Colour Development of Green Coffee Bean during Batch Roasting in Fluidized Bed Roaster

(By: Mohamad Taib Miskon & Nurul ‘Uyun Ahmad)

“While enjoying a cup of an aromatic, astonishing, hot and refreshing coffee drink.”

Roasting has been one of the most important step in a coffee production as it transforms the tasteless green coffee bean into a delicious cup of Joe. It is an act of introducing an amount of heat to a batch of green coffee bean to trigger complex chemical reaction [1] as well as colour and physical change [2]. There are various types of coffee roasting methods such as using the traditional hot pan, drum roaster and fluidized bed or hot air roaster.

This article presented the colour development of green Ethiopian coffee bean during roasting in a Fresh Roast SR500 coffee roaster with Artisan Roaster Scope. Figure 1.0 depicted the colour changes of the bean over temperature progression during the roasting process. The process took about 15 minutes and it can be divided into six key stages;

Stage 1: Drying (at minutes: 0 – 2.15)

The roasting process started with the bean temperature at 33°C and it endured the drying phase for about 2.15 minutes as the beans’ colour changed from green to yellow. During this stage, the beans were absorbing heat from the hot air or also known as an endothermic process. 

Stage 2: Yellowing (at minutes: 2.15 – 6.33)

At this stage, more water was removed from the bean and the bean colour was changing from yellow to brown. The size of the bean was also expanding rapidly due to the build-up of gas pressure inside the bean [3].

Stage 3: First Crack (at minutes: 6.33 minutes)

The first crack was determined by the audible popping sound, indicating the beginning of the beans’ exothermic reaction.

Stage 4: Roast development (at minutes: 6.33-9.09)

At this stage, the beans’ flavour and sweetness started to develop [4] and the process continued for about 3 minutes.

Stage 5: Second Crack (at minutes: 9.27)

The second crack occurred at minutes of 9.27 and the heater was turned off shortly after to allow the execution of the cooling phase.

At this stage, the bean experienced the second crack whereby the oil’s bean were encapsulated to the surface of the bean. The bean produced was less acidic, smoky yet aromatic, and the authentic flavour has developed.

Stage 6: Cooling (at minutes: 9.36-15.00)

The bean must be cooled quickly to stop the roasting process.

[1]       A. N. Gloess et al., “Evidence of Different Flavour Formation Dynamics By Roasting Coffee From Different Origins: On-Line Analysis With PTR-ToF-MS,” Int. J. Mass Spectrom., vol. 365–366, pp. 324–337, 2014.

[2]       J. Daniel Bustos-Vanegas et al., “Developing Predictive Models For Determining Physical Properties of Coffee Beans During The Roasting Process Kinetic Charcoal Cooling: Computer Simulation and Technological Applications View Project Harvest Process View Project Developing Predictive Models For Determining Physical Properties Of Coffee Beans During The Roasting Process,” Ind. Crop. Prod., vol. 112, pp. 839–845, 2018.

[3]       R. Eggers and A. Pietsch, “Technology I: Roasting,” Coffee Recent Dev., pp. 90–107, 2008.

[4]       L. Poisson, I. Blank, A. Dunkel, and T. Hofmann, The Chemistry of Roasting-Decoding Flavor Formation. Elsevier Inc., 2017.