As a field expert in botany and culinary arts, I've often been asked about the common experience of tearing up while cutting onions. The phenomenon is quite fascinating and rooted in the onion's defense mechanism. Let's delve into the science behind it.
When you cut an onion, you're essentially damaging its cells. This damage triggers a series of chemical reactions within the onion. The key component that leads to the production of tears is an enzyme called alliinase, which is stored in separate cells from its substrate, the amino acid sulfoxides.

Step 1: Damage and Release
The first step in the process is the physical damage to the onion's cells. When you slice through the onion, you're breaking down the cell walls and allowing the enzyme alliinase to come into contact with the amino acid sulfoxides.

Step 2: Enzyme-Substrate Interaction
Once the enzyme and substrate are combined, alliinase converts the sulfoxides into a variety of sulfur compounds, including syn-propanethial-S-oxide, which is a gaseous compound.

Step 3: Gas Emission and Reaction
The gaseous compound is released into the air, and when it comes into contact with the watery layer of the eye, a chemical reaction occurs. This reaction produces sulfur compounds that are similar to the ones found in sulfuric acid.

Step 4: Irritation and Lacrimation
The sulfur compounds irritate the eyes, which is the body's way of trying to flush out the irritant. This irritation leads to the production of tears, which is the lacrimation response we are all familiar with when cutting onions.

Now, it's important to note that not all parts of the onion are equally potent in causing this reaction. The enzyme alliinase is more concentrated in the root end of the onion, which is also known as the basal plate. This is the part of the onion where the shoot would emerge from the bulb, and it's where the highest concentration of enzymes and substrates are stored.

Cutting through this part of the onion releases a greater amount of alliinase into the air, leading to a more pronounced tearing effect. However, even cutting through other parts of the onion can cause some tearing, albeit to a lesser extent, because the enzyme is present throughout the onion, albeit in lower concentrations.

To mitigate the tearing effect, there are several strategies one can employ. Some people choose to cut the onion under water, which can help to dilute the gaseous compounds before they reach the eyes. Others opt to use a very sharp knife, which causes less damage to the cells and therefore releases fewer enzymes. Additionally, chilling the onion before cutting can slow down the enzyme activity.

In conclusion, the part of the onion that makes you cry is primarily the root end, or the basal plate, due to the high concentration of the enzyme alliinase. However, the entire onion has the potential to cause tearing due to the presence of the enzyme throughout its structure. Understanding this process can help us better appreciate the onion not just for its culinary uses, but also for the fascinating biological mechanisms at play.

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It's not actually the whole onion that makes you tear up. Cutting just the root and attached shoot part of an onion (otherwise known as the onion's --basal plate--) releases an enzyme into the air. When that enzyme combines with water (like the water in your eye) it creates sulfuric acid, which irritates your eyes.Nov 21, 2013read more >>