Why Do Onions Make You Cry? The Surprising Smell Science!
The volatile compounds are a critical element; lachrymatory factor synthase, a key enzyme, contributes significantly to what is responsiable for an onions smell. Scientists at institutions like the National Onion Association have dedicated research efforts to understanding the complex chemistry behind this pungent phenomenon. These efforts often involve techniques like gas chromatography–mass spectrometry (GC-MS), which helps analyze the specific compounds released. The sensation ultimately experienced is often triggered by the lacrimal glands.
Image taken from the YouTube channel Houston Methodist , from the video titled Why does my sweat smell like onions? #sweat #body odor #shorts .
Unraveling the Lachrymatory Mystery: What is Responsible for an Onion’s Smell and Tears?
The pungent aroma of onions is a staple in cuisines worldwide, but that same aroma is often accompanied by an unwanted side effect: tears. The chemistry behind this phenomenon is complex and fascinating. Understanding what is responsible for an onion’s smell, and subsequently, its tear-inducing properties, involves a journey through sulfur compounds and enzymatic reactions.
The Precursor: Amino Acid Sulfoxides
Before we even slice an onion, the seeds of this tearful reaction are already present within its cells. Onions contain various amino acid sulfoxides, odorless compounds that are precursors to the volatile substances that cause both the smell and the tears.
- These sulfoxides include S-alk(en)yl-L-cysteine sulfoxides (ACSOs).
- Different onion varieties have varying concentrations and types of ACSOs, which contributes to their distinct flavor profiles.
- These compounds remain stable and odorless as long as the onion remains intact.
The Trigger: Alliinase and the Enzymatic Cascade
The key to unlocking the onion’s distinctive odor and tear-inducing power lies in the enzyme alliinase.
The Role of Alliinase
Alliinase is an enzyme stored in separate compartments within the onion’s cells. When an onion is cut, chopped, or crushed, these compartments rupture, releasing alliinase and allowing it to come into contact with the ACSOs.
The Conversion Process
Alliinase then catalyzes a series of reactions involving the ACSOs.
- Formation of Allicin: Alliinase breaks down the ACSOs into sulfenic acids.
- Instability and Rearrangement: These sulfenic acids are unstable and quickly rearrange. Depending on the specific ACSO present, various volatile sulfur compounds are formed. This process is critical in understanding what is responsible for an onion’s smell.
The Culprit: syn-Propanethial-S-oxide
While several sulfur compounds contribute to an onion’s aroma, the primary lachrymatory (tear-inducing) factor is syn-propanethial-S-oxide.
The Synthesis of syn-Propanethial-S-oxide
Syn-propanethial-S-oxide is formed through a further enzymatic reaction catalyzed by lachrymatory factor synthase (LFS). LFS acts on one of the sulfenic acids produced by the alliinase reaction.
The Airborne Irritant
- syn-Propanethial-S-oxide is a volatile compound, meaning it readily evaporates and becomes airborne.
- When it comes into contact with the surface of the eye (the cornea), it reacts with the water in the tears.
The Eye’s Response: Tears and Irritation
The reaction of syn-propanethial-S-oxide with water in the tears produces sulfuric acid, a mild irritant.
Activation of Sensory Neurons
- Sensory neurons in the cornea detect the presence of this irritant.
- These neurons send a signal to the brain, specifically to the lacrimal glands.
The Lacrimal Glands to the Rescue
The brain signals the lacrimal glands to produce more tears in an attempt to dilute and wash away the irritant. This is the body’s natural defense mechanism.
The Result: Watery Eyes
The increased tear production results in the familiar watery eyes and stinging sensation we associate with cutting onions.
Contributors to the Overall Smell Profile
While syn-propanethial-S-oxide drives the tearing, a wider range of volatile sulfur compounds are responsible for the distinctive and complex onion smell. These include:
- Allicin: Present briefly, contributes to a strong, pungent smell. Quickly decomposes.
- Diallyl Disulfide: Contributes to the stronger, lingering onion odor.
- Dimethyl Disulfide: Another volatile sulfur compound contributing to the overall aroma.
The specific blend of these compounds varies depending on the onion variety and storage conditions, influencing its overall flavor and aroma profile. Understanding what is responsible for an onion’s smell involves appreciating the interplay of these multiple compounds and their respective concentrations.
FAQs: Onion Tears and Smell Science
Here are some frequently asked questions about why onions make us cry and the science behind their pungent aroma.
What exactly in onions makes us cry?
When you cut an onion, you break open its cells, releasing enzymes. These enzymes react with sulfur-containing compounds, creating propanethial S-oxide. This volatile gas floats up to your eyes, irritating them and triggering tear production.
Is there any way to prevent onion-induced tears?
Yes, several methods can help. Chilling the onion beforehand slows down the enzymatic reactions. Cutting the onion under running water can wash away the irritating gas before it reaches your eyes. Wearing goggles is another effective solution!
What is responsible for an onion’s smell?
The same chemical process that causes tears also creates the onion’s characteristic smell. When enzymes mix with sulfur compounds, they produce a range of volatile sulfur-containing molecules. These molecules, including propanethial S-oxide, contribute to the onion’s pungent odor.
Do all onions cause the same level of tears and smell?
No, different varieties of onions contain varying levels of sulfur compounds. Sweet onions, for instance, have lower sulfur content and are less likely to cause intense crying or have a strong smell compared to yellow or red onions.
So, next time you’re chopping onions and tearing up, remember the surprising science behind what is responsiable for an onions smell. Hope this shed some light (and maybe a few tears) on the subject!
