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Why does cutting onions make our eyes tear?

Q. Why does cutting onions make our eyes tear?

A. When an onion is cut, certain (lachrymator) compounds are released causing the nerves around the eyes (lacrimal glands) to become irritated.
These compounds consist of methionine and cystine that are part of the amino acid family. (Protein is made up of several of these amino acids grouped together.)
When an onion is sliced or diced, the onion's cells release these compounds into the air. When this occurs, "enzyme" works to alter the amino acids into lachrymator compounds. This form of sulfuric acid irritates the nerves around the eyes making them tear.

How can we prevent this reaction?

  1. Use a sharp knife to cut the onion to reduce the amount of damage to the onion cells.
  2. Cool the onion in a refrigerator to suppress the scattering of the lachrymator.
  3. Soak the onion in water to dissolve the amino acids. (Cutting the onions in half or quarters before soaking them is even more effective.)
  4. Wear swimming goggles or ski goggles while cutting the onion.
  5. Use a ventilator or fan to blow the allyl sulfide away.

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What is lachrymator?

Irritation to the tear glands causes it to secrete tears continuously without being influenced by the intent and emotion to cry. The chemical compounds released by the onions are allyl sulfide and they also exist in garlic and spring onions.

What is amino acid?

An organic compound containing an amino group (-NH2), a carboxylic acid group (-COOH), with over 80 natural side groups, of which over 20 compounds being able to form proteins.

What is enzyme?

A high polymer produced by living organisms and functioning as biochemical catalysts. There are several types of enzymes with each of them displaying a definite chemical reaction.

What is a catalyst?

A substance that alters a chemical reaction without being affected itself.