Why Do Onions Make You Cry?

Sep 2, 2011 9:01 am

The Food Scientist tackles a persistent question

chopping an onion
Photo: TheDarkThing on Flickr
Try chilling that onion before you slice.
 

What are onion tears?
The unwanted tears that come as a result of chopping onions, whether yellow, white or red.

What causes onion tears?
When it comes to shedding tears, there are three different types. The first, known as basal tears, are the involuntary kind that provide lubrication needed for efficient eye function. The second, and perhaps the most recognizable type, are psychic tears. Fans of movies like Old Yeller or The Notebook are surely familiar with them. Unlike with basal tears, these are released when a stimulus invokes an emotional reaction like sadness. While the resulting waterworks can sometimes occur uncontrollably, it’s by no means involuntary.

The third and most important type when talking about onions, are known as reflex tears. As the name implies, these occur as a reflex to some external stimulus acting on the eye. Forms of stimuli range from being cut by a sharp object to inflammation in the presence of natural irritants like dust or pepper. With onions, it’s the naturally occurring sulfur compounds known as sulfides that are responsible for causing the characteristic reaction. The compound to blame? syn-propanethial S-oxide. Sulfides are converted to syn-propanethial S-oxide enzymes called allinases, which are released when the cells of the onion are damaged by the blade of your knife. This potent lachrymatory agent is then released as a gas and causes the stinging sensation many of you know all too well.

The tears that follow are released not because the pain is too much to handle, but because your body is attempting to dilute or wash away this onion-derived irritant. Keep in mind that not all onions are created equal. The sweet vidalia onion, for instance, contains low levels of sulfide and is known for its subtle, and some say sweet, onion taste. As expected, this translates to a weaker lachrymatory reaction.

How can you prevent onion tears?
Even the strongest chefs are not immune to the powerful effects of the onion. Besides keeping a box of tissues on-hand, try chilling your onions before chopping. The theory behind this method is that the colder the onion, the less sulfide gas that will evaporate when its cut. If your dignity is of no concern, you could also try a pair of onion goggles, but I recommend saving your money for more important kitchen gadgets.


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