This is the main process that transforms the raw green beans into a bed of aromatic coffee beans that win the world around. Basically, there are 3 stages in coffee roasting: drying, roasting and cooling. Drying is when the bean loses its moisture and mass. A homogeneous loss of water content at a steady rate will form the base for a consistent subsquent roasting stage when heat transfer leads to browning effect (Mallaid reaction) and caramelization of sugars within the coffee beans. Depending on the roast style desired, the next stage, ie. cooling, has to kick in immediately to bring down the temperature as fast as possible to prevent the beans from further subjected to roasting within the bean cells. Beans can be cooled either with rapid cold air or water-quenching. Both methods affect the rate of eventual loss of carbon dioxide (aroma) and moisture content.
Technically speaking, a lot of events happen during roasting that requires a skillful roast master to manage the vast number of variables to carve out the final signature roasted coffee. Roasting is a process of heat and mass transfer with endothermic and exothermic reactions. At the beginning of roasting, not much can be observed on the surface of the beans because heat is slowly conducted from the surface towards the centre of the bean as the internal bean temperature reaches the evaporation of the bean moisture. As the cell walls within the bean structure are still firm at this stage, vapor build-up due to pressures makes the beans expand. Soon, the internal bean temperature increases faster than the external heat source, the stress from the pressure build-up overcome the tensile strength of the beans, resulting in the 1st series of cracking sound. From this point onwards, the roast process can stop depending on the required roast style. Here it is important to increase the heat transfer gradually and step-wise to maximize the formation of the volatile compounds. Conventional roasting determines the end of roast based on the sound, smell and color intensity of the roasted beans. This reliance on the senses demands a very experience roaster to accurately cut the heat source at the right time. With technological advent, roasters are now deploying multi-stage and profile roasting to not only pull off the best desired roast accurately, but able to achieve this consistently throughout the batches.
In summary many changes happen to the bean in roasting:
Color – Changes from pale green to whitish-yellow to different shades of brown
Bean surface – Texture from hard moist to wrinkled to dry shiny with taints of oil depending on the roast style. Appearance of oil or shiny surface indicates a dark roast with a shorter shelf-life
Mass and volume – Mass is greatly reduced to almost half, whereas volume increases up to 80%.
Structure of bean – Hard, firm cell walls to porous and brittle structure due to large amount release of carbon dioxide
pH value – Increases from 4.9 to more than 5.4 depending on roast style (light to dark), roasting method (slow or fast) and coffee bean process (dry or washed)
Volatile content – Aroma reaches its maximum at medium roast
