There are as many home-grown tips for preventing tears while chopping onions as there are cures for hiccups. I thought I’d seen them all but today I came across an article from WonderHowTo that suggested a perfectly ridiculous way to keep onions from making your cry: Cut out the core.
First, the author of this article assumes that everybody discards the core of the onion anyway. Therefore, if cutting it out before slicing an onion prevents tears, why not? Well, I have heard of a few weirdos who insist on discarding the core but in reality, hardly anyone does this. Go ahead and watch a bunch of cooking shows. You’ll see plenty of onions being sliced and chopped. You won’t see anyone discarding the “core.”
Even if removing the core prevented onions from making your eyes water, you’d have to be a skilled surgeon to be able to excise this bit precisely enough to not end up with pre-watered eyes before you even begin cutting up the onion. From a practical sense, I think most of us would rather just brave a few minutes of stinging eyes than to go to all this trouble before even beginning to get to the chopping part.
And, if you do remove this middle part, it will simply make dicing the onion more difficult. You see, properly mincing an onion is not hard at all, and, if you have a very sharp chef knife and use the proper method for chopping your onion, you’ll get it done much faster and thus you will not expose your eyes to as much irritant.
But, the real reason I mentioned this silly advice is because of the premise. According to the article, the reason this works is that the core of the onion contains sulfur-based gas. When this is released into the air and makes contact with the water in our eyes, it forms sulfuric acid.
Wow! Sulfuric acid in your eyes. That’s terrible. It would be if it were true. The truth is that all the cells of the onion contain sulfur-based compounds call amino acid sulfoxides. When the cells of the onion are intact, these compounds will not irritate your eyes. In other words, until you slice into an onion or otherwise damage the cells, the tear-inducing effect is not actually present in the onion.
When you do slice through the cells of the onion, enzymes in the onion called alinases are released. These begin to break down the amino acid sulfoxides to form sulfenic acids.
Although there are different sulfenic acids that are produced, one in particular is problematic for our sensitive eyes. It is called 1-propenesulfenic acid. Another enzyme in the onion called lachrymatory factor synthase converts this acid into syn-propanethial-S-oxide. This is the stuff that causes you to cry. This compound can easily spread out through the air and make contact with your eyes, to which it is an irritant. Those tears are your eyes way of trying to remove the irritant. Hey, if onions just made you tear up, it wouldn’t be so bad. The problem is the burning and stinging.
Wearing glasses does help somewhat, but the irritant can still get around the glasses and into your eyes. When I always wore contact lenses, I found onions to be barely any problem. However, if you have to chop a whole lot of onions and you are wearing contact lenses, they can make matters worse. Although at first, they may provide some protection, once the compound eventually begins to make contact with the sensory neurons in your eyes, the contacts will just make it harder for your eyes to flush it out.
My uncle used to swear by holding a wooden match in your mouth while chopping onions, burny tip out. This does nothing but it makes you look cool.
There are lots of other tips, of course. A common claim is that the sharper the knife, the less crying you will do. The idea here is that a sharper knife ruptures fewer cells. It would also seem, then, that the less finely you chop your onions the less crying you’ll do. In reality, to cut an onion efficiently you’ll need a very sharp knife.
Freezing onions for around 15 minutes before chopping probably has the best chance of helping to prevent tears as the reactions between the enzymes and amino acids which lead to the irritant will happen more slowly at lower temperatures. Speaking of slowly, as I’ve already mentioned, the best way to prevent at least most of the irritating effect of chopping onions is to learn to chop them up efficiently. So read the article I linked above.