Slicing through an onion damages cells, causing enzymes and other substances that are normally kept apart to spill out and react together. In standard onions the result is a sulphur-containing chemical called syn-propanethial-S-oxide, which resembles tear gas. This forms an irritating acid when it comes into contact with water in your eyes. Some research groups have created onions that are genetically modified to lack an enzyme that leads to syn-propanethial-S-oxide, but these have not yet made it to market.
The tearless onions – Sunions – now in shops were created by repeatedly cross-breeding milder varieties containing lower levels of pyruvate. This substance is a by-product of the same reaction that forms syn-propanethial-S-oxide and also has a good measure of pungency. ED
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