What happens in the roaster?

Anna Kerwick @ 2019-08-26 17:04:18 +1000

What happens between the green unroasted coffee beans and the brown ones that you buy is quite a complex process, but below we’ve broken it down into a short summary of the phases.

Firstly, in the warm up phase of the roast, the green bean will start to pale. Drum roasted beans take some time to heat up, whereas air roasted beans warm up quickly because of the more efficient transfer of heat.

In the early yellow stage the coffee is losing moisture in the form of steam vapour (not visible) and it will emit a humid, hay-like smell as the coffee takes on heat.

Yellow-tan stage

  • Browner colour, marbling appearance
  • First ‘toasty’ smell (like toasted grain or bread), less humid air coming off the coffee

Light brown stage

  • Some bean expansion visible, central crack has opened slightly
  • Releases some silverskin or chaff

Brown stage

  • Right at the door of the first crack
  • Coffee has browned to a little more even colour

First crack begins

  • Very first popping sounds can be heard (similar to popcorn popping)

First crack underway

  • Coffee appears mottled and uneven in colour, expands in size and shows visible cracks
  • First crack is an exothermic reaction – the beans are giving off heat
  • The beans quickly become endothermic – a roaster that is not adding enough heat will stall the roast at this point, creating coffee that will taste “baked”

First crack finishes

  • This is considered a “light roast” when the first crack is done and the roast is stopped
  • Bean surface is smoother, but still has darker line marks like a finely etched pattern

Light plus roast

  • Time is allowed for an even bean surface to develop, with softer edges
  • Whole time between light – medium – dark should only be 4 – 4.5 minutes depending on profile

Medium roast

  • On the verge of the second crack
  • Beans have a slight sheen with softer edges

Medium plus roast

  • Coffee has barely entered the second crack – the roast is stopped after the first audible snaps are heard
  • Second crack may continue into the cooling phase – called “coasting”. The more effective and rapid the cooling, the better the ability to stop the roast at the desired stage
  • Bean is a bit fuller, more small cracks on the face (flat side) of the bean

Medium to dark roast

  • Origin character gets eclipsed by roast character
  • More body and flavour than lighter roasts, but with a reduced acidity
  • All Merlo blends are roasted to this stage. This is done to maximise the shelf life of the coffee while not compromising on the flavour characteristics of the coffee.

Full dark roast

  • Only 30 seconds later, the coffee bean will begin to carbonise and will burn and turn to ash.