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How Do Onions Make the Toughest Men Cry?
Onions are used in almost every dish we all love and enjoy. Despite its range of flavors, onions are notoriously known for making us cry. This is not a tragic tale of nature, but the beauty of the chemistry of onions!
What is an onion composed of?
Onions belong to the Allium family, which also includes garlic, leeks, and shallots. They are rich in water, carbohydrates, fiber, vitamins, and minerals. The main chemical compounds responsible for their flavor and aroma are the sulfur-containing compounds, specifically sulfoxides, and thiosulfinates. Onions get sulfur for such compounds through the soil. Usually, soils with more sulfur make onions with stronger flavors and odors. It also depends on the variety you’re growing, as some onions simply take up less sulfur than others; there also is a limit up to how much sulfur an onion can take.
Onions are literally designed to protect themselves against us eating them – but it does not stop us. Within each onion cell, there is a sealed vacuole that is filled with enzymes. When an onion gets damaged, these enzyme-filled vacuoles break open, releasing their contents, which then mix with other chemicals inside the onion cells. This causes a whole cascade of chemical processes to happen rapidly. Soon, as we know, our eyes start to fill with tears.
Sulfoxides are stable when the onion is intact. They only undergo significant chemical changes when the onion is damaged. Onions contain compounds known as S-alk(en)yl-L-cysteine sulfoxides (ACSOs). The most abundant of these is isoalliin, but other ACSOs like propiin and methylallyl sulfoxide are also present.
What is going on at the chemical level?
Looking at the chemical processes in detail, when you cut an onion, you break the cells, allowing the enzyme alliinase to come into contact with ACSOs. The role of alliinase is to catalyze the conversion of these sulfoxides into sulfenic acids. Sulfenic acids are highly reactive and rapidly condense to form thiosulfinates. Thiosulfinates responsible for the pungent aroma of fresh onions. Thiosulfinates can further break down into a variety of sulfur-containing compounds, including diallyl disulfide and diallyl trisulfide, which contribute to the characteristic odor and flavor of onions.
Moving on to our number one enemy: the tear factor. A specific compound known as syn-Propanethial-S-oxide causes this effect. This is created from 1-propenyl sulfenic acid, which is converted into syn-Propanethial-S-oxide by an enzyme, lachrymatory-factor synthase. The researchers have reasoned that alliinase helps to change the shape of the starter molecule (ACSOs) so that the second enzyme could go to work on the compound formed (sulfenic acids).
During the enzymatic conversion of sulfoxides, only some of the 1-propenesulfenic acid undergoes a further reaction to form the syn-Propanethial-S-oxide, also known as the lachrymatory factor. It is a volatile sulfur gas which is released into the air, it forms sulfuric acid when it comes into contact with the moisture in our eyes. This irritates the eyes, causing lacrimal glands to produce tears as a defense mechanism, trying to flush out the irritant.
What does cooking an onion do to it?
Cooking onions triggers further chemical changes, transforming their flavor and aroma.
When onions are heated, thiosulfinates break down into simpler sulfur compounds, such as sulfides and polysulfides, which are less pungent but contribute to the savory, sweet, and complex flavors of cooked onions. When onions caramelize, the Maillard reaction causes the sugars and amino acids in the onion to react to form new flavor compounds. This enhances the sweetness and depth of the dish.
Not only about the chemicals behind the flavor, but cooking onions also reduces the concentration of syn-Propanethial-S-oxide, which is why cooked onions don’t make us cry. At high temperatures, the bonds between the enzymes weaken, which causes the enzyme to denature and show a significant drop in its activity. Likewise, heat denatures the alliinase enzyme and lachrymatory-factor synthase enzyme, preventing the formation of new lachrymatory compounds.
Not only cooking reduces the tear factor, you can try chilling the onion. Cold temperatures can slow down the enzyme activity, so the enzymes cannot create syn-Propanethial-S-oxide as rapidly, which can help reduce our tears. You can also cut the onions under running water as the water carries away the lachrymatory factor before it reaches your eyes. In order to reduce release of syn-Propanethial-S-oxide, you need to reduce the damage to cells. So, you should use a sharp knife to protect as many cells as possible. Although it may not be as fashionable, goggles protect your eyes from tearing up as it prevents syn-Propanethial-S-oxide from ever meeting your eyes.
Why are onions good?
Onions may make us cry unwillingly, but they are also boosting our health.
Onions are rich in flavonoids, particularly quercetin, which have strong antioxidant properties. These compounds help neutralize free radicals in the body, reducing oxidative stress and lowering the risk of chronic diseases. The sulfur compounds like thiosulfinates, have anti-inflammatory and antimicrobial properties, so they aid in the prevention of infections and the reduction of inflammation. The sulfur-containing compounds in onions help in reducing blood pressure, improving cholesterol levels, and preventing platelet aggregation; they improve our cardiovascular health. The chemicals in onions could possibly the inhibit cancer growth, as well as improve our sleep and mood. They are also a very good source of dietary vitamin C, which is a building block of collagen, onions may well play a role in hair and skin health.
The chemistry of onions is a fascinating interplay of sulfur compounds and enzymatic reactions that give rise to its unique flavor, aroma, and even its ability to make us cry. Whether raw or cooked, onions owe their culinary versatility to these intricate chemical processes. Beyond the kitchen, the compounds responsible for an onion's characteristic properties also offer a range of health benefits. Finally, you can just wait for the no-cry onion to hit the shelves as researchers have been working on a genetically-modified onion without the lachrymatory-factor synthase enzyme responsible for producing syn-Propanethial-S-oxide. Next time you have anything with onions, make sure to enjoy it well now that you know the chemistry behind it!
References:
- The Flavour Science of an Onion - FoodCrumbles
- Why Do Onions Make Us Cry? : The Salt : NPR
- Propanethial-S-oxide: how chopping onions makes you cry | Podcast | Chemistry World
- Why onions make us cry (snexplores.org)
- Why Onions Make You Cry - The New York Times (nytimes.com)
- Why Do Onions Make You Cry? | Britannica
- This Enzyme Is Why Onions Make You Cry | Smithsonian (smithsonianmag.com)
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