why do we cry when cutting onions when we are stressed?

The Deep Dive

The tears shed while cutting onions are a direct result of a sophisticated chemical defense mechanism. When an onion's cells are damaged by a knife, enzymes called alliinases convert sulfur-containing amino acid sulfoxides into sulfenic acids. In onions, one of these acids rapidly rearranges into syn-propanethial-S-oxide, a volatile compound that becomes airborne. Upon contact with the moisture on the eye's surface, it reacts to form a mild sulfuric acid solution, irritating the corneal nerves. This irritation activates the lacrimal reflex, prompting the lacrimal glands to produce tears to flush out the irritant. Stress enters the equation by engaging the body's fight-or-flight response. The release of cortisol and adrenaline during stress increases blood flow and heightens nerve sensitivity, particularly in the eyes. This can reduce blink rate, allowing more gas to accumulate and intensify the irritation. Furthermore, stress lowers the threshold for emotional responses; the physical discomfort from the onion can trigger a more pronounced crying reaction as the body is already in a heightened state of alert. Thus, the interplay between the onion's biochemical arsenal and the body's stress physiology creates a synergistic effect, making tears more abundant and immediate.

Why It Matters

This knowledge has practical applications in everyday life, especially in cooking. Understanding the biochemistry can lead to effective strategies to reduce eye irritation, such as using sharp knives to minimize cell damage, chilling onions to slow enzyme activity, or cutting near ventilation. It also highlights the profound mind-body connection, demonstrating how mental states like stress can exacerbate physical reactions. For individuals prone to stress, managing it through techniques like deep breathing can make kitchen tasks more comfortable, enhancing overall well-being and efficiency in food preparation.

Common Misconceptions

A widespread myth is that crying when cutting onions is an emotional response, similar to sadness. In reality, it is a purely chemical reflex triggered by syn-propanethial-S-oxide irritating the eyes, with no emotional component involved. Another misconception is that stress causes onions to release more of this irritant gas. However, stress does not alter the onion's enzymatic processes; instead, it affects the human body's physiological response by increasing eye sensitivity and reducing blink rate, thereby amplifying the perceived effect without changing the onion's chemistry.

Fun Facts

• Onions evolved to produce syn-propanethial-S-oxide as a defense mechanism to deter herbivores, making them less appealing when damaged.
• Refrigerating onions for about 30 minutes before cutting can reduce the release of the irritating gas by slowing down the enzymatic reactions that produce it.