976). Pungency and other flavors are formed from the hydrolysis of S-alk(en)yl cysteine sulfoxide precursor molecules by alliinase following cell disruption. This cleavage produces S-alk(en) sulfenic acid, pyruvic acid, and ammonia. Schwimmer and Weston’s method quantifies pyruvic acid formation from the enzymatic cleavage of the flavor precursor molecules. Pyruvic acid reacts with 2,4-dinitrophenyl hydrazine to produce a colored derivative, which can be measured spectrophotometrically. Although pyruvic acid is a relatively stable compound in this reaction, it is also a common metabolic product in the plant that can upwardly bias enzymatic hydrolysis measurements. As a result, pyruvic acid background levels are determined by thermal deactivation of the enzyme system before analysis. To determine the amount of pyruvic acid formed enzymatitally, the background concentration is subtracted from total pyruvic acid. Schwimmer and Weston’s method, however, is time-consuming and cumbersome, which makes routine and systematic evaluation of breeding material and experimental treatments costly. We propose a routine method for evaluating onion pungency that reduces time and costs. Processing a single bulb in our laboratory using Schwimmer and Weston’s method took ≈74 min-58 min to extract and purify the onion juice for total and background measurements, and 16 min to quantify pyruvic acid. To shorten the time required to extract and purify the juice, we designed and devel-

RELATIONSHIP BETWEEN PUNGENCY, SOLUBLE SOLIDS, AND SUSCEPTIBILITY TO NECK ROT IN ONIONS

A field study was conducted to determine the relationship between pungency, soluble solids content, and susceptibility to neck rot in onions. `Golden Cascade', `Sweet Amber', `Valdez', and `Vega' onions were planted in a field with low soil S content. Sulfur, as coarse-ground calcium sulfate, was applied as a band before planting. After harvest, yield was determined and a sample of jumbo onions was taken from each plot to determine pungency, dry matter content, and soluble solids content. Healthy bulbs were returned to storage and evaluated for neck rot after 4 months. Yield, grade, and neck rot incidence after storage were not affected by S treatments. However, there was a trend toward lower neck rot incidence at the highest S application rate (160 lb/acre). Pungency of jumbo onions increased after the application of S as gypsum. `Sweet Amber' and `Valdez' were less pungent than `Golden Cascade' or `Vega'. Neck rot susceptibility was evaluated with an inoculation test of detached bulb scales. Growth and sporulation of the neck rot pathogen Botrytis allii were reduced by S application. Pungency and neck rot susceptibility were negatively correlated.

Relationship between Pyruvate Analysis and Flavor Perception for Onion Pungency Determination

Individual onion (Allium cepa L.) bulbs were evaluated for pungency by measurement of enzymatically produced pyruvate and by flavor perception. In four separate experiments, pyruvate values were highly and significantly correlated to mean sensory ratings. Correlation coefficients (r) were 0.92, 0.84, 0.95, and 0.79, and regression coefficients (R2) were 0.84, 0.71, 0.91, and 0.62. The high correlations indicate that pyruvate analysis can be used as a reliable selection technique for pungency in onion breeding programs.

Onion germplasm interacts with sulfur fertility for plant sulfur utilization and bulb pungency

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.

A METHOD TO MEASURE SULFUR-CONTAINING VOLATILES IN ONIONS USING A GAS CHROMATOGRAPH.

A 50 g sample taken as a horizontal section from the mid-height of an onion bulb was blended with 100 g water for 1 min in a closed plastic mason jar. A 0.5 ml of a headspace sample was drawn and injected into a Perkin Elmer 8500 GC equipped with FPD for detection of sulfur compounds. The major volatiles tentatively identified in onion were thiopropanal S-oxide, methyl propyl disulfide, dipropyl disulfide, and propyl allyl disulfide. We observed significant variation of peak pattern and height depending on position in a bulb, among bulbs within variety, and between varieties. These results seemed to comply well with taste test. There were no significant correlations between total peak height and bulb weight, soluble solids, or pyruvate concentration in juice extract. Our investigation suggested that this procedure provided better understanding and measurement of onion pungency than pyruvate analysis.

Effects of Herbicides on the Dry Matter Content, Total Soluble Solids, and Pungency of Onion1

Abstract Demand for dehydrated onions, Allium cepa L., is becoming considerable in world markets. Any changes in dry matter, total soluble solids, or pungency of onions may affect quality of the dehydrated product. Several studies have been made on the influence of herbicides on weed control and onion yield (2, 4, 5) but little has been reported on the effects of herbicides on quality characteristics. The purpose of this study was to evaluate the effects of chlorbromuron (N'-(4-bromo-3-chlo-rophenyl)-N-methoxy-N-methylurea), chloroxuron (N’-4-(4-chlorophenoxy) phenyl-N,N-dimethylurea), propachlor (2-chloro-N-isopropylacetanilide), CP-50144 (2-chloro-2′,6′-diethyl-N-(methoxymethyl) acentilide), Sindone 0263 (1, 1-dimethyl-4,6-diisopropyl-5-indanylethylketone + 7-indanylisomer), propanil (3’, 4′-dichloropropionanilide), and prometryne (2,4-bis(iso-propylamino)-6-(methylthio)-s-triazine) on dry matter content, total soluble solids, and pungency of onions.

Effects of Herbicides on the Dry Matter Content, Total Soluble Solids, and Pungency of Onion1

Abstract Demand for dehydrated onions, Allium cepa L., is becoming considerable in world markets. Any changes in dry matter, total soluble solids, or pungency of onions may affect quality of the dehydrated product. Several studies have been made on the influence of herbicides on weed control and onion yield (2, 4, 5) but little has been reported on the effects of herbicides on quality characteristics. The purpose of this study was to evaluate the effects of chlorbromuron (N'-(4-bromo-3-chlo-rophenyl)-N-methoxy-N-methylurea), chloroxuron (N’-4-(4-chlorophenoxy) phenyl-N,N-dimethylurea), propachlor (2-chloro-N-isopropylacetanilide), CP-50144 (2-chloro-2′,6′-diethyl-N-(methoxymethyl) acentilide), Sindone 0263 (1, 1-dimethyl-4,6-diisopropyl-5-indanylethylketone + 7-indanylisomer), propanil (3’, 4′-dichloropropionanilide), and prometryne (2,4-bis(iso-propylamino)-6-(methylthio)-s-triazine) on dry matter content, total soluble solids, and pungency of onions.