The act of slicing an onion transforms a mild vegetable into a source of sharp odor and eye irritation. This pungent release is a rapid chemical defense mechanism inherent to the Allium genus. The resulting smell is a complex bouquet of volatile sulfur compounds, which are only created when the onion’s cellular structure is broken. Understanding this process requires looking closely at the stored components and the enzyme-driven reaction that produces these signature molecules.
The Onion’s Stored Chemical Components
The intact onion bulb is odorless because its reactive components are kept physically separated within the cells. The precursors to the smell are sulfur-containing molecules known as S-alk(en)yl cysteine sulfoxides (ACSOs). These ACSOs are non-protein amino acid derivatives; trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide (PRENCSO) is the most abundant type found in onions.
These odorless precursors are stored in the cytoplasm of the onion cells. The activating enzyme, alliinase, is sequestered elsewhere, held apart in the cell’s vacuole. This cellular compartmentalization ensures that the defensive compounds are only produced when the tissue is damaged. The concentration of ACSOs determines an onion’s final pungency, influenced by the cultivar’s genetics and the sulfur content of the soil.
The Initial Enzymatic Trigger
When the onion is cut, the cell walls rupture, causing the cytoplasmic and vacuolar contents to mix. This breach brings the stored S-alk(en)yl cysteine sulfoxides into contact with the alliinase enzyme. Alliinase acts as a catalyst, hydrolyzing the ACSOs to initiate the chemical cascade.
This enzymatic cleavage rapidly breaks down the ACSOs, producing an unstable intermediate called a sulfenic acid, along with pyruvate and ammonia. The primary sulfenic acid generated is 1-propenyl sulfenic acid, which sets the stage for both the smell and the tear factor. Since the enzyme reaction rate is temperature-dependent, chilling the onion slows alliinase activity, delaying the production of these irritants.
Distinct Chemical Pathways for Smell and Tears
The unstable 1-propenyl sulfenic acid intermediate follows two distinct chemical pathways to create the characteristic onion experience. The first pathway involves the spontaneous, non-enzymatic rearrangement and self-condensation of the sulfenic acids. This reaction forms volatile sulfur compounds known as thiosulfinates, which quickly decompose into mono-, di-, and trisulfides. These resulting organosulfur compounds are volatile and responsible for the onion’s pungent odor and flavor.
The second pathway is responsible for the tear-inducing effect. A separate enzyme called Lachrymatory Factor Synthase (LFS) intercepts some of the 1-propenyl sulfenic acid. LFS catalyzes the conversion of this acid into a highly volatile molecule: syn-propanethial S-oxide. This compound is the lachrymatory factor that rapidly vaporizes into the air. When it contacts the moisture on the eye’s surface, it creates a mild form of sulfuric acid, which irritates the sensory nerve endings and triggers the protective reflex to produce tears.
Controlling the Pungency Through Preparation
Understanding these chemical pathways allows for methods to reduce the intensity of the odor and tear factor. Chilling the onion before slicing slows the kinetics of the alliinase and LFS enzymes, reducing the rate of volatile compound production. Using a sharp knife minimizes the rupture of cell walls, decreasing the mixing of enzyme and substrate.
Cutting the onion under running water or near a strong vent limits the concentration of volatile irritants reaching the eyes. The water dissolves the water-soluble syn-propanethial S-oxide before it vaporizes, while ventilation carries the gas away. Applying heat, such as cooking, denatures the alliinase and LFS enzymes, stopping the chemical reaction. This stabilizes the less volatile sulfur compounds, resulting in a sweeter, milder flavor.