The Scientific Process Behind Onion Tears
The iconic tear-inducing effect of chopping an onion is the result of a fascinating and complex chemical defense mechanism evolved by plants in the Allium family. For years, the blame was placed entirely on the enzyme alliinase, but more recent research has uncovered a second, crucial enzyme known as lachrymatory factor synthase (LFS). The two-part enzymatic reaction occurs when the onion's cells are ruptured, such as by slicing, dicing, or crushing, causing two previously separate components to mix and trigger the cascade of events that leads to tears.
The Role of Alliinase: The First Step
Within an intact onion, odorless amino acid sulfoxides are stored separately from the enzyme alliinase. The moment a knife blade damages the onion's cellular structure, this physical barrier is broken, allowing the alliinase enzyme to come into contact with the sulfur-based amino acid sulfoxides. This initial reaction, catalyzed by alliinase, swiftly converts the sulfoxides into an intermediate compound known as sulfenic acid. Sulfenic acid is unstable and will eventually rearrange on its own, but the presence of LFS accelerates the process towards a specific, more potent irritant. This initial step is similar across many allium vegetables, contributing to their distinct flavors.
The Critical Function of Lachrymatory Factor Synthase
The discovery of lachrymatory factor synthase (LFS) in 2002 was the key to understanding the tear-producing mechanism. LFS intercepts the sulfenic acid produced by alliinase and, in a critical second step, rearranges it into a volatile gas called syn-propanethial-S-oxide. It is this gaseous compound that wafts into the air and reaches our eyes. When syn-propanethial-S-oxide interacts with the water in our eyes, it forms a mild sulfuric acid, which irritates the sensitive nerves, stimulating the lachrymal glands to produce tears as a protective flushing mechanism. The pungency and flavor of the onion are also largely a product of these organosulfur compounds, which act as a deterrent to pests.
Comparison of Onion Enzymes
| Feature | Alliinase | Lachrymatory Factor Synthase (LFS) |
|---|---|---|
| Function | Breaks down amino acid sulfoxides | Rearranges sulfenic acid into lachrymatory factor |
| Location | Segregated within the onion cells | Stored separately within the onion cells |
| Trigger | Cell damage from cutting or crushing | Present when sulfenic acid is formed |
| Product | Sulfenic acid (an unstable intermediate) | syn-propanethial-S-oxide (the irritant gas) |
| Result | Contributes to flavor compounds | Causes eye irritation and tears |
| Pungency Role | Initiates the flavor-forming cascade | Creates the specific tear-inducing gas |
Minimizing Tears While Cutting Onions
Several methods can help minimize the release of the tear-inducing compounds. The goal is to either reduce the enzymatic reaction rate or prevent the volatile gas from reaching the eyes. The following list offers some proven techniques:
- Refrigerate the onion: Chilling the onion for 30 minutes before cutting it slows down the enzymatic reactions, limiting the amount of gas produced.
- Use a sharp knife: A sharper knife causes less cell damage, which means fewer enzymes and precursors are released into the air.
- Cut under water or a fan: Submerging the onion in water during cutting traps the volatile compounds, preventing them from becoming airborne. Cutting under a range hood or next to a running fan also helps blow the irritating gases away from your face.
- Cut off the root last: The root end of the onion tends to have the highest concentration of sulfur-based compounds. Cutting it off last or leaving it intact for as long as possible can help reduce the tear-inducing effect.
Flavor vs. Crying: The Genetic Factor
Not all onions produce the same level of lachrymatory factor, which is why some varieties are milder or sweeter than others. The amount of sulfur the plant absorbs from the soil, as well as its genetic makeup, influences the concentration of the precursor compounds and, consequently, the pungency and tear-producing potential. This has led to the development of “tearless” onion varieties like Sunions, which have been bred over generations to reduce the concentration of the volatile compounds responsible for eye irritation.
The Effect of Cooking on Onion Enzymes
Cooking onions, whether sautéing, grilling, or boiling, effectively neutralizes the enzymes responsible for the chemical reactions. Heat denatures the enzymes, rendering them inactive. This is why cooked onions have a much milder, sweeter flavor and do not cause eye irritation. The longer the cooking time, the more the pungent sulfuric compounds are broken down, leaving behind sweet, caramelized flavors from the onion's natural sugars. To learn more about the chemical processes in the Allium family, visit the U.S. National Institutes of Health website.
Conclusion
The presence of both alliinase and lachrymatory factor synthase (LFS) is what defines the unique and irritating chemistry of the onion. Alliinase kicks off the process by converting sulfur-based compounds into sulfenic acids, which LFS then transforms into the potent volatile gas, syn-propanethial-S-oxide. This gas is the final product that irritates our eyes and causes tears. Understanding this two-step enzymatic reaction provides the scientific foundation for the various methods people use to reduce crying while preparing onions.
