Pure chemistry: the science of coffee roasting
Placed in a hot roaster, a coffee bean undergoes a magical transformation through chemical reactions and emerges completely changed, both chemically and physically. This happens because we expose a coffee seed containing more than 900 chemical compounds to very high temperatures in a short time. Inside the roaster, coffee magic is happening!
Can you feel the chemistry?
Heating sets off hundreds of chemical processes and reactions simultaneously inside the small body of a green bean. Imagine a sudden fireworks display: in a single second, some substances are released, others break down and new ones form. At the same time, water within the bean turns to steam, rapidly increasing the pressure. It rises and rises until the bean's outer structure breaks with a loud snap, known as first crack.
This cracking releases newly formed substances. As the chemical compounds responsible for the coffee's characteristic aroma escape into the surroundings, we can smell spices, honey or chocolate — notes that may help us recognise coffees from South America, for example. During the processes now taking place, including the still-intriguing Maillard reaction, complex compounds such as amino acids undergo changes that produce simpler substances, including aldehydes, sugars, fats, acids, furans and melanoidins.
Interestingly, the flavour of the coffee depends on how these processes, including the Maillard reaction, unfold inside the roaster. Their duration and temperature help determine the development of sweet and acidic notes. Acids associated with sweet-and-sour flavours break down quickly at high temperatures. This is why coffees roasted for a long time at high heat lose the floral and citrus freshness that we particularly value in African coffees.
Where does that fleeting aroma come from?
Newly formed substances contribute to different aspects of the drink depending on whether they are volatile or non-volatile. Volatile compounds, which are released into the air, account for aroma; non-volatile compounds, which remain in the coffee, contribute to flavour and texture.
What can we find in our coffee? Caffeine, of course, which contributes to the drink's psychoactive effects and its influence on dopamine receptors, associated with the so-called happiness hormone. Caffeine levels change little during roasting, while subsequent extraction — brewing — affects how much reaches the cup. Contrary to popular belief, a light, delicate pour-over can deliver more caffeine than a powerful espresso.
The bean also contains important non-volatile substances: sugars contributing natural sweetness, fats influencing body, and melanoidins determining colour. Among the volatile compounds are aldehydes, associated with refreshing fruity aromas; furans, responsible for caramel notes; and guaiacol, which can give certain coffees a surprisingly sharp, peppery note.
From green to cinnamon: the colours of roasted coffee
Coffee also changes physically during roasting, most obviously in colour. At first, the green bean slowly turns yellow and slightly golden, over around ten minutes, before moving more quickly towards a light brown often described as cinnamon. Its colour indicates the stage of roasting and the reactions taking place.
The transition from green to yellow, for example, signals the breakdown of chlorophyll. Progressively darker browns are associated with melanoidins and the release of fats and volatile substances. Recognising the roasting stage from the bean's colour is a useful skill. By judging whether it is green, yellow or cinnamon-coloured, a roaster can control the development of the desired flavour profile.
As you can see, working with coffee beans in any roastery involves solid scientific knowledge. Organic chemistry, physics and biochemistry are just some of the disciplines that help explain coffee's magic. And since scientific facts are easier to absorb with a little stimulating caffeine in our system, explore the secrets of applied coffee science with a cup of our high-caffeine coffee in hand!