Onions grow underground, where many other critters live and search for food. When an onion is sliced or broken open, sulfoxides and onion enzymes are emitted in the form of sulfenic acid, according to ophthalmology experts at Texas A&M College of Medicine. As that acid and the onion enzymes interact, they create syn-propanethial-S-oxide gas. That gas is the molecule that wafts up to your eyes, reacts with the water naturally covering your eyes, and generates sulphuric acid and other irritating substances that can induce tears. (The same happens to animals that try to eat onions underground as they’re growing.) Our eyes are very sensitive since they’re involved with such an important task: vision.
A splash of citrus juice or vinegar alters the pH enough to hamper the enzyme’s ability to function as usual.
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