Classic Drum Roaster
Classic drum roasters heat up the raw coffee by passing on the heat from its walls to the beans by direct contact, or conduction. In this process, the coffee is kept moving with a paddle or by rotating the drum so that the beans are roasted evenly. However, in many roasters, hot air is also added in order to achieve a more homogeneous result meaning the beans are roasted by convection. The Classic Drum roaster is still the preferred method of roasting specialty coffee.
The Roaster of choice at Altitude Coffee is a Mill City 3kg Gas Drum Roaster. This means that the roaster can potentially roast up to 3kg per batch. Each batch can take anywhere between 11 to 17 minutes depending on the profile you are hoping to achieve. This roaster has many adjustments that can be made before and or during the roast to achieve the desired result such as gas flow adjustment, airflow adjustment and also drum speed adjustment. This roaster also features an auxiliary fan for drawing cool air over the roasted beans once dropped from the drum into the cooling tray.
1. DRYING STAGE
The coffee bean has a humidity of 8–12%. We need to dry it before the actual roasting starts. Drying stage typically lasts 4–8 minutes with traditional drum roaster. The temperature in the end of drying stage is typically 160 ⁰C. The drying stage is also important for collecting energy for the bean because the last stage of roasting is exothermic meaning it produces heat.
2. BROWNING STAGE
From 160 ⁰C the coffee starts to smell like cooked pasta ,hay or baked muffins. This is when the aroma precursors are starting to convert to aroma compounds. At the browning stage the Maillard Reaction begins which is responsible for the browning. This is the stage when the roast naturally slows down to ensure flavour development. At the end of the browning stage the coffee starts to pop. This is called the first crack and the development stage starts.
3. DEVELOPMENT STAGE
In the beginning of development stage the reaction becomes exothermic and the coffee cracks. During drying and browning stages the bean has collected energy that makes the coffee explode. Development time is when the wanted aroma compounds are developing. If we do not slow down the roast at development stage, we easily get coffee that is smoky tasting and the flavour is too sharp.