1. How Cutting an Onion Makes an Enzyme Create Tear-Inducing Chemicals
Inside onion cells, there are materials that can become tear-inducing chemicals. When you cut an onion with a knife, the cells break open. This allows the materials and enzymes to meet. Through several steps, they turn into the substance that stings your eyes.
In 2002, researchers from House Foods Corporation reported finding the enzyme responsible for the final step of this process [1]. It is called lachrymatory factor synthase, or LFS. This enzyme works on a middle substance created by another enzyme already in the onion, known as alliinase. The LFS enzyme finishes the job to create the tear cause.
This means tears are made by two enzymes working together: alliinase and LFS. However, for a long time, no one knew about this "second" enzyme [6].
2. What Researchers Believed About Tear Formation Before 2002
Before the year 2002, researchers believed that once alliinase broke down the onion's material, the tear-inducing substance would form naturally on its own [2].
This idea seemed to make sense. If one enzyme starts the reaction and the rest happens by itself, there is nothing else to look for. It appeared that the job was done once alliinase acted.
However, a clue from another vegetable shook this belief. Scientists began to wonder if their explanation was complete [2].
3. Why Garlic Has the Same Enzyme but Does Not Make You Cry
Cutting garlic does not make your eyes water. But garlic also contains alliinase, the same enzyme found in onions [3].
If alliinase alone was enough to create tear-inducing chemicals, then cutting garlic should also sting your eyes. Researchers tested this by adding garlic's alliinase to onion material (called PRENCSO). The result showed that the tear-inducing substance did not form [3].
From this experiment, scientists concluded that onions must have another enzyme that was still unknown. By simply comparing two vegetables that share one enzyme but react differently, they found the missing piece of the puzzle.
4. Why Scientists Overlooked the Enzyme Even After Finding the Chemical
The tear-inducing chemical itself had been identified in the 1970s. However, the enzyme that creates it was overlooked [5]. The inventor of the tearless onion stated that isolating and studying this enzyme was not difficult [5].
The discovery happened by accident. Researchers at House Foods were investigating why mixing onion and garlic paste sometimes turns green. This green change rarely happens in retort curry packages, so they studied the cause [4].
While studying this color change, they noticed the existence of LFS. They were not directly chasing the mystery of tears. Instead, the enzyme responsible for tears appeared while they were answering a different "why" question about food chemistry [4].
5. How Silencing the Enzyme's Gene Reduced Tears in Onions
If scientists found the missing enzyme, what would happen if they stopped it? In a 2008 study, researchers created onions where the LFS gene was silenced using RNA interference [6].
In six of these onion strains, LFS activity dropped significantly. When these onions were damaged, the amount of tear-inducing chemical decreased greatly. Instead, the middle substance turned more into other sulfur compounds [6].
It is important to read the numbers carefully. The reported "up to 1/1544" refers to how much the enzyme activity dropped. It does not mean the amount of tears was reduced to 1/1544 [6]. House Foods explains that they have sold these onions as fresh produce under the name "Smile Ball" since 2015 [4].
In 2013, this research won an Ig Nobel Prize in Chemistry [4]. One expert suggested that tear-inducing chemicals might help protect the onion from insects and microbes [3].
6. How to Compare Cutting an Onion and Garlic at Home Safely
You can try this comparison at home with an adult's help. Ask a family member to cut an onion and garlic for you. An adult must use the knife [3].
Notice if your eyes sting and how strong the smell is. Compare the two vegetables. Even though both have alliinase, onions make you cry while garlic does not. This difference was the clue that led researchers to find LFS.
To record more data, compare not just the stinging sensation in your eyes, but also how long it takes for the smell to become noticeable [3].