Q: Why does cutting onions make you cry?
A: When you cut into or squish an onion, but not when you simply peel them, you release certain liquids which are usually isolated from one another. Some of the chemicals in these fluids combine to form the hemical syn-propanethial-S-oxide; the S stands for sulfur, and onions are especially good at absorbing sulfur. Syn-propanethial-S-oxide is volatile, so some of it turns into a vapor which is irritating, especially to the eyes. When someone’s eyes are irritated the lacrimal glands are stimulated to produce tears to rinse the irritant away. This is why cutting onions make us cry.
If this sounds similar to tear gas, that’s because it is! The most common type of tear gas contains 2- chlorobenzaldene malononitrile, which is an irritant to the mucous membranes of the nose, mouth and lungs causing respiratory symptoms and/or a runny nose, as well as to the eyes causing tearing. So, it is reasonable to think of onions as nature’s tear gas.
Onions evolved this mechanism to protect themselves from predatory herbivores. There are actually many ways that plants protect themselves from predators, whether insects, animals or other plants that want to eat/destroy them. For example:
folding their leaves inward and drooping when they are touched so that they appear less tempting as a food source.
having evolved to look like rocks or other inanimate objects.
Some plants pay hard to get by:
communicating with other plants nearby by releasing an organic compound, telling their neighbors they have encountered a predator.
coating themselves with a substance, such as a waxy chemical, making them harder to eat.
forming a tough outer coating, making it hard for a predator to bite through them.
simply having evolved to taste bad.
Some plants call in the cavalry by:
giving off chemicals that attract birds, ants, wasps, bees or moths that may help ward off/harass herbivore predators.
Some plants fight back by:
producing toxic chemicals that stop other plants close by from thriving.
producing venom/chemicals that cause unpleasant reactions if they are touched, such as the itch caused by urushiol made by poison ivy.
producing a sticky substance that adheres to the legs of insect predators, trapping them.
Some plants punish predators who have eaten them by:
having evolved to become poisonous to some herbivores, for example by forming hydrogen cyanide compounds that are toxic, or by forming other chemicals, such as digitalis, that cause a heart attack.
Bottom line: plants have developed many different methods to protect themselves from predators. Onions making you cry is just one example.
Certain varieties of onion are less sulfurous than others, and so are less irritating and cause less crying. A group of scientists in Japan engineered a type of onion that has less of the chemicals that start the reaction described above, hence minimizing the chain of events that causes crying. Unfortunately, much of the taste of onions which make them a food commonly used in many recipes and a nice ingredient in salads, etc., is lost in these onions.
So, a little crying when you are preparing onions for your favorite dishes may be the price you need to pay for your desired culinary results. Cutting onions when they are cold from the refrigerator, onions get mushy when they thaw after being frozen, may minimize the chemical reactions described above and may help minimize tearing. Using a sharp knife to minimize the trauma that liberates the chemical-laden fluids, keeping the onions you are cutting as far away from your eyes as possible, being efficient when doing the cutting and doing the preparation under a vent hood are other techniques that may help. Crying over spilt milk: that’s a different story.
Jeff Hersh, Ph.D., M.D., can be reached at DrHersh@juno.com
Want news like this sent straight to your inbox? Head over to MetroWestDailyNews.com to sign up for News Headlines and make sure you never miss a thing. You pick the news you want, we deliver.