Why does onion make us cry?

Why does onion make us cry?

Cutting an onion makes us cry because it releases specific chemical compounds that irritate our eyes, triggering a natural tear response.

When an onion is sliced, its cells are ruptured, causing various enzymes and sulfur-containing compounds to mix. This interaction leads to the rapid formation and release of volatile, airborne lachrymator compounds. These compounds are derived from precursors within the onion, including amino acids like methionine and cystine. Once airborne, these irritants come into contact with the sensitive nerves around the eyes, specifically stimulating the lacrimal glands. The irritation prompts the eyes to produce tears as a protective mechanism, flushing out the foreign substances and alleviating the discomfort.

The Science Behind the Tears

The process involves a fascinating biochemical reaction that unfolds rapidly:

1. Cell Damage: When you cut an onion, you physically break open its cells.
2. Enzyme Release: This damage causes the release of enzymes, such as alliinase, which are usually separated from their substrates within the intact onion cell.
3. Chemical Reaction: The enzymes react with sulfur-containing compounds (like the amino acid sulfoxides, which are formed from precursors such as methionine and cystine). This reaction initially forms sulfenic acids.
4. Lachrymatory Factor Synthase: Another enzyme, lachrymatory factor synthase, rapidly converts these sulfenic acids into a highly volatile gas known as propanethial S-oxide. This is the primary lachrymator compound responsible for the irritation.
5. Eye Irritation: Propanethial S-oxide drifts into the air and, upon contact with the moisture in your eyes, transforms into a mild sulfuric acid. This acid irritates the delicate nerve endings in your eyes, particularly within the lacrimal glands.
6. Tear Production: Your brain registers this irritation and sends signals to the lacrimal glands to produce tears. These tears act as a natural defense mechanism to dilute and wash away the irritant, protecting your eyes from harm.

Tips to Reduce Onion-Induced Tears

Fortunately, there are several methods you can employ to minimize the eye-watering effects of cutting onions:

• Chill the Onion: Place the onion in the refrigerator for 30 minutes or the freezer for 15 minutes before cutting. Cold temperatures slow down the enzymes' activity, reducing the release of the irritant gases.
• Use a Sharp Knife: A very sharp knife causes less cell damage, thereby releasing fewer irritating compounds into the air compared to a dull knife that crushes cells.
• Cut Under Ventilation: Slice onions near an open window, under a powerful range hood, or near a fan to direct the gases away from your face.
• Submerge in Water: Cut the onion under running water or in a large bowl of water. The water absorbs the sulfuric compounds before they can become airborne and reach your eyes.
• Wear Protective Eyewear: Ski goggles, swimming goggles, or specialized onion goggles create a physical barrier, preventing the gases from reaching your sensitive eyes.
• Light a Candle: Some believe that lighting a candle nearby can help burn off the sulfur compounds, though scientific evidence for this method is limited.
• Cut Near Steam: The steam from a pot of boiling water can help absorb the irritant gases, similar to cutting under running water.

By understanding the chemical reaction at play and employing these simple techniques, you can chop onions with far fewer tears, making your cooking experience more enjoyable.