Feel the heat! An introduction to the art of coffee roasting

Coffee is familiar to us all: aromatic, dark-brown beans, smooth to the touch, releasing a bouquet of flavours and aromas when covered with hot water. Yet the fresh Arabica or Robusta beans that arrive at our roastery are not wonderfully fragrant — quite the opposite. Their coffee essence is hidden deep inside, and roasting is what brings it out.

Why roast coffee?

Roasting is the process that creates the coffee we know. In the roaster, green beans containing only a small number of aromatic compounds become a sensory powerhouse, with around 700 such compounds developing. Heating the raw beans transforms their chemical components, including amino acids, carbohydrates and sugars. Through wonderfully named processes such as the Maillard reaction, aromatic compounds develop that create the flavour and fragrance associated worldwide with the magical black brew we prepare in an elegant dripper.

Hearing the colours change

The most obvious change during roasting is, of course, colour. Beans turn yellow, then gradually browner, revealing an increasingly developed roast. Their size and shape change too. Interestingly, we can observe the process without looking inside the roaster, because the changes can also be heard.

As temperature rises, pressure builds inside a bean until it reaches a critical point and causes a characteristic crack, rather like popcorn popping. Gases released during the first and second cracks tell the roaster that further aromas are developing. Unfortunately, the distinctive aromas of roasted coffee come from volatile substances that soon escape. To enjoy the full aroma, brew your coffee within a few weeks of roasting!

The roaster's secret

Roasting green coffee requires extensive knowledge, experience and often an individual “coffee instinct”. Why? Coffee is a highly individual raw material: different beans need different roasting conditions. We sometimes joke that a coffee roaster, like a bomb-disposal expert, gets only one mistake! Even a small adjustment can completely change the beans' flavour.

Roasters are often reluctant to reveal the settings that brought the most delicate blackcurrant notes out of a Kenyan Nyeri. Like the secrets of medieval guilds, the techniques of master roasters are closely guarded. At night, the roasters themselves are locked in soundproof safes in the roastery basement... or so the story goes!

How to roast without burning out

Despite the mystery, some principles are familiar. Small, hard beans grown high in the mountains may need a higher temperature and shorter time in the drum than softer lowland beans, such as Brazil Santos. Incorrect settings on the control panel — shortening the roast by two or three minutes or raising the temperature by just 2°C — can leave coffee too acidic, insufficiently sweet or simply over-roasted and too bitter.

Mastering roasting often takes years. Matching the conditions to the coffee to develop the most interesting bouquet is an art we are proud to refine at BLUEBERRY ROASTERS.

Each roast lets us create coffee with a new flavour profile that might win over our customers' taste buds. Want to know how roasting influences flavour? Below is a handy guide you can use when asking BLUEBERRY ROASTERS about our coffees. We hope you will return to it often!

The basic ways to roast green coffee

LIGHT — beans are roasted at relatively low temperatures, up to around 200–205°C, preserving their natural acidity and fruity aromas. Light roasts are also associated with a lively caffeine kick. You enjoy our African coffees roasted this way because their fruity, floral notes are a central attraction.

MEDIUM — a slightly higher temperature, around 210–220°C, produces a cup with an almost ideal balance of bitterness and acidity. This is one of the most popular approaches, highlighting the complexity of many coffees' aromatic bouquets. It is often described as offering a gentler caffeine character than lighter roasts.

DARK — this is truly “playing with fire”, as it is easy to over-roast the beans. At around 220–250°C they become almost black and visibly oily. As roasting intensifies, acidity becomes less noticeable and bitterness grows more pronounced, while the brew is traditionally associated with a lower caffeine content.