Why chopped onion is bitter after a while

T V Jayan New Delhi | Updated on August 20, 2018 Published on August 20, 2018

New sulphur compounds are formed when onion is cut   -  s-cphoto

Scientists point to new sulphur compounds formed when onion is cut

Ever wondered why onion tastes awful when diced pieces are exposed to the air for a while? Science may have an answer to explain this foul taste associated with the bulbous vegetable.

A team of Czech chemists has found that leaving chopped onion pieces exposed to air for as little as 30 minutes leads to the formation of new sulphur compounds, hitherto not known to exist. The scientists, led by Roman Kubec of the University of South Bohemia in the Czech Republic, reported their findings in American Chemical Society’s Journal of Agricultural and Food Chemistry last week.

Its characteristic taste and flavour have made onion one of the most popular vegetables worldwide. It is an important ingredient of most meals of nearly all nationalities. In the food industry, the bulbous vegetable plays an essential role in the production of numerous products, such as soups, canned foods, pickles, salads and meat products like sausages.

But, as onions are chopped and processed for cooking, a bitter taste could leave the resulting meal unsavoury sometimes. Very little, however, is known about why this particular phenomenon occurs, despite its significant negative impact on the taste of processed onion. There have been only two papers dealing with this puzzling phenomenon, in the past, according to the scientists.

In the first paper, published more than 50 years ago, Sigmund Schwimmer, a scientist with the US Department of Agriculture, concluded that the bitter ingredient was not derived from isoalliin, the key precursor of the sulphur sensory-active compounds which cause eyes to water and sting. However, in a subsequent paper, Schwimmer opined that isoalliin may have some role in the formation of a bitter taste. But the identity of the substances that contributed to the foul taste remained elusive, prompting Kubec and colleagues to probe further.

It runs in the family

The team processed onions with a kitchen juicer. Freshly obtained juice was not bitter, but after 30 minutes, it developed a strong bitter taste. The group performed sensory-guided high-performance liquid chromatography to identify the compounds that formed over time in the onion juice. The researchers discovered nine groups of new sulphur compounds in the onion juice, which they dubbed allithiolanes.

These compounds form spontaneously when the onion is damaged.

Interestingly, the allithiolanes are not limited to onions but are also present in other onion family vegetables such as leeks and garlic.

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Published on August 20, 2018
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