From a lovely coffee cherry to a delicious smelling brown bean, the journey from the tree to your coffee pot produces a myriad of chemical changes. Roasters roast coffee in order to change the chemical makeup of the green coffee bean in a manner which produces the optimum amount of aroma and flavor. Aroma is the smell of the dry, roasted coffee. Flavor is what we call the taste of the coffee after it is brewed.
Roasting is an art which takes practice and precision. Different coffees require different methods of roasting them. The temperature when the beans are added to the roaster might need to be higher or lower than another coffee in order to produce the best roast. Also, the length of time that the beans are roasted will impact the flavor and aroma of the finished product.
When coffee is roasted, it goes through a number of changes. The green bean will expand in size. In fact, it nearly doubles in size for most coffees! The color changes during the roast from a green or yellow bean to a brown or black roasted bean. The coffee beans lose the moisture they contained as a green bean, therefore, the roasted bean is approximately half as dense as it was before roasting. As the coffee roasts, it actually becomes sweeter as the sugars within the bean caramelize. The longer the beans are roasted, the more the sweetness is roasted out. The beans also becomes more acidic which adds the lovely bright flavor that is so often desired in coffee.
While roasting, the beans actually smell like bread cooking! I was surprised the first time I tried to roast coffee. The room smelled more like a bakery than a coffee shop. After attending roasting classes, I learned that there is actually phase that is called the "bread phase" of the roasting process.
Roasting changes the bean significantly, but it is also what creates the body, aroma and flavor that we enjoy from each cup.