Cutting an onion and finding your eyes burning and watering is a very common phenomenon. It's not simply due to the vegetable's intense odor, but rather to a chemical reaction that occurs when its tissues are damaged.
Onions actually have a full-fledged defense system. When cut, the cells break, and substances that normally remain separate come into contact. This reaction produces volatile compounds that can quickly reach the eyes and cause irritation.
Tearing, therefore, is nothing more than a protective response of our organism.
What happens when we cut an onion
Inside the onion cells are sulfur-containing compounds and specific enzymes. As long as the vegetable remains intact, these substances are kept separate in different compartments.
When the knife blade ruptures the cells, enzymes come into contact with the sulfur compounds and initiate a series of chemical transformations. Among the products formed is a volatile substance called lacrimal factor.
This molecule disperses easily in the air and reaches the eyes within moments.
The more cells are damaged, the greater the amount of substance released. This is why a very finely chopped onion often tends to provoke more tears than one cut into large pieces.
Why do eyes start to water?
The surface of the eye is coated with a thin tear film. When the volatile compound produced by the onion reaches this area, it causes irritation.
Nerve endings on the ocular surface quickly detect the stimulus and send a signal to the lacrimal glands, which increase tear production to dilute and remove the irritant.
The burning sensation and tearing are therefore a normal defense reaction.
It's incorrect to imagine that a significant amount of a truly aggressive acid forms in the eye, as is sometimes reported. The phenomenon is more complex and depends primarily on the irritating effect of the tear factor on the ocular surface.
Why onions use sulfur compounds
Sulfur-containing compounds contribute to the onion's characteristic aroma and are part of its natural defense mechanisms.
When tissue is damaged, the substances produced can deter insects and other organisms that might feed on the plant. It's a chemical system that activates precisely following a cut, bite, or injury.
The same group of compounds also contributes to many of the aromas that develop during cooking. Heat alters the substances present in the onion, transforming the pungent flavor of the raw product into progressively sweeter and softer notes.
A sharp knife really makes a difference
Using a sharp knife is one of the simplest ways to reduce tears.
A sharp blade cuts through the tissue cleanly and crushes fewer cells. A dull knife, on the other hand, requires more pressure and tends to compress the pulp, releasing more cellular fluid.
This does not completely eliminate the problem, but it can limit it.
A good knife also offers an important practical advantage: it allows you to work more quickly and with greater precision.
Cool the onion before cutting
Placing the onion in the refrigerator for about 20-30 minutes before cutting it can help reduce the discomfort.
Low temperatures slow chemical reactions and reduce the volatility of some substances. Consequently, fewer irritants can reach the eyes quickly.
It's not necessary to freeze onions. Excessive freezing could alter their texture and make them less pleasant to work with.
Whole onions also shouldn't be regularly stored in the refrigerator for this reason alone. Under normal conditions, they prefer a cool, dry, dark, and well-ventilated place.
Ventilation helps
Cutting onions near a running extractor hood can significantly reduce the amount of volatile substances reaching your face.
Even an open window or a gentle breeze can be helpful. The goal is to remove the fumes from the cutting board before they rise into your eyes.
A small fan can be positioned at the side, as long as the airflow is directed away from the face. A direct stream of air directly into the eyes could increase dryness and discomfort.
Should the root be left intact?
A very common tip suggests leaving the root section until the end of the cut. This method can be helpful, but not because all the irritant is concentrated exclusively in that spot.
Leaving the root attached keeps the layers together and allows for more orderly handling. It also avoids having to immediately cut off an additional part of the bulb.
You can then remove the top, cut the onion in half, peel it, and proceed with cutting, keeping the root as a support. This will only be removed at the end.
Do sweet onions make you cry less?
In many cases, yes, but this isn't an absolute rule. Varieties considered sweet often tend to have a less pungent flavor and a different balance of sulfur compounds.
This may result in reduced production of the tear factor, but the outcome varies depending on the cultivar, soil, growing conditions, and storage.
It is therefore not enough to look at the color of the onion to predict how much it will make you cry.
White, golden, red, or spring onions can show very different intensities even within the same category.
Do freshness and storage influence the effect?
The onion's condition can also affect the reaction. Storage changes the moisture content, consistency, and tissue composition.
A very dry or old onion may behave differently than a freshly picked one, but there is no simple relationship according to which a fresher onion always makes you cry more or less.
Growing conditions also play an important role. The availability of sulfur in the soil, for example, can influence the quantity and type of aromatic compounds present in the bulb.
Traditional remedies: which ones really work?
Many popular tricks promise to completely eliminate tears: holding a spoon in your mouth, chewing bread, breathing only through your mouth, or lighting a candle.
These systems do not offer reliable results.
More useful, however, are measures that directly affect the production or dispersion of irritants: cool the onion, use a sharp blade, and work in a well-ventilated area.
Tightly fitting safety glasses are the most effective method when chopping large quantities of onions, because they physically prevent fumes from reaching your eyes.
What to do if the burning sensation is intense
When your eyes start to burn badly, it is best to stop cutting for a moment and move away from the work area.
Don't rub your eyes, especially with hands that have just touched the onion. Your fingers should be thoroughly washed with soap and water.
The discomfort usually subsides quickly. If necessary, the eyes can be gently rinsed with clean water.
Severe or persistent pain, especially if onion juice has directly entered the eyes, requires greater attention.
In conclusion
Onions make you cry because cutting them breaks down their cells and triggers a chemical reaction that produces a volatile irritant. When this substance reaches your eyes, your body responds by increasing tear production to dilute and keep it away.
Cooling the onion, using a very sharp knife and working near an extractor hood or an open window are the simplest measures to limit the phenomenon.
The tears caused by onions, therefore, are not only due to their smell: they are the result of the encounter between the plant's natural defense system and the protective system of our eyes.
