Compound library · Biosynthetic pathways
Sulphur compounds
Enzymes cut sulphur precursors the moment tissue is damaged
4 compounds in the library
4 compounds follow this pathway Left of the dashed line are compounds lighter than any essential-oil component — they are gone before the main axis begins.
Onions, garlic, chives, cabbage and shiitake all store odourless sulphur-containing molecules, together with an enzyme that can cut them. The two are kept in separate compartments of the cell.
When you slice, crush or bite, the compartments break and the two meet. The enzyme works within seconds, and the smell appears almost instantly. A whole onion has almost no smell; a freshly sliced one stings the eyes.
This is the pathway of instant reaction, quite unlike the microbial pathway, which needs days. It also differs from the fatty-acid pathway in its raw material: fatty acids are cut from cell membranes, while here the precursors are derivatives of sulphur-containing amino acids.
Heat then transforms these products further. That is why raw onion is pungent and fried shallot sweet — two different groups of compounds, one formed from the other.
Compounds that follow this pathway
| Compound | Chemical family | Boiling point | Threshold |
|---|---|---|---|
| Dimethyl sulfide | Other compounds | 37 °C | 0.300 ppb |
| Dimethyl trisulfide | Other compounds | 170 °C | 0.100 ppb |
| Dipropyl disulfide | Other compounds | 194 °C | — |
| Lenthionine | Other compounds | 240 °C | — |
Three axes, three questions
This library classifies compounds along three axes, and each answers a different question.
Boiling point tells you when you smell a compound — early or late on the smelling strip. Chemical family tells you how it behaves — stable or fragile, and how to recognise it on an analysis. Biosynthetic pathway tells you where it comes from.
The third axis is the explanatory one. The other two describe how compounds cluster into groups; this one says why they cluster that way.