The characteristic smell of onions is primarily due to the presence and emission of sulphur compounds and their derivatives.
The Chemistry Behind Onion's Distinctive Aroma
Onions, belonging to the Allium genus, are well-known for their pungent and unique aroma. This aroma is not just a simple scent but a complex interplay of various chemical compounds.
The key points regarding the smell of onions are:
- Sulphur Compounds: The fundamental source of the onion's signature smell lies in sulphur and its derivatives. When an onion is cut or damaged, specific enzymes within its cells react with these sulphur-containing compounds, leading to the formation of volatile molecules that are released into the air.
- Amino Acid Sulfoxides: More precisely, the chemicals responsible for the characteristic flavors (and thus the smell) of Allium vegetables are a class of compounds known as amino acid sulfoxides. These compounds are stable when the onion is intact but undergo rapid transformation when the plant tissue is disrupted.
How the Smell is Released
When you slice or crush an onion, its cell walls are broken. This action mixes enzymes (like alliinase) with the amino acid sulfoxides stored in separate compartments within the onion's cells. This enzymatic reaction produces sulfenic acids, which are highly unstable. These sulfenic acids then spontaneously rearrange to form volatile sulphur compounds, including thiopropanal S-oxide, which is the lachrymatory factor responsible for eye irritation, and various thiosulfinates and disulfides that contribute to the distinctive aroma.
Key Compounds Responsible for Onion Aroma
Compound Type | Description & Role in Smell | Example Effect |
---|---|---|
Sulphur Compounds | The overarching class of chemicals responsible for the pungent odor. | Primary source of the onion's characteristic scent. |
Amino Acid Sulfoxides | Specific precursors to the volatile sulphur compounds. | Convert into pungent compounds when onion is cut. |
The Allium Family Connection
The genus Allium includes other pungent vegetables such as garlic, chives, leeks, and shallots. These vegetables also owe their characteristic aromas and flavors to similar sulphur-containing compounds, although the specific balance and types of compounds vary, giving each its unique profile. Understanding these chemical processes helps explain why the smell of onions can be so strong and pervasive, affecting both the air and, sometimes, our hands after handling them.