The Chemistry of Roasting
Roasting coffee is science just as much as it is an art. The science behind roasting is the chemical changes that occur to the aromatics, acids, and flavor components of the bean that are altered during the roast process which change the flavor, acidity, aftertaste and body of the coffee as desired by the roaster which is the artistic aspect of roasting.
The transformation from raw green beans to finished roasted coffee is a complex process in the production of coffee. When turning raw bean into roasted coffee the beans undergo a significant transformation that gives rise to hundreds of new compounds that we enjoy.
Green coffee has moisture content of 10%-13% but during the roasting process the heat applied to the beans reduces the moisture content by upwards of two thirds. 1 kilogram of green coffee is usually reduced to 800-900 grams of the final product, roasted coffee.
For the next steps I’d like to define a couple of words and how they relate to coffee:
Endothermic – Is occurs during the initial stages of roasting when the raw coffee beans begin to absorb heat from the roaster.
Pyrolysis – Is a step that occurs during roasting when the coffee is heated to a high enough temperature in which the flavor, aromatics, and color changes. Primarily it alters the sugar compounds in the coffee. Pyrolysis occurs on the outside and inside of the coffee bean. It is the cause of the light browning on the exterior of the coffee and internally it alters the flavor components as well as destroys toxic or unpleasant taste.
Exothermic – this is the opposite of endothermic where the coffee bean releases heat and results in additional pyrolysis.
During the first stage of roasting (1-5 minutes) of roasting the raw beans collect heat, the scientific terminology for this is called endothermic. The green beans are slowly dried to become a yellow color and the beans begin to smell like toast or popcorn.
As the beans collect heat they double in size. The growth caused by the heat forces the bean to facture and a cracking noise is heard, which we refer to as first crack. During this stage the noise heard sounds similar to kernels of popcorn transforming into popcorn.
This is followed by the initial stages of pyrolysis, the changing of the exterior color and internal flavor. The color changes from light brown to a medium brown.
Thereafter, an additional stage of endothermic occurs to the bean where they collect more heat during the roast process until an exothermic reaction occurs and the beans fracture again, this is called second crack. Second crack is less audible than first crack. Following second crack the beans enter a second stage of pyrolysis.
To sum the roasting process up
1) Raw beans undergo an endothermic reaction
2) Once the beans collect enough heat an exothermic reaction occurs known as first crack
3) The first stage of pyrolysis occurs which alters the color and flavor components
4) This is followed by the beans undergoing a second endothermic reaction
Once the beans collect enough heat, an exothermic reaction occurs which is followed by another stage of pyrolysis.