After the green beans have been fermented and extracted from the seed, the next step in making coffee is the coffee roasting process. The beans are slowly dried so that they become yellow in color and give off an odor very similar to toast or popcorn. When the heat reaches 205ºC, the bean will double in size and crack. AS the temperature slowly rises to 220ºC, the color starts to change to light or dark brown. BY this time, the bean has lost about 13% of its weight.
The second step in the roasting process is to increase the heat to 230ºC when the beans will crack a second time and become medium brown in color and look oily. During the heating from 170º upwards, the sugars in the beans start to caramelize. The darker the beans turn in this process, the more they caramelize. This is the most important part of the roasting process. Too much caramelization will result in coffee that tastes too sweet and not enough will produce bitter tasting coffee. The roasting process should end after the first crack and a just a little past the second crack.
You need to watch the coffee beans carefully when you roast them yourself. When the oily sheen appears on the beans, this is the development of caffeol, the oil that give coffee its aroma and taste. Roasting the beans too much after this appears has a great effect on how the coffee tastes when you make a brew. For espresso, it is important that the roasted beans be just right. They should not be too light nor too dark.