Streamlining Onion Pungency Analyses

Abstract

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-