Onions have long plagued cooks, prying open millions of tear ducts during their illustrative career. But why do onions make us cry? And is there any way to stop it?
The world eats a lot of onions. Over 100 billion pounds a year, in fact. But each time these delicious little numbers are sliced, there’s a price to pay. It’s a worthy price, in our opinion, but the tears that come with cutting an onion must have dumfounded people for centuries.
So why do onions make you cry?
Thankfully, nowadays it can now be easily explained through a rather simple chemical reaction.
As part of the Allium genus, alongside the likes of garlic and shallots, onions are grown as bulbs in the ground and absorb a lot of sulfur. The sulfur is used to produce amino acid sulfoxides which in turn produce sulfenic acids inside the onions cells. When you cut open the cells, the acids are released and mix with previously separated alliinase enzymes, forming an irritant known as “syn-propanethial-S-oxide” (SPSO). The process is called “lachrymatory synthase”.
Believed to have developed as a defense mechanism to protect onions in the ground, the irritant now causes most harm when rising off kitchen chopping boards. Our eyes, as a defense mechanism of their own, react to the sulfuric acid and produce tears designed to rinse the irritant away.
Are all onions the same?
The amount an onion makes you cry depends on how sulfurous the onion is. Sweet onions, for example, contain less sulfur and therefore produce less SPSO when their cells are broken. Sadly, the high-sulfur onions are the classic options: white, yellow and red.
Scientists have, however, begun developing tear-less onions. Using RNA interference, they were able to reduce the lachrymatory synthase activity “1,554 fold”. The report, conducted by a group of scientists in 2008, “is the first demonstration, to our knowledge, of gene silencing in an Allium species”. Despite the positive results, the doctored onions are not yet widely available.
What remedies work?
People will have their own techniques that they swear by. From putting a spoon in your mouth, to wearing goggles, to chilling the onions before cutting, each wife’s tail is corroborated by varying degrees of scientific support.
Chilling the onions beforehand does seem to have some sort of evidence behind it. If you freeze the alliinase enzymes, it stops them the mixing with sulfuric acid, diminishing, in turn, the production of SPSO. Soaking them in water, meanwhile, is said to absorb some of the gases.
Goggles do also prevent the irritant from getting in your eye. You’ll look a bit weird, and hampering your vision while slicing food doesn’t seem a great idea, but at least if you lose the tip of a finger, you won’t be crying already.
