Coffee beans are originally seeds matured in coffee cherry. They are of green color and they don’t smell like coffee. Thanks to the roasting process, color changes and around one thousand different aroma compounds are developed, resulting in what we call coffee flavors.
There are three main stages in roasting : drying stage, browning stage and development stage.
1. Drying Stage
This first step begins as soon as the green bean coffee enters the roaster until it changes color. The coffee bean then changes from green to yellow, this is called the “Stucker reaction”. This phenomenon is explained by the evaporation of the water that was in the grain.
The humidity of a coffee bean is around 10%. It needs to be dry before the actual roasting starts. This stage typically takes no more than 10 minutes with traditional drum roaster. The temperature in the end of this stage is around 160 ⁰C. A special care to not burn the beans should be taken especially with drum roasters, which could produce too much heat in the beginning of the process.
2. Browning Stage
At 160 ⁰C the coffee starts to smell like toasted bread. Aroma precursors are thus starting to convert to aroma compounds. Drying continue to happen during this browning stage.
The browning process happens thanks to a chemical reaction known as the Maillard reaction. In Maillard reaction, hundreds of different aroma and color compounds are produced from reducing sugars and amino acids reaction. This stage is crucial for flavor development so the longer it takes, the better are the flavors. In the end of browning stage, the first crack happens and the coffee starts to pop.
The beans have changed color from yellow to cinnamon at the end of the browning stage.
3. Development or Roasting Stage
During development stage, the aromas that have been produced during the previous stage start to develop and stabilize. It is also during this stage that coffee gets its final color. Duration and temperature for this stage are determined by the roaster. If it is not properly controlled, the resulting coffee will taste smoky and its flavor will be too sharp.
Depending on the needs in terms of desired flavor, the development stage typically takes between 15% and 25% of the total roast time.
Coffee flavors quality will depend on many factors. One of the most important factors that impacts the result flavor is the roast intensity (or roast level, roast degree) or how much the coffee is roasted. In general, dark roasted coffee tastes more bitter, having rather burnt flavors, whereas light roasted coffee tastes more acidic, having rather fruity flavors. The more the roasting continues, the less fruity compounds we have. We then have more sulfuric compounds, which results in roasty and burnt flavors. Generally speaking, light roasted coffee retains the raw coffee character better.
With quality roasting, coffee beans will have a good balance of fruit acids, oils, caramelized sugars, and simple sugars.
Roasting Duration – Slow vs Fast
Although the roast intensity is the most important factor impacting the resulting coffee’s flavor profile, the duration of each roasting stage and the total duration of the whole roasting process is also a major factor. Roasting faster is better to get more aroma compounds however special care to not burn the beans!
There are cases, though, where fast roasting is not recommended, due to to roaster design or coffee’s characteristics. If we need to control which flavors to develop and don’t want to develop some flavors, we need to adjust the roast profile. As an example, acidity is a desired flavor but on espresso blends many people prefer low acidity. When roasting slower, we give more time to organic acids to break down and obtain a less acidic coffee. This is when slow roasting is recommended.