Chemistry of coffee roasting

What actually happens when you roast your green beans? If you fill up your roasting chamber, set the roasting time and heat and press start, you and I are roasting alike. I have a felling about the roasting time, for example if I roast for press pot (14-15 min.) or espresso (16-18 min.) by 241 degrees. During the roast I add/cut additional time to the roast, by judging the color of the coffee beans. To ensure perfect results each time, you have to know the basic chemistry behind the roasting process, so you can navigate better.

Lets begin with the obvious, the outside of the coffee beans. This is usually the best known knowledge about home roasting and its a great place to start. When introducing the coffee beans to the heat from the roaster nothing appears to happen in the first couple of minutes, the beans simply remains grayish green. The beans will then attend to get a slightly yellow color and sending scent of grassy notes. This scent will change along with the roasting time and it will go from grassy to a scent of bread or grain. Finally the scent will change from bread/grain to a aroma of coffee along with the beans color slightly turning darker, this will happen within 2-15 min. depending on the volume of beans roasting and the intensity of the heat in the roasting chamber.

First crack, is a well known cause of the roasting in the coffee community and it is the definite start of the roast transformation. The coffee beans transformation is initiated by the beans internal sugars that are caramelizing and thereby making the bound up water inside the beans starting to split off from the carbon dioxide, causing the crackling sound from the beans.

Different roasting colors from www.sweetmarias.com

You can actually terminate your roasting progress after the first crack but I would not suggest that because it will leave you with a very sour cup. The above schedule shows some of the different color of roasted coffee beans, but the darkening color of the beans is an effect mainly caused by the caramelization of sugars. The caramelization of the coffee beans sugar is popularly called the coffee flavor oils and determines the balance of the cup, sour vs. sweet.

Second crack, is properly caused by the breakdown of the woody structures of the bean and sets a more powerful wave of crackling. The second crack is also a signal about the coffee beans suitability for espresso. A espresso roast is usually terminate just before, under or just after the second crack, because the beans are at the peak of depth, sweetness, aroma, body and complexity and low on acidity.

The outside characteristics of the roasted coffee bean is a great navigation tool for determine your roast, because it will be more precise than the set roasting time and heat. The inside characteristics is listed below in order of the roasting process.

1. Forces water out of the bean
2. The beans dry out and it expands its woody parts (making a reducing of the total weight of the bean by 14 to 20 percent).
3. Initiate a continuous transformation of some sugars into CO2 gas.
4. Drives off some volatile substances (incl. a small part of the caffeine). 
5. Caramelizes a portion of the beans sugars.

Finally most people do not know that coffee contains more substances then wine, and coffee contains 700 to 850 different substances with has been identified in late 2003. This does apply for the roasted coffee, but the green bean contains more than 2.000 substances, which makes it one of the more complicated of natural flavorings considered by food chemists. 

// Hendrup