why do we cry when cutting onions?

The Deep Dive

The act of cutting an onion triggers a fascinating chain of biochemical events that culminate in the familiar sensation of tearing up. Onions belong to the Allium genus and contain sulfur-based compounds stored in their cells. When the onion's tissue is damaged by cutting, the enzyme alliinase is released and interacts with sulfur-containing amino acids, such as S-1-propenyl-L-cysteine sulfoxide. This reaction produces intermediate compounds that are then converted by lachrymatory factor synthase into syn-propanethial-S-oxide, a volatile sulfuric gas. This gas, also known as the lachrymatory factor, easily evaporates and rises into the air. Upon reaching the moist surface of the eyes, it dissolves and forms a mild solution of sulfuric acid, which irritates the nerve endings in the cornea. The irritation activates the trigeminal nerve, sending signals to the brain's lacrimal nucleus, which in turn stimulates the lacrimal glands to secrete tears. These tears serve to dilute and wash away the irritant, providing relief. This mechanism is thought to have evolved as a defense mechanism in onions to deter herbivores. Interestingly, not all alliums produce this compound; for example, garlic releases allicin, which has different properties. The process is temperature-sensitive, which is why chilling onions can reduce tear production by slowing enzymatic activity. Understanding this biochemistry has led to the development of onion varieties with lower lachrymatory potential and kitchen tools designed to minimize cell rupture.

Why It Matters

Understanding why onions cause crying has practical benefits in everyday cooking and food preparation. By knowing the chemical process, individuals can employ effective strategies such as using sharp knives to minimize cell damage, chilling onions to slow enzyme activity, or cutting them under running water to wash away irritants. This knowledge enhances kitchen efficiency and comfort, reducing the unpleasant experience of tears. Beyond cooking, it illustrates how plant biochemistry interacts with human physiology, offering insights into sensory biology and reflex mechanisms. Additionally, this awareness can inform the development of hypoallergenic onion varieties, contributing to agricultural innovation and improved food experiences for sensitive individuals.

Common Misconceptions

One prevalent misconception is that the smell of onions is what causes tears, but in reality, it's a chemical reaction that occurs when the irritant gas contacts the eyes, not the nasal passages. Another myth is that certain tricks, like holding a matchstick in your mouth or cutting onions near a flame, can completely prevent tearing; however, these methods are largely ineffective as they don't address the enzymatic production of syn-propanethial-S-oxide. Some believe that only raw onions cause tears, but cooking onions denatures the enzymes, reducing or eliminating the irritant. The correct understanding involves the specific biochemical pathway that produces a lachrymatory agent, and only methods that block this production or the vapors, such as wearing goggles or using a ventilation fan, are truly effective.

Fun Facts

• Onions are one of the oldest cultivated plants, with evidence of their use dating back over 5,000 years in ancient Egypt.
• The lachrymatory factor in onions is so potent that it has been studied for potential use in non-lethal weapons or as a natural pest deterrent in agriculture.