ROAST­ING

Cosmos - - Coffee -

We have dis­cussed where the cof­fee bean comes from; but how does the bean be­come the cof­fee we know and love to drink?

First, the cof­fee cherry needs to be pro­cessed and dried out so the bean can be re­moved. Then the bean needs to be sun-dried. This leaves a green cof­fee bean ready for roast­ing.

The roast­ing process takes place in a heated steel drum. The beans are con­tin­u­ously ro­tated and heated un­til they grad­u­ally be­come darker in colour. The darker they are, the stronger the roast.

As the tem­per­a­ture of the drum in­creases, a num­ber of chem­i­cal re­ac­tions oc­cur. These lead to the break­down of su­gars into ‘aro­matic com­pounds’ – spe­cial ring-shaped mol­e­cules that gives the bean its colour and flavour. The time the bean spends roast­ing in the drum de­ter­mines how strong these flavours are.

It is all about time and tem­per­a­ture. The first chem­i­cal re­ac­tion, known as the ‘Mailard Re­ac­tion’, oc­curs at 148 °C. This is the ini­tial con­ver­sion of su­gars and pro­teins into dif­fer­ent groups of mol­e­cules known as ‘fu­rans’, which pro­vide a burnt flavour, and ‘hy­drox­ymethyl­fur­furals’ (HMF), which cre­ate the sweet aro­mas.

The next re­ac­tion, called the ‘first crack’, oc­curs at 195 °C. It is an en­dother­mic re­ac­tion, ab­sorb­ing energy. This causes chloro­phyll to break down. As mois­ture is turned into steam, pres­sure builds within the bean that even­tu­ally rup­tures it, re­sult­ing in a ‘pop­ping’ sound.

The ‘sec­ond crack’ oc­curs at 225-230 °C. This re­ac­tion is both en­dother­mic and exother­mic – re­leas­ing energy. Pres­sure in­creases with heat, break­ing the struc­ture of the bean. Com­pounds adding ad­di­tional flavour are also formed, such as car­bon diox­ide and ni­tro­gen ox­ide.

CREDIT: ME­LANIE MITTMANN / GETTY IMAGES

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