What happens in the process of roasting coffee beans?

Roasting is something that is easy to go wrong with, especially if you are less experienced or just starting out. Roasting beans is a skill on its own, but as with any skills, it can be learned. However the end result doesn’t depend only on the process itself. If you want to get a good roast you have to start with beans that have been skillfully selected and dried.

There are different ways to process beans. Some bean processors use the wet process method where they use a wash to remove the fleshy fruit from the bean and to separate different kinds of beans. Density differences in the bean will cause some to float higher, making for easier removal or separation.

Others use a slower, more expensive dry-process also known as unwashed or natural coffee. In dry process coffee cherries are dried in the sun and then the dried skin, pulp and parchment are removed from the beans. It may take up to 4 weeks before the cherries are dried to the optimum moisture content. Beans, processed this way will have a more subtle acid profile, while the acidity of wet-processed beans is more striking. Some acidity in coffee is desirable. The alternative is a flat, lifeless cup and who wants to drink coffee like that?

Now let’s take a look at what happens to beans as they heat up during roasting?

During the bean roasting process aromatics and acids, along with other flavor compounds, are produced in different concentrations.

In the first stage the beans absorb heat and the green beans are slowly dried to a yellowish tinge. ‘Green’ doesn’t necessarily refer to the color as such, but simply to the beans that are unroasted or raw. If done properly, the beans will have an odor that is similar to a toast or popcorn.

From about 170°C-200°C (338°F-392°F) sugars in the bean will begin to caramelize, aided by the increase in temperature of the moisture enclosed by the skin. That’s just one reason why it’s important for beans to have the proper moisture content, which comes from correct drying. Caramelized sugars are less sweet, so reaching the right amount is important for the final brew.

At about 205°C (400°F), beans will expand to approximately double their original size and become light brown and at the same time lose about 5% of their original weight. As the temperature rises to about 220°C (428°F), beans will lose about 13% more weight and release some carbon dioxide (CO2).

When the temperature increases to around 230°C (446°F), the roasting beans become medium-dark brown and take on an oily sheen. Oftentimes there will be a loud pop as the beans enter the ’second crack’ phase.

Here roasters have to be very careful not to overdo it. Volatile aromatic compounds are boiled off and the oils on the outside of the bean can combine with oxygen in the air. That process can strip the bean of desirable flavors and lead to a burnt taste.

The goal is to reach proper balance of bitterness, acidity and a host of other attributes making up the final flavor profile.

In tasting guides coffee connoisseurs will sometimes see the term ‘body’, as if its meaning were self-evident. ‘Body’ despite what it suggests, does NOT refer to the actual thickness or viscosity of the liquid. That attribute is the result of the kinds of proteins and fibers in the brew.

Used as tasters do, it refers to the feel on the tongue when rubbed on the roof of the mouth. It’s the result of the fat content in the drink and that – apart from growing conditions that home roasters can’t control – is determined largely by the roasting.

Too light a roast will leave too high a concentration of bitter compounds in the final product. Too dark will produce an excessively chocolatey, burnt taste. You have to do a little bit experimenting here to find the right balance that suits your taste. That is also the main benefit of roasting the beans yourself – you can get them exactly the way you want them. As the saying goes – practice makes perfect. So practice and experiment – it’s worth it.