Coffee roasting is a process during which various chemical and physical changes take place inside coffee beans due to heat, and thereby the structure of the coffee transforms, and so does its taste. The changes that occur inside the coffee can be connected to a certain temperature, so the easiest way to control and monitor the process of roasting is by measuring the time elapsed and the internal temperature of coffee. However, this is feasible only when using industrial coffee roasting machines. You need to find a different method when using a roaster designed for home use.
During roasting, the internal structural transformations of coffee are accompanied by a variety of external alterations. The colour, oiliness, and scent of coffee transform. The coffee beans crack at two stages of roasting that are clearly audible (the first crack is louder, it sounds like popcorn popping, the second is fainter, just some little snaps). When roasting at home, you should observe these external changes, and determine the wanted degree of coffee roast accordingly.
Three main types of roasts - light, medium, and dark - are differentiated in coffee roasting, and within that additional categories are defined (their names vary by regions and counties). The degree of roast is not determined by colour, but on the basis of which stage the coffee reaches during the process of roasting. Evidently, the longer the coffee roasting takes, the darker colour is obtained. However, this colour may differ by coffees, thus the colour cannot serve as a relevant reference point for defining the roast level.
Now let's see the main phases of coffee roasting!
1. The heating of green coffee
Green coffee begins to absorb heat, as a result of which it turns light yellow. At this point, we do not smell any particular scent.
The yellow beans swell slightly and start turning brown slowly. Some silver skin (chaff) is released from the coffee beans.
3. The first crack begins
The coffee starts giving out sounds similar to popcorn popping. The surface of coffee beans is wrinkled, their colour is pale, and the browning subsides. Normally, one can observe very diverse, light and dark beans at this level of roasting.
4. The first crack ends
The frequency of cracks decreases. There can be several seconds between the final cracks. The surface of the beans gets smoother, and one can smell a pleasant, sweet scent. The coffee obtains a nice, even, mid-brown colour.
5. Between two cracks
The coffee smells stronger, and more aromatic. The beans get browner slowly but steadily, their surface is shiny, but not oily yet. The weight loss (due to loss of moisture content and other reasons) is between 13 and 18%, as a result of which the spinning of coffee can accelerate within the coffee roasting machine.
6. Second crack
The cracks re-start, but with quieter, shallower pops. The roasted scent gets stronger and stronger. Oil appears on the surface of coffee beans while their colour darkens further.
7. The second crack ends
The surface of coffee is now glossy, oily, and its colour gets slowly similar to black. There is a strong roasted, burnt smell. One should not roast the coffee darker than this, because there is a risk that it will catch fire.
For a better understanding of the relationship between the roasting process and roast levels we have made a diagram where we also indicated changes to the distinctive traits of coffee, such as flavour/aroma, acidity, and body.
Depending on what kind of coffee one would like to drink, the roasting should be stopped at different stages. Generally speaking, the light roasting (Cinnamon, Light roast, American) is characterised by acidity and light body, and this is the stage where the taste characteristics of coffee varieties can be savoured the best. The most popular roast level is the medium one (City, City+, Full City) as the acidity of the coffee is already suppressed by the sweetness of caramelized sugar, the coffee is adequately full-bodied and has a nice roasted flavour, but the taste characteristics of coffee types are still present in their flavour. Darker roasting (Full City+, Vienna, French, Italian) results in full-bodied coffee with roasted flavour and low acidity. The latter is usually recommended for espressos.
When tasting roasted coffee one should always note that the quality of the coffee drink (i.e. the cup quality) depends not only on kind of roasting, but also on the quality of raw coffee (the place of origin, type, and processing method), the fineness of the grind, and the brewing method.