Onions are a common vegetable found in kitchens worldwide, adding distinct flavors to many dishes. Their unique chemical makeup often sparks curiosity, particularly regarding the compounds responsible for their pungent taste and the tears they can cause.
The Tear-Jerking Compound
When an onion is cut, its cells are ruptured, releasing enzymes and sulfur-containing compounds. An enzyme, lachrymatory factor synthase (LFS), acts on amino acid sulfoxides within the onion. This enzymatic reaction converts them into unstable sulfenic acids. These sulfenic acids then rearrange into syn-propanethial S-oxide.
Syn-propanethial S-oxide is the primary lachrymatory factor, meaning it causes tears. This airborne compound floats up from the cut onion and makes contact with the eyes. Upon reaching the eye, it reacts with water to form a mild sulfuric acid. This acid irritates sensory nerves, prompting the lachrymal glands to produce tears as a protective flush.
Other Acidic Components in Onions
Beyond the tear-inducing compound, onions contain various organic acids and sulfur-containing amino acids that contribute to their flavor and nutritional profile. Glutamic acid is the most prevalent organic acid. Other organic acids include citric acid, malic acid, tartaric acid, oxalic acid, pyruvic acid, and fumaric acid.
Onions are also rich in sulfur-containing amino acids, such as S-propenyl-L-cysteine sulfoxide (PRENCSO) and S-methyl-L-cysteine sulfoxide. These amino acids serve as precursors for the volatile sulfur compounds responsible for the onion’s characteristic taste and aroma. These naturally occurring compounds are distinct from the reaction product that causes eye irritation.
Overall Acidity of Onions
The acidity of a substance is measured by its pH level, indicating whether it is acidic, neutral, or alkaline. Onions are generally considered mildly acidic. Their typical pH range falls between approximately 5.0 and 7.0.
This pH level contributes to the overall chemical environment within the onion, influencing its stability and how its various compounds interact. The specific pH can vary slightly depending on the onion variety and growing conditions.
Transformations During Cooking
Applying heat to onions initiates chemical transformations, altering their flavor and reducing pungent effects. Cooking deactivates enzymes, such as alliinase and lachrymatory factor synthase, which produce syn-propanethial S-oxide. This enzymatic deactivation explains why cooked onions do not cause eye irritation.
Heat also breaks down volatile sulfur compounds that give raw onions their sharp taste. As these compounds dissipate, natural sugars become more noticeable. Complex carbohydrates break down into simpler sugars, like fructose and glucose, increasing perceived sweetness. Caramelization and Maillard reactions, involving sugars and amino acids, further enhance sweet and savory flavors, giving cooked onions a milder, sweeter profile.