Why do garlic, onions, and other foods have sulfur-related flavors and odors?

Many foods, such as garlic and onions, possess distinct flavors and odors because they are rich in sulfur, which they absorb from the soil and incorporate into various amino acids.12

When these plants are cut or crushed, their cell structures are broken, allowing previously separated enzymes to interact with these sulfur-rich compounds. This biochemical reaction produces volatile, sulfur-containing substances that provide the characteristic sensory profiles of these foods:312

• Garlic: The enzyme alliinase converts alliin into the intermediate 2-propenesulfenic acid, which then forms allicin. Allicin is the primary flavorant responsible for garlic's pungent smell and taste. Further processing with heat or water can transform allicin into various disulfides and polysulfides, which also contribute to garlic's flavor profile.345
• Onions: A similar process occurs when an onion is cut, converting sulfur-rich precursors into 1-propenesulfenic acid. This compound rearranges into (Z)-propanethial S-oxide, a volatile "lacrimatory factor" that causes eye irritation and tearing.31

These organosulfur compounds are essential to the unique gustatory and olfactory characteristics of many other foods, including chives, leeks, and various mushrooms.2