We all know that onions can cause us to tear up when we cut them. But why does this happen?

As always, we turn to our best friend when we need a logical explanation - Science!

Within each variety of vegetables belonging to the Allium family resides a class of organic molecules called amino acid sulfoxides. These molecules help give the onions their specific bitter flavors.

Within the tissues of these same onions are enzymes called allinases. When these enzymes are released, be it through slicing, crushing, piercing, whatever, they react with the amino acid sulfoxides, converting them to sulfenic acids (RSOH). The sulfenic acids are very unstable, and often will often re-arrange their molecular structure to form syn-propanethial-S-oxide(H₇O₃S₂). It is this chemical that causes tearing. From the Scientific American:

Its effects on the eye are all too familiar. The front surface of the eye--the cornea--serves several purposes, among them protection against physical and chemical irritants. The cornea is densely populated with sensory fibers of the ciliary nerve, a branch of the massive trigeminal nerve that brings touch, temperature and pain sensations from the face and front of the head. The cornea also receives a smaller number of autonomic motor fibers that activate the lachrymal (tear) glands. Free nerve endings detect syn-propanethial-S-oxide on the cornea and drive activity in the ciliary nerve--which the central nervous system interprets as a burning sensation--in proportion to the compound's concentration. This nerve activity reflexively activates the autonomic fibers, which then carry a signal back to the eye ordering the lachrymal glands to wash the irritant away.

There are several ways to prevent or mitigate the causes of tearing. One, you could have brain surgery that would block any sensory information sent from the ciliary nerve. However, that may be a tad impractical.

Water is the best route. Cutting onions under water, or soaking the onions prior to slicing will work. Choosing onions with higher water content can also lessen the tearing. This means purchasing onions that haven't been dry cured. Any "named" onions, such as "Vidalia" or "Walla Walla", will do. Dry cured onions inlcude the generic white, yellow, and red onions.

If the only options available to you are the generic whites, yellows, and reds, it would be the white onions which should have higher water content, and red onions having the least amount.

Another way to prevent or lessen tearing is to preventing an excessive amount of allinases from being released. The best way to do that is to cut the onion in such a way that minimally damages the tissue. In other words - the sharper the knife the better.

Finally, cut the onions in a well ventilated area. The vapors released from the onion can be dispersed quickly with a fan in the area.

So all of you out there cutting red onions with a butter knife in a windowless room, cut it out. You're only asking for trouble.