Why do purple onions make us cry?

The pungency and tear-inducing properties of onions, especially purple onions, are largely attributed to the presence of a compound called syn-propanethial-S-oxide (SPSO). Here's why purple onions specifically tend to cause more tears:

1. Higher Concentration of SPSO: Purple onions generally have a higher concentration of SPSO compared to other varieties of onions. This compound is responsible for the characteristic pungent odor and eye-watering effect of onions when cut or crushed.

2. Volatile Nature of SPSO: SPSO is a volatile compound, meaning it easily vaporizes and disperses into the air when the onion is cut. These vapors travel through the air and reach our eyes, triggering the tear response.

3. Tear-Producing Mechanism: When SPSO vapors come into contact with the cornea of our eyes, they react with the water present on the eye's surface. This chemical reaction produces sulfuric acid, which irritates the nerve endings in the cornea, causing a burning sensation and triggering the production of tears.

4. Individual Sensitivity: The intensity of the tear-inducing effect can vary from person to person. Some individuals may be more sensitive to SPSO than others, making them more prone to shedding tears when cutting purple onions.

To reduce the tearful experience while cutting onions, a few techniques can be employed:

- Chilling the onion before cutting: Cold temperatures can slow down the release of SPSO vapors.

- Using a sharp knife: A sharp knife creates cleaner cuts and releases fewer SPSO vapors.

- Cutting under running water: The water helps capture and wash away the irritating vapors.

- Avoiding direct contact: Wearing goggles or glasses can protect your eyes from the vapors.

Despite the tears they may bring, purple onions are a flavorful addition to many culinary creations. Their distinct taste and vibrant color enhance various dishes, making them a popular choice in cooking.