Sulphur compounds
The pungent sulphur molecules in hing, garlic and onion that turn sweet and savoury with heat.
Also: thiols, asafoetida aroma, disulfides, allium smell
Sulphur-containing molecules provide the aroma of the allium family and of asafoetida. Raw hing is rich in sulphur compounds, which smell harsh and rotten-egg-like. Gentle frying in oil transforms them into milder, sweeter compounds with a savoury, onion-like smell, a change cooks recognise by smell alone. Garlic and onion work the same way, from sharp raw to rich and sweet when cooked. The effect explains why Jain and some Hindu cooks use hing as a substitute for onion and garlic. Kala namak, black salt, also carries sulphur, so its smell is distinctive, like boiled egg.