What Causes Irritation of Eyes and Tearing While Cutting Onions

Onion (Allium cepa) is one of the plants of Allium group. Garlic is another well-known vegetable from the same group which we use in our cooking. Onion contains vitamins, it has chemicals with anti-fungal and antibacterial properties and it also reduces the risk factors and diseases associated with modern life such as high blood pressure, high blood sugar and abnormal levels of cholesterol.

In spite of all the benefits, which are not known to many, onion is infamous for its pungent vapor which causes burning sensation in our eyes and tearing.

Here is how the onion vapor causes burning sensation in our eyes and lacrimation (tearing):

  • Onion contains chemicals known as amino acid sulfoxides apart from dozens of other organic compounds.
  • Onion also contains enzymes known as Allinases.
  • When the onion is intact, amino acid sulfoxides and allinases are separately stored in the onion cells.
  • When the onion is cut or crushed, its cells are disrupted and the enzymes allinases come in contact with amino acid sulfoxides and convert them to sulfenic acids.
  • Sulfenic acids are of many types. One of them 1-propenesulfenic acid, is converted to Lachrymatory factor (syn-propanethial-S-oxide) by another enzyme LFS (lachrymatory factor synthase).
  • Lachrymatory factor (syn-propanethial-S-oxide) is a gas which spreads quickly in the available space.
  • It is this Lachrymatory factor which when it comes into contact with the eyes, irritates the sensory nerves which we perceive as burning sensation.
  • In order to wash this offending chemical away from the eyes, there is a reflex increase in the activity of lacrimal (tear) glands (lacrimal, lachrymal and lacrymal are all correct spellings), causing profuse watering/tearing from the eyes.

Scientists in New Zealand have developed a type of onion which does not have the enzyme LFS responsible for synthesis of lachrymatory factor using bio-technology which inactivates the genes for making those enzymes. According to these scientists, it may take another 10 years for such ‘tear-free’ onions to be available in our local stores. Till then, you can try these methods to prevent tearing while cutting onions:

1. The irritant gas is water-soluble and it does not reach your eyes if you cut them under running water.
2. For the same reason you can cut the onions submerged in a container of water.
3. You can use a fan to blow the gas away or outside the window while cutting the onions.
4. Refrigerating the onions for about 30 minutes before cutting helps reduce the formation of the offending gas as the chemical conversion by the actions of enzymes is slowed down significantly.

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