Cutting onions is a routine task in kitchens around the world, yet it often brings an unexpected side effect—tears. While many assume it is simply due to the strong smell, the real explanation is rooted in chemistry. Onions contain special compounds that are released when their cells are broken, leading to the production of an irritant that triggers our eyes to water. Understanding why onions make us cry involves exploring plant biology, sulfur chemistry, and the body’s protective reflexes.
Onions (Allium cepa) are bulb vegetables belonging to the allium family, which also includes garlic, leeks, and chives. Like many plants, onions have developed chemical defense systems to protect themselves against pests and animals. These defenses are stored in different compartments within the onion’s cells. When the onion is whole, these compounds remain separate. However, when the onion is cut or crushed, the cell walls break, releasing the stored chemicals and initiating a chain of reactions.
Onions are rich in sulfur-containing molecules known as amino acid sulfoxides. These compounds are odorless and harmless while contained within intact onion cells. The real transformation occurs when cutting or chopping disrupts the onion’s tissues, allowing these molecules to come into contact with specific enzymes.
When an onion is sliced, the enzyme alliinase is released. This enzyme acts on the amino acid sulfoxides, breaking them down into sulfenic acids. These sulfenic acids are unstable intermediates that quickly rearrange or react with other molecules. In onions, they lead to the production of a highly irritating compound known as the lachrymatory factor.
The lachrymatory factor in onions is a volatile chemical called syn-propanethial-S-oxide. Here’s how it causes tears:
This reflexive tearing is a protective mechanism, similar to how eyes water when exposed to smoke, dust, or strong fumes.
The irritation happens almost instantly because the lachrymatory factor is volatile, meaning it evaporates rapidly at room temperature. As soon as the onion is cut, the compound diffuses into the air and reaches the eyes within seconds. This explains why even chopping onions from a distance can sometimes cause eye irritation.
Several factors affect how strongly onions make us cry:
Over time, people have developed strategies to reduce the tear-inducing effect of onions. Some of these include:
From an evolutionary perspective, the production of irritating compounds is thought to be a defense mechanism. By producing chemicals that cause discomfort, onions discourage animals, insects, and microorganisms from eating or damaging them. What makes us cry actually serves as the onion’s protective shield.
The chemistry of onion tears has inspired scientific applications. For instance, researchers have studied the lachrymatory factor pathway to develop genetically modified onions that do not trigger tearing. Additionally, understanding how sulfur compounds react helps food scientists control flavors and odors in culinary processes.