Chopping onions unleashes a “chemical defense that onion plants have to protect themselves against insects and microbes,” says UW-Madison horticulture professor Irwin Goldman. We’re just innocent bystanders, it seems.
Goldman explains that one compartment inside onion cells contains an enzyme, called allinase, while another compartment holds the enzyme’s substrate: a suite of sulfur compounds known as ACSO for short. Because sulfur is an essential nutrient, the onion stores sulfur from the soil as ACSO for later use.
When onion cells rupture – whether through an insect’s nibble or a knife’s cut – allinase and ACSO mix together and react, producing another set of sulfur compounds called thiosulfinates. In addition to giving onions their familiar taste and odor, thiosulfinates repel pests that attack onion bulbs underground.
They aren’t the chemicals that cause tears to well, however. Before the thiosulfinates are produced, the reaction of allinase and ACSO releases a volatile chemical that wafts into the air and reacts with the water in our eyes. “Basically it produces something like sulfuric acid in your eye,” says Goldman.
The mild acid irritates the eye’s nerve cells, stimulating tears that help wash it away. Chilling onions before chopping can reduce the crying, says Goldman, because “this is an enzyme-mediated process, and low temperatures slow enzymes down.”
Sweet onion varieties, like Vidalia, grown in the low-sulfur soils of Georgia and Texas also show less of this activity and are less pungent, he adds.