A Simplified Pyruvic Acid Analysis Suitable for Onion Breeding Programs

Abstract

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