Roasting Coffee-The Technical Way
"The manner in which coffee is roasted is of great importance to its flavour. If roasted too little, it will be weak and insipid; if too much, the taste will be bitter and unpleasant."-Eliza Leslie, 1837, Directions for Cookery
You can use several methods for roasting coffee at home: in the oven, on the stove and by using an air-pop popcorn maker. You can even purchase an inexpensive home roaster.
Most of the information provided below comes from the Coffee Research Institute's website (coffeeresearch.org).
Coffee Roasting is a chemical process by which aromatics, acids and other flavour components are either created, balanced or altered in a way that should augment the flavour, acidity, aftertaste and body of the coffee as desired by the roaster (which is you in this case).
Roasting Coffee Beans
The first stage is endothermic*. The green beans are slowly dried to become a yellow colour and the beans begin to smell like toast or popcorn.
The second step, often called the first crack, occurs at approximately 205ºC (401ºF) in which the bean doubles in size, becomes a light brown colour and experiences a weight loss of approximately 5%.
In the next step, the temperature rises from 205ºC (401ºF) to approximately 220ºC (428ºF), the colour changes from light brown to medium brown and a weight loss of approximately 13% occurs. The resulting chemical process is called pyrolysis and is characterized by a change in the chemical composition of the bean as well as a release of CO2.
The second step is followed by a short endothermic period which is followed by another exothermic** step called the second crack. This second pyrolysis occurs between 225-230ºC (437-446ºF) and the roast colour is defined as medium-dark brown. The second pop is much quicker sounding and the beans take on an oily sheen.
Espresso potential is maximized in roasting when you maximize the sweetness and aroma of the coffee while minimizing the bitterness and acidity. Most people focus on the latter and therefore roast extremely dark, yet without sweetness and aroma, the espresso will never be palatable. This explains the unpopularity of straight espresso and the popularity of espresso based drinks where either milk or other flavours are used to replace the sweetness that was lost by roasting darkly.
From 170-200ºC (338-392ºF) the sugars in coffee begin to caramelize. From tasting pure sugar versus its caramelized component it is evident that uncaramelized sugar is much sweeter. The dark colour of coffee is directly related to the caramelization of the sucrose in coffee. Therefore, to maximize sweetness you want to minimize the caramelization of sucrose, yet you do not want to roast too lightly or bitter tasting compounds will not thermally degrade. Stop the roast somewhere between the end of the first crack and less than half way through the second crack. Do not roast well into or past the second crack. The Coffee Institute recommends a roasting chamber temperature somewhere between 205-215ºC (401-419ºF). If you roast it a couple of minutes past the second crack, the beans will be burnt and will be ruined. You can roast it as light or as dark as you like and there will be some experimentation on your part to get to your perfect roast!
Once you have completed your roast, you have to cool the beans. Place them on a baking sheet in a thin layer and let them cool. Place them in an air tight container and use them as soon as you can. Coffee tastes best when it's freshly roasted and used up quickly.
*endothermic means a chemical change that is accompanied by an absorption of heat.
**exothermic means a chemical change that is accompanied by a liberation of heat.