As a roaster you may encounter coffee beans coming from different regions of the world and as similar as all the beans might appear at a glance (particularly to a layperson) a roaster knows how big of a difference it is.

Around the globe several countries produce coffee. These are all located somewhat along the equator often referred to as the “bean belt”. But too hot weather (as the word equator brings to mind) and Arabica; the high quality bean won’t grow. So here high altitude plays an important role in Arabica production as well as its quality.
The colder the climate available – harder the coffee beans turn out to be. That’s because the fruit (coffee cherry) on the coffee tree matures relatively slowly allowing more sugar development and so; richer the taste turns out to be in the beans.

But altitude is not the only factor that affects the flavor of course. Varieties and cultivars vary in the flavors too. Bourbon coming from the same region has different notes than Geisha. Then there are processing methods that too have an effect on the coffee flavor.

All factors combine and the roasters face the challenge to skillfully roast batch of beans that are different from one another. So there is no one size fits all approach to coffee roasting (as it is with so many other things in life) and here are a few insights from experts in the field on what different stages of roasting exactly do to the beans and how they might be used to bring out the best a coffee has to offer.

So first when green beans are fed to a roasting machine they start to absorb heat and lose moisture. The greenish shade of the beans start turning into pale yellow and the detectable smell is kind of grassy at this point. This can be referred to as the initial drying stage of the roast and at this stage not increase in size of the beans can be observed neither the Malliard reaction has begun.

Malliard reaction can simply be explained as the reaction among sugars and amino acids that happens during cooking – say meat or roasting coffee beans in this particular case. Toasty aromas and brown color are what we get to smell and see when the milliard takes place. But before all this happens coffee beans are still in their drying stage and extending that stage can highlight the sweetness a coffee has to offer.

In contrast extending the roasting time (the later stage) begins to turn the sweetness into bitterness and carefully extending the roast can result into more caramel like sweetness. Whereas extending the initial drying stage brings out sweetness without caramel like notes.

As the roast continues coffee beans absorb more heat, color starts shifting from pale yellow towards more tan – light brown shades and the aromas become toastier. Beans also expand in size. This is because the inner moisture of the beans is considerably heated by now and not only that causes the beans to expand physically but also while escaping, hot steam; crack open the beans – leading to audible popping; referred to as the first crack.
The popping sound is pretty much similar to popcorn popping. There are two cracks that occur while roasting coffee. Second crack is audible too but is subtle than the first crack. During the first crack it is the bean that cracks opens while during the second crack it is the structure of the seed/beans starts to develop cracks.

Another important change that occurs during the first crack is the caremalization of the sugars. It is important to keep in mind at this point the beans begin to give off heat that is; the process becomes exothermic now but it is important to keep the heat supply going into the roast as well. If at this point roast is stopped beans quickly become endothermic that is; start absorbing heat. If there is no external heat supply at this point the roast will stop and the developed flavors will be “baked”.
On the other hand if the first crack is extended and the internal bean temperature is maintained properly (between 340°F and 400°F) it gives rise to a more syrupy mouthfeel.
The first crack then proceeds into the second crack as the beans absorb more heat and the internal bean temperature gets somewhere around 446°F. The longer the coffee is roasted at this point the more the acidity gets muted eventually. Because of caramelization sweetness begins to move from malty to candy and then proceeds into the cocoa like bitterness.

With varying coffee variety, origin and altitude the bean matrix or material also varies and so what part of roasting process can develop the best flavor profile for a coffee can only be understood with time and experience. It is best to do the due research when experimenting with a new coffee like where is it from and what other roasters have to say about it and using that as a base experiment with extending different parts of the roast.