Slice into an onion and, within a few seconds, your eyes begin to feel the burn. How does such a little veggie pack such a big punch?
There are three types of tears: emotional (crying or weeping), basal tears and reflexive tears.
Humans cry emotional tears as a response to emotional stress, suffering, mourning and physical pain. Unless you are having an extremely bad day, the tears you experience when you cut into an onion have nothing to do with emotion.
Basal tears are a protective film that typically covers the eye at all times. These tears lubricate the eye and eyelid.
If you have ever experienced sore eyes after crying, you can blame basal tears. Emotional tears tend to be more “watery" and can temporarily dilute or wash away the basal tear film when we cry. The result is often dry, painful eyes.
Reflexive tears result when foreign particles or irritating substances irritate our eyes. Common culprits include smoke, wind, dust and, yes, onion vapors.
So what is it about the onion that causes such a tear-jerking reaction? When an onion goes under the knife, cells are ruptured, which causes a chemical reaction.
The resulting gas is highly irritating to most eyes. When the eye processes irritation, neurons send up all kinds of flares to the brain asking for help flushing out the invaders. Reflexive tears to the rescue!
You can reduce or prevent eye irritation when dicing onions in several different ways. Chopping an onion under running water or submerged in a tub of water will prevent the gas from spreading and reaching the eyes.
Sticking an onion in the fridge or freezer prior to slicing will limit the enzyme's ability to activate, reducing the amount of irritant gas released.
Lastly, start chopping from the top! The onion's root has the highest concentration of enzymes. Cutting from top to bottom will lessen your exposure to the strongest enzyme release.