Uncovering the Chemistry Behind Why Onions Make You Cry

[Zero-G]

Everyone knows onions make you cry , but do they actually do any damage to your eyes? How can one stop their eyes from getting sore when cutting up these cruel vegetables?

 

[honestrosewater]

Maybe someone else can explain why - I find that cold air (like from the freezer) helps to relieve the stinging and watering.

 

[hypatia]

how stuff works


From How Stuff Works, read more at the above link...
When you slice through an onion, you break open a number of onion cells. Some of these cells have enzymes inside of them, and when they are sliced open, the enzymes escape. The enzymes then decompose some of the other substances that have escaped from sliced cells. Some of these substances, amino acid sulfoxides, form sulfenic acids, which then quickly rearrange themselves into a volatile gas.

 

[Moonbear]

There's really no avoiding it. The best thing you can do to reduce the amount of volatile compounds released is to use a very sharp knife so you get faster, cleaner slices that cut through the cells but don't crush them, squirting up the contents, as you cut. Beyond that, some swear by chilling the onions first in the fridge. I also find just working near the kitchen sink and frequently washing everything with cold water helps send some of the volatiles down the drain rather than giving them a chance to get airborne.

I've never had the pleasure of having a kitchen with a downdraft vent on the counter, but that might help if it's sucking the volatiles down and away from you instead of typical kitchen vents that suck the air up, which would be toward your face.

 

[Monique]

There is some type of onion that does not release much of the annoying volatiles, try a few varieties and see which one works for you. Onions personally never bother me, cut them up in a certain way so that it does not fall apart.

My person technique is the following:
Take off excess roots and the outer skin of the onion, slice onion in half straight through the bottom (do not take its bottom off, otherwise you're in more trouble than you need to be), lay both sides down on the side you cut off (so gasses can't escape), then make 3-4 incissions from the bottom to the head (don't go through the bottom, you want the onion to stay together), more incissions for finer onion dices, now you just slice the whole onion in the perpendicular direction, starting at the head and working down to the bottom (note: there is no need to slice in any other direction, since you've made use of the layered texture of the onion).

Now, if you did it correctly the onion half will still be intact lying on the board, after you have sliced all your onions you take them up and put them in the pan.

 

[Ouabache]

Some great answers have been posted.. I'd like to cover a few things that may not have been mentioned yet..

I seem to recall from organic chem, that onions contain allyl sulphide which ultimately converts to syn-ropanethial-S-oxide, a volatile gas (by enzymatic reaction as hypatia pointed out). So the genus of this plant is quite appropriate Allium (which includes other aromatics e.g. garlic, scallions, shallots, and leeks).

When this gas dissolves in saline tears that normally bathes your eyes, it forms a mild sulphuric acid, which as you can imagine, is irritating and painful. (the ref hypatia gave, also notes this).. It is making your tears acidic which is not healthy to your eyes. Your body's reaction is to flush out this acid by crying. (you can accelerate this process by flushing your eyes with water). I remember reading (don't recall the publication) tears created from exposure to onions, and those shed with normal emotion, were compared. Those created by emotional release, contained more compounds (body's toxins?) than those generated from onions. The implication is that emotional release of tears is beneficial to (emotional and physical) health, giving all the more reason not to hold them back.

You probably remember from grade school, that molecules will change from liquid to gaseous phase to a greater degree, with an increase in temperature. Similarly, a decrease in temperature will also decrease this phase change. It makes sense then, as you lower the temperature of an onion, less gas will be evolved. Cold air from your refrigerator or freezer condenses the volatile gas back to liquid phase and less reaches your eyes. I think that is why your eyes may feel better when you stick your head in the freezer after chopping onions. There may also be a biological explanation, perhaps the cool air mildly anesthetizes your eyes (speculation: slowing down physiological reactions such as enzyme activity and pain signals to your brain)

For particularly irritating onions, I refrigerate them, prior to use and that helps immensely. Also, keep you head away, rather than directly over your cutting area. Running cold water also condenses the volatile gases by cooling the microenvironment.