What exactly causes onions to make a person cry? The answer involves a combination of chemistry and enzymes. The trigger behind the tears is an organic acid compound, syn-propanethial-S-oxide.
The Cause: Sulfur Compounds
The primary sources of tears induced by cutting onions are sulfur compounds. Onions belong to the Allium family of vegetables, including onion, garlic, scallion, shallot, leek, and chives. They contain a variety of sulfur-rich compounds, naturally healthy for the body and skin.
Q. What's the difference between an onion and a freezer?
A. One make your eyes water ... the other makes your water ice.
When an onion is sliced, its cellular structure is disrupted, leading to the release of enzymes known as alliinases. In various proportions alliinases exist in all plants of the Allium family. Alliinases act on a compound, alliin or allicin, to convert it to syn-Propanthial S-oxide.
Syn-propanethial-S-oxide is part of the onion's natural defense mechanism. By releasing this when damaged, the onion deters predators in its environment. This defense mechanism serves an essential purpose in the onion's survival and propagation.
Tear-Inducing Process
When the insidious oxide meets the eye, it irritates the sensitive sensory nerves in the eyes. In response to this irritation, the tear glands produce tears as a defense mechanism. This process can be encapsulated:
Cell Damage: Cutting the onion damages its cells, releasing enzymes and sulfur compounds.
Formation of Gas: The enzyme alliinase converts alliin into propanthial S-oxide.
Formation of Eye Irritant: When gas contacts eye, it begins to form a thin watery solution. This is mild sulfuric acid.
Tear Production: In immediate defense the eye makes tears to flush out the irritant.
After-Effects: The eye is temporarily irritated but not permanently damaged.
The Role of pH Levels
While syn-Propanethial-S-oxide is the primary irritant, the pH level of the onion also plays a minor role in its tear-inducing properties. Onions are typically acidic, with pH levels ranging from 5 to 6. Anything lower than 7 is acidic.
This acidity can enhance the perception of irritation, magnifying the effect of sulfur compounds on the eyes. It's mainly the sulfur compounds responsible for the crying.
Can We Avoid the Tears?
Many cooks have developed various strategies to minimize crying while chopping onions. Here are some popular methods:
Chill the Onions: Refrigerating the onion before chopping can slow down the enzymatic reaction, leading to a lower release of irritants.
Use a Sharp Knife: A sharp knife will cut through the onion cleanly, reducing cellular damage and, consequently, the release of sulfenic acids.
Cut Under Water: Chopping onions under running water or submerged in a bowl can prevent the gas from escaping to reach the eyes.
Wear Goggles: Wearing protective goggles can completely shield eyes from the effects of the irritating gas.
Light a Candle: The flame neutralizes the gas traveling through the air by consuming its oxygen components. In this way it eliminates odors too.
Allicin is also responsible for benefits found in onions, garlic and other members of the Allium family. It can reduce heart disease, decrease blood pressure, prevent platelet aggregation, and fight inflammation.
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