Why do onions make you cry?
To answer this question, one needs to understand what the microscopic structure of an onion looks like. Onions are composed of millions of tiny cellular sacs (similar to the individual sacs of an orange or grapefruit, only much smaller). When an onion is cut, millions of onion cells are sliced apart, and these sacs burst.
When these cellular sacs break open, they release enzymes (e.g., Alliinase) that react with amino acid sulfoxides (e.g., “Prensco”) to form sulfenic (sulfur-based) acids. These acids quickly vaporize into propanethial S-oxide, more commonly known as “lachrymatory factor,” a volatile and noxious sulfurous gas. Lachrymatory factor is unstable, and when it reaches our eyes, it reacts with the water in them to make sulfuric acid. In response to the presence of sulfuric acid, your eyes will water (you “cry”) as your body tries to dilute out the acid.
Ironically, sulfenic acids (e.g., 1-propenylsulphenic acid) are also responsible for much of an onion’s flavor, taste, and pleasant aroma. The more you chop an onion the more flavor you will get through the breakage of cellular sacs, but the lachrymatory factor released will also make your eyes water.
How can you chop an onion without crying?
Every chef wants to maximize the taste he extracts from his onions, but nobody likes tearing up while cutting onions. Fortunately, there are some simple rules you can follow to deal with the irritating effects of onions:
1. Keep your head as far away from the onion as you can. This will let the sulfuric fumes disperse before they reach your eyes.
2. Use a sharp knife when cutting onions. Using a sharp knife to slice onions reduces the cell-breaking that occurs when a blunt knife is used to cut onions and minimizes noxious gas release.
3. Chill or cook the onions before cutting them. Cooking an onion or peeling and chilling it in the fridge before slicing it changes the composition of the onion in a way such that fewer sulfuric compounds will be released into the air when they’re cut.
4. Cut the onion next to a strong draft from a fume hood, fan, or window. This will help pull the fumes away from your eyes.
5. Cut onions in a pot of water, under running tap, or near a steaming kettle. The water will absorb and neutralize sulfuric gases that onions release before they get to your eyes.
6. Soak the onions in water before cutting them. The enzymes that create the sulfur-based acids are denatured (destroyed) by the water-air boundary, and if you soak in salt water, the ions in the salt water denature (permanently inactivate) the enzymes.
7. Put vinegar on the chopping board. Vinegar also denatures the onion enzyme that creates sulfur-based acids.
8. Breath with your mouth instead of your nose. Breathing with your mouth instead of your nose helps to prevent the sulfuric fumes from reaching your eyes. As you inhale, you suck the gas in (the gas is drawn towards your tongue and throat) and as you exhale, you blow it away. This keeps a lot of the gas from ever reaching your eyes.
9. Light a candle. A lit candle set near the cutting board will draw the sulfuric gases released by the onion away from your eyes and into its flame.
10. Cut “away” from the body/eyes. This will keep onion juice, filled with concentrated sulfuric acid-producing enzymes, from spraying in your eyes.
11. Have some food in your mouth. Many people have reported that chewing bread, sugar, or gum helps avoid tears when cutting onions.
12. Cut very carefully and avoid the root of the onion. Avoid cutting the root of the onion unless you really need to since that is the place where the highest concentrations of acid-making enzymes are located.
13. Wear goggles or contact lenses. Goggles and contact lenses create a barrier between the surface of your eyes and the sulfurous gases released.
Keep these tips in mind the next time you cut an onion and you should have no problem staying tear-free. Bon Apetit!
