Why onions can taste bitter

Onions are commonly used in a number of meals, both cooked and raw. But you may have noticed they not only bring tears to yours eyes but can also leave a bitter taste in your mouth.

This happens when they are chopped or processed, but it was not known why. Published in ACS's Journal of Agricultural and Food Chemistry, scientists have found it is down to previously unknown compounds they called allithiolanes.

Previous research has shown that onions release a compound called lachrymatory factor when cut that causes eyes to water. Several sulfur-based compounds are also formed when the onion is chopped, but none of these caused a bitter taste.

Roman Kubec and colleagues processed onions with a kitchen juicer and found fresh juice was not bitter, but it developed a strong bitter taste after 30 minutes. After performing sensory-guided high-performance liquid chromatography, the researchers discovered nine groups of new sulfur compounds — allithiolanes. These compounds form spontaneously when the onion is damaged.

“Allithiolanes possess an unpleasantly bitter taste with detection thresholds in the range of 15–30 ppm,” the study paper stated.

Further testing revealed they were also present in other popular vegetables such as leeks and garlic.

Image credit: ©stock.adobe.com/au/Olga Lyubkin