The Chemistry of an Onion

The latest in the series of food chemistry graphics looks at the chemistry of onions – specifically, what causes their odour, and why chopping onions will make your eyes water. Interestingly, none of the compounds that cause these effects are present in the intact onion; rather, when the cell walls of the onion are damaged by chopping, an enzyme released produces a range of compounds as a defence mechanism, which act as irritants.

There are a wide range of compounds produced by these reactions, many of which are unstable and only exist fleetingly before breaking down further. If the onion is ingested, these compounds are eventually broken down into allyl methyl sulfide, shown below, which can be removed from the body by exhalation – giving rise to the characteristic ‘onion breath’. Garlic and onion are of course members of the same family (the allium family), so it’s no surprise the same compound is responsible for ‘garlic breath’.

You can download a free pdf of this graphic here. The other graphics in the food chemistry series can be found on the site’s infographics page. Meanwhile, if you want to take a more detailed look at the chemistry of onions, follow the links provided below.

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

A modified version of this graphic will appear in ‘The Curious Chemistry of Food & Drink’, available to pre-order now!

References & Further Reading

• Onion, Garlic & Chives – Picture It Chemistry
• Why do onions make you cry? The chemistry & health benefits of onions – Something About Science
• Organosulfur natural products in chemistry – Musah Research Laboratory
• Allium thiosulfinates: chemistry and biological properties – Noureddine Benkeblia & Virginia Lanzotti