Why do onions make us cry?

To stop greedy predators from eating the bulbs, onions release volatile chemicals that cause our eyes to well up.

In the first year of their life cycle, they form a tasty bulb that serves as an energy store.

Onion scientific name 

Allium cepa

Eye irritationEdit

The chemical at the heart of our discomfort is called propanethial S-oxide, which is also known as lachrymatory factor (LF). The technical term for our tear glands is "lacrimal glands," and LF is a chemical that causes tears.

Only three other molecules with similar tear-inducing properties have been found to date, and they are all produced by plants.

Freshly cut onions often cause a stinging sensation in the eyes of people nearby, and often uncontrollable tears. This is caused by the release of a volatile gas, syn-propanethial-S-oxide, which stimulates nerves in the eye creating a stinging sensation. This gas is produced by a chain of reactions which serve as a defence mechanism: chopping an onion causes damage to cells which releases enzymes called alliinases. These break down amino acid sulfoxides and generate sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, the lacrimatory factor synthase, producing the syn-propanethial-S-oxide. This gas diffuses through the air and soon reaches the eyes, where it activates sensory neurons. Lacrimal glands produce tears to dilute and flush out the irritant.

Eye irritation can be avoided by cutting onions under running water or submerged in a basin of water. Leaving the root end intact also reduces irritation as the onion base has a higher concentration of sulphur compounds than the rest of the bulb. Refrigerating the onions before use reduces the enzyme reaction rate and using a fan can blow the gas away from the eyes. The more often one chops onions, the less one experiences eye irritation.

The amount of sulfenic acids and lacrimal factor released and the irritation effect differs among Allium species. In 2008, the New Zealand Institute for Crop and Food Researchcreated "no tears" onions by using gene-silencing biotechnology to prevent the synthesis of lachrymatory factor synthase in onions.

When we start chopping, the cells inside the onion are broken up. As a consequence, an enzyme called allinase is released, which produces the chemicals that are subsequently broken down into flavor molecules. These give onions their characteristic taste.

Some of the chemicals involved in this reaction are turned into LF by LF synthase. When LF comes into contact with the front of the eye, or the cornea, nerve endings located here signal to the brain that an irritant has arrived on the scene. This, in turn, leads to signaling back from the brain to the tear glands.

Tears and blinking are subsequently initiated to remove the pesky irritant from the cornea.

syn-Propanethial S-oxide (C₃H₆OS), a member of a class of organosulfurcompounds known as thiocarbonyl S-oxides (formerly "sulfines"), is a liquid that acts as a lachrymatory agent (triggers tearing and stinging on contact with the eyes). The chemical is released from onions, Allium cepa, as they are sliced. The release is due to the breaking open of the onion cells and their releasing enzymes called alliinases, which then break down amino acid sulfoxides, generating sulfenic acids. 

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, which is rearranged by another enzyme, called lachrymatory factor synthase, to produce syn- propanethial-S-oxide.

Composition

Most onion cultivars are about 89% water, 9% carbohydrates (including 4% sugar and 2% dietary fibre), 1% protein, and negligible fat(table). Onions contain low amounts of essential nutrients and have an energy value of 166 kJ (40 Calories) in a 100 g (3.5 oz) amount. Onions contribute savoury flavour to dishes without contributing significant caloric content.