Why do we cry anytime we eat onions?

You know the familiar feeling: tingly, stingy eyes and tears followed. Why is this humble onion, a key ingredient in your next culinary development, going to cause you such discomfort?

Cutting Onions
Onions act in self-defense, and tears are the consequence.

Existence is all about survival and reproduction for plants and animals, alike. Onions are no exception. In order to avoid greedy rodents from consuming the bulbs, onions release volatile chemicals that make our eyes well up.

Onions are perennial plants and are part of the family Allium. This means that they form a tasty bulb in the first year of their life cycle which serves as an energy store. The plant grows a flower and seeds the next year, so that it may reproduce.

To protect the onion from attack, sophisticated chemistry is required and scientists have recently established how the enzyme responsible for this process works.

Tear factor

At the root of our discomfort is the chemical called propanethial S-oxide, also known as lachrymatory factor (LF). The scientific name for our tear-glands is “lacrimal glands,” and LF is a tear-causing chemical.

To date, only three other molecules with similar tear-inducing properties have been found, and all are plant-produced.

Marcin Golczak, Ph.D. – an assistant professor at the Department of Pharmacology at Case Western Reserve University in Cleveland, OH – and colleagues have recently discovered the enzyme structure that produces LF in onions: lachrymatory synthase factor (LF synthase).

The cells within the onion are torn off as we start chopping. As a result, an enzyme called allinase is released which produces the chemicals which are subsequently broken down into molecules of flavor. These give their characteristic taste to the onions.

Some of the chemicals involved in this reaction are converted by LF synthase into LF. As LF comes into contact with the front of the eye, or the cornea, the nerve endings here suggest to the brain that there is an irritant on the scene. That, in effect, contributes to the tear glands being signaled back from the brain.

Thereafter, crying and blinking are performed to eliminate the irritating corneal irritant.

The elusive enzyme

Knowing the enzyme structure and function is an complicated science. While LF synthase had been described as early as 2002, no one has been able to demonstrate how it actually functions until now.

Prof. Golczak and his colleagues converted the enzyme solutions into microscopic crystals which they could then imagine using X-rays. This helped them to determine the enzyme’s 3-D structure and to identify the small pocket in which the LF chemical conversion occurs.

Detailed knowledge of this chemical process fills a fundamental knowledge gap and provides scientists with a greater understanding of the onions’ biochemical potential.

Harnessing health benefits

Onions provide a variety of benefits for the body. Studies indicate a correlation between the chemicals in onions and cancer growth inhibition, as well as sleep and mood improvement.

Since they are also a very good source of dietary vitamin C, a building block of collagen, onions may well play a role in the health of the hair and skin.

Two groups of scientists have already been working on producing tear-free onions to save us from the pain of tears. One of these recent onions, associated with cardiovascular disease, has been shown to prevent platelet aggregation.

The next time you get your chopping board ready to cook up a meal full of nutritious onions, think of the amazing biochemical processes that will unfold before your teary eyes.

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