To grate onions without tears, chill the onion for about 30 minutes before grating, or grate it under running water to prevent the irritating compounds from reaching your eyes.
Onions release a chemical called propanethial S-oxide when cut or grated. This chemical irritates the eyes, causing them to water as a defense mechanism. Several methods can minimize this effect:
Chilling the Onion: Refrigerating the onion for at least 30 minutes before grating slows down the release of the irritating compounds. The cold temperature reduces the volatility of the enzymes responsible for the tear-inducing reaction.
Grating Under Running Water: Grating the onion under a gentle stream of cold water helps to wash away the propanethial S-oxide before it reaches your eyes. Ensure the water doesn't splash excessively.
Using a Food Processor: A food processor with a grating attachment can quickly grate the onion with minimal exposure to the irritating fumes. The enclosed environment helps contain the chemicals.
Cutting Near a Fan or Open Window: Increasing ventilation helps to disperse the irritating compounds away from your face.
Wearing Goggles: If all else fails, wearing kitchen goggles or even swimming goggles provides a physical barrier to protect your eyes.
Avoid cutting the root end of the onion until the very end. The root end contains a higher concentration of the tear-inducing compounds. Cutting it last minimizes their release during the grating process.