Although in concentrated form, furfuryl mercaptan is a foul smelling compound. In 1926 it was already patented as coffee’s main odor active compound. It is a furan molecule substituted with a sulfanylmethyl group and is a clear colourless liquid when pure, but it becomes yellow coloured upon prolonged standing.
flavorscientist.com: Furfuryl mercaptan (otherwise known as coffee mercaptan) is the characterizing component of coffee. Coffee flavor and Furfuryl mercaptan are generated by a roasting process and a chemical reaction called the Maillard reaction. The Maillard reaction is one of the most important reactions in food flavor development. The Maillard reaction is between a carbonyl group (sugar) and an amino compound (protein). Degradation of the condensation products of this reaction give a number of oxygenated compounds; such as furans, pyrazines, pyrroles, oxazoles, thiophenes, thiazoles and other heterocyclic compounds.
Furfuryl mercaptan is found in other roasted foods: beef, pork, chicken and popcorn.