The average home cook chops an onion just before using it, but those who cut onions in advance—whether for home or industrial cooking—have long noticed a peculiar occurrence: Waiting a while to use onions after cutting them often results in their developing a bitter taste that was not previously present when they were freshly cut. [mobile-ad name=”Advert 1″]This bitterness prompted an inquiry by Czech Professor Roman Kubec of the Department of Applied Chemistry at České Budějovice’s University of South Bohemia, who led a team in investigating its source. In a paper published in the Journal of Agricultural and Food Chemistry, Kubec and his team explained they had identified the source of bitter off-taste in processed Allium cepa onions—nine groups of previously unidentified sulfur compounds they named allithiolanes A through I. They did this by processing onions through a juicer, verifying that fresh juice was not bitter, and noting it developed an off-taste after 30 minutes. Using liquid chromatography, the team identified the compounds as they developed spontaneously over time after the onion was damaged. The team also identified the spontaneous development of the same compounds in leeks and garlic.
“For a long time, the only studied sulfur compounds of onion were the lachrymatory factor and thiosulfinates that are associated with its typical pungency and biting taste,” Kubec tells Food Quality & Safety. “Allithiolanes, on the other hand, are a group of sulfur-rich heterocyclic species with considerably higher molecular weight. Thus, allithiolanes do not affect significantly the smell of onion (they possess only a faint odor), but they have a striking impact on its taste.”
The formation of allitholanes, Kubec explains, is entirely spontaneous, triggered immediately when the onion bulb is cut or otherwise damaged. [mobile-ad name=”Advert 2″]“Because allithiolanes are formed from the very same precursor (named isoalliin) as are the other odor- and taste-bearing species of onion, it seems impossible to completely avoid their formation in heat-untreated onions,” says Kubec. “Their formation may be slowed by inactivation (e.g., by elevated temperature or acidification) of alliinase which is the enzyme that catalyzes the cleavage of isoalliin. However, the best advice I can give to consumers is to cut the onion immediately before use to ensure that the amount of allithiolanes does not reach detectable levels.”
In what may be frustrating for those in the food production industry who work with onions and hope to be able to avert the development of allithiolanes in cut and stored onions, Kubec says there is not an obvious industrial application to the study at present.
“I do not expect any immediate impact of our discovery on the food production industry,” he says. “At the moment, we only made the very first step by the identification of the bitter principles. In the near future, we will focus on the evaluation of biological properties of allithiolanes. It may turn out that although these species taste awfully bitter, they can also exhibit significant health-promoting activity.”