Every one might have experienced tears when onions were cut. The chemistry inside onions is the reason for the tears. When they grow, they absorb sulfur from the earth. Sulfur absorption helps form amino acid sulfoxides. When an onion is cut, its cells are broken and they release their contents. Amino acid sulfoxides form sulfenic acids. This volatile sulfur compound reacts to form sulphuric acid. The sulfuric acid burns, stimulating the eyes to release more tears to wash the irritant away.
Cooking onion does not affect our eyes. This is because cooking inactivates the enzymes which cause tears. Refrigerating cut onion slows reactions and changes the chemistry inside the onion which helps avoiding tears while cutting. The sulfur-containing compounds also leave a characteristic odor on the fingers.
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