Compound Interest: onion

Infographic on the chemistry of onions. As onions are sliced, they release a class of enzymes, allinases, which break down amino acid sulfoxides. A specific compound produced during this process is 1-propenesulfenic acid. 1-propenesulfenic acid is rearranged by another enzyme, called lachrymatory factor synthase, to produce syn-propanethial-S-oxide. Production of this gas peaks 30 seconds after mechanical damage to the onion, and it stimulates sensory neurons in the eye causing a stinging sensation; the eye produces tears to flush it out. Thiosulfinates are the primary flavour and odour producing molecules in an onion. These compounds are not present in intact bulbs, but are formed via enzymatic reaction from sulfur amino acids. Allicin is one of these compounds, which in turn quickly breaks down to form other sulfur-containing compounds. — Click to enlarge

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.

Tag: onion

The eye-watering chemistry of an onion