What are the properties or chemicals that onions may have that make people cry when cutting them?
In: Biology
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As a defense mechanism against being eaten, onions release a gas that acts as an irritant when cell walls are damaged. That sulfuric gas reaches your eyes and the tear glands in your eyes activate to help try and clear the irritating gas from your eye. Peppers are another plant with a similar defense mechanism that humans just happen to enjoy eating. Birds don’t feel the spice though, which allows them to eat the peppers and spread the seeds, which is advantageous to the plant.
To give you a bit more information to answer the part of your questions about the chemicals involved, onions are part of a family called alliums with other plants such as garlic and leeks. When the cell walls are damaged these types of plants release alliinases, which are enzymes that break down to create that spicy flavor that raw garlic and onions have, and also make a sulfenic acid. That acid is what gets aerosolized, effectively becoming a weak organic tear gas (lacrimators). These are extremely volatile chemicals and break down rapidly, which is why fresh onion and garlic tastes so different to garlic prepared longer in advances or when cooked (heat accelerating the chemical reactions).
There’s lots of things people say to do to not cry when cutting onions, but there’s only a couple things that actually work.
* Keep your knife sharp. A dull knife damages more cells making more gas.
* Keep the onion together as much as possible when cutting. If you allow the onion pieces to fall and scatter across the cutting board they have more available surface area to release the gas.
* Wear goggles to protect your eyes or have a fan to blow away the fumes. Looks dorky but who cares in your own home?
Syn-propanethial-S-oxide. It is produced by the onion when certain enzymes come into contact with the air.
This chemical is quite volatile and breaks down to produce sulfuric acid when it comes into contact with water. Like the water in your eyes.
The amount of sulfuric acid released is not enough to be seriously dangerous, but it is enough to cause a pain reaction and to get your eyes to start producing more water to flush it out.
I use swimming goggles and then cut half a dozen onions at once. You won’t feel anything in your eyes.
Onions absorb sulphur from the environment which their cells use in various amino acids. When you cut an onion these sulphur rich amino acids combine with enzymes and form a volatile compound of sulphuric acid.
In short, you’re getting tiny droplets of sulphuric acid into your eyes, which stings. Then your eyes water which makes the sulphuric acid turn into hydrogen sulphate which also stings.
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