1. Why the Tear-Causing Compound Only Appears After an Onion Is Cut
It is tempting to think that the source of tears is already stored inside the onion. However, it is explained that the tear-inducing compound itself does not exist in the onion before you cut it. Instead, it is first created from material substances only after the tissue and cells are broken [1]. Before cutting, the onion contains a precursor substance called PRENCSO. When a knife breaks the cells, this substance is broken down by an enzyme called alliinase to create propenesulfenic acid. Then, an enzyme called lachrymatory-factor synthase (LFS) acts on it to create the compound that irritates the eyes [2]. Before cutting, these materials and enzymes are in separate places; they only meet when you cut.
This second enzyme was announced by researchers from House Foods Group in the British science journal *Nature* on October 17, 2002. Until then, it was thought that tear-inducing compounds formed naturally after being broken down by alliinase. It is reported that this was a chance discovery during experiments [2]. If tears are made by reactions that happen after cutting, could other conditions change how much you cry? Next, we look at what makes spring new onions different from regular ones.
2. How New Onions Differ From Regular Onions
New onions sold in supermarkets in spring give the impression of having thinner skin and being softer and juicier than usual onions. Reading explanations from the Agricultural and Food Industry Promotion Organization (ALIC) reveals one reason for this. Most onions sold in Japan are yellow onions, which are known for their sharpness and high storability. It is common to dry them for about 1 month after harvest before shipping. On the other hand, new onions produced in prefectures in early spring are explained as having less sharpness and being easy to eat raw or in salads [3]. Note that this 1-month drying period is a general rule for yellow onions and does not apply to all varieties.
New onions are shipped immediately after harvest. Because they have high water content and spoil easily, it is recommended to eat them as soon as possible [4]. House Foods’ guide also states that new onions, which are not dried, do not keep well, so they should be stored in the vegetable compartment of the refrigerator [5]. So far, we know that new onions are described as "shipped quickly without drying," "high in water content," and "having less sharpness." Nowhere does it say they cause fewer tears. Having less sharpness and causing fewer tears are different things.
3. Why It Is Hard to Compare How Many Tears Different Onions Cause
To compare how much tearing occurs, you first need to measure "how many tears came out" in a consistent way. There are individual differences in perception, and the same person might react differently depending on their health or how dry their eyes are. There are also methods to measure the amount of tear-inducing compound with machines. However, there are reports that the measurement method itself affects the results. In a study where two types of onions were stored for about 4 months and measured during storage, the relationship between the amount of tear-inducing compound and its precursor substance varied by variety and changed during storage. Furthermore, it is stated that the time between cutting and measuring greatly changed the results [6].
This study did not compare new onions with stored onions. It discussed two varieties kept for a long time, so one should be cautious about connecting this directly to new onions. However, it shows that three conditions—variety, storage method, and timing of measurement—affect the results, which are points to watch when comparing. From current materials, we can definitely say only that new onions are described as having less sharpness and more water content. Regarding tearing, you can check for differences in feeling by cutting them at home. By the way, some people have actually made onions that do not cause tears.
4. How the Low-Tear Onion "Smile Ball" Was Made by Stopping the Enzyme LFS
If the enzyme LFS does not work, fewer tears should be produced. Based on this idea, House Foods developed an onion called "Smile Ball" and has sold it as fresh produce since 2015. It is explained that because almost no tear-inducing compound is created when cut, you do not cry while cooking, and it feels almost not sharp even when eaten raw [7]. This is not a new onion, but a variety improved through breeding. Do not mix this up with the topic of new onions.
Researchers in New Zealand also created onions where LFS activity was reduced by suppressing gene function (RNA interference). They reported in 2008 that enzyme activity dropped to about 1/1,544th of normal levels in the most effective case, and very little tear-inducing compound was produced even when damaged [8]. The figure of 1,544 times is the upper limit for the onion where activity dropped the most; not all onions dropped that much. Interestingly, the paper states that when LFS was stopped, some sulfur-containing compounds from the material flowed into different chemical substances [8]. Not crying means the reactions inside the onion changed to a different path.
These two examples show that the presence or absence of tears is decided during the reaction process. On the other hand, these studies do not tell us if new onions decide their tear output in the same way. Still, you can compare how they feel when cut using onions you have at home.
5. How to Compare New and Stored Onions Safely With an Adult
If you have both new and stored onions, try cutting them together with an adult. An adult should use the knife.
The five conditions to align are: the knife, the cutting method (same thickness), the time spent cutting, whether they were cooled before cutting, and ventilation status. House Foods’ guide lists cooling before cutting and wetting the cut surface with water as tear countermeasures [5]. This time, apply these to both onions equally so you only see the difference.
Record how tears feel on a 3-point scale: "not at all," "a little," or "quite a lot." Do not decide after one try; do it again on another day to check if the result is the same. As seen in Section 3, if conditions are not aligned, you cannot tell what the difference is.
New onions spoil easily, so eat them quickly after cutting. It is explained that soaking thin slices in water softens the sharpness [4], so you can also compare taste separately from tears. If your eyes feel uncomfortable, do not rub them; wash with water and consult an adult if it continues.
House Foods’ announcement is a good entry point to read about the mechanism of tear formation in Japanese [1]. How the official definition of new onions is written in Ministry of Agriculture, Forestry and Fisheries materials is a question worth investigating next.