The Chemistry of Coffee Roasting

Nothing affects the flavour of coffee more than how the green bean is roasted. If you were to grind and brew raw green beans, it would not taste like coffee. Roasting is the key stage where the characteristics of the taste, aroma & final flavour of the bean is developed. The length of time of the roasting process also determines whether the coffee will end up being a “cinnamon”, “city”, “espresso”, “French” or “Spanish” Roast. In simplified terms, coffee is described as “light”, “medium” or “dark” roasted.

As heat is applied to the bean a chemical change in the physical structure and composition occurs. Water evaporates, starches convert to sugars, and the sugars caramelize. The beans increase in size from 25-35%. Throughout the roasting process, the hard shell of the bean begins to crack, or pop like popcorn. They lose 15-22% of their weight, mostly through this evaporation.

Gradually the green beans turn a yellowish colour, then darken to a deep rich brown. It is during this colour change that a number of chemical reactions occur, causing the beans’ sugars and proteins to interact with each other. It is these changes, and the release of caffeol, or coffee oil, that are essential in bringing out the flavour and aroma of the beans. The darker the beans are roasted, the more oil they produce.

Flavourful acids form as the beans turn into a medium-dark roast. As the roasting progresses toward a darker roast, these same acids will now begin to break down, and the sugar components will start to caramelize. A darker roast has more body and an intense richer flavour to the palate. That is why espresso beans are characteristically low in acidity, rich in body, and sometimes caramel-like (caramel, after all, is just roasted sugar).

The lighter the roast, the more flavour acids, resulting in interesting flavours and sparkle. Lighter roasts are lighter in body because the roast has not produced caramelized sugars or caffeol.

Medium roasts have less acidic snap; they are richer, with a more rounded flavour. Here, coffee oils begin to appear.

At the dark roast stage, all acidic tones disappear; the beans are oilier; there is a definite bittersweet, chocolaty flavour; the brew is rich and full in body and texture.

Roast levels also affect caffeine levels. As a general rule of thumb: the lighter the roast, the more caffeine is retained. Roasting darker essentially burns off more caffeine.

Knowing Your Coffee Roasts

Do you get confused when hearing about different styles of roast coffee?

Many people grind their beans themselves, to ensure a fresh and flavourful cup of coffee. But not as many people roast their own. Whether you are a home-roaster or not, knowing the different roast levels and their taste characteristics can be helpful when purchasing coffee.

In general, lighter roasts are sharper and more acidic than darker roasts. Darker roasts have a fuller flavour. Beans that have been over-roasted will take on a burned, smoky or charcoal flavour. Also, there is less caffeine in the darker roasted coffees than in the lighter ones. The roast alone doesn't determine the resulting coffee taste or quality. The origin of the beans makes a big difference. A bean from Ethiopia will taste differently than a bean from India, even if they are both French roast.

What exactly does roasting do?

The sugars, fats and starches that are within the bean are emulsified, caramelized and released. This creates the delicate coffee oil. This oil is what gives coffee its distinctive aroma and taste.

Roast Levels Explained

These are the basic roasting classifications used by professionals to designate the darkness of roasts. Many are used interchangeably, so be careful.


Cinnamon, New England, Light, Breakfast


American, Medium Brown, City, Brown

Medium Dark

Full City, Vienna, Velvet


Italian, Espresso, European


Espresso, Italian, Continental

Very Dark

French, Dark French, Spanish

Cinnamon: The bean is light brown, and dry (no oil visible). The flavour is baked or "bready", like toasted grain. There will likely be definite sour tones. There is not much body in cinnamon roasted coffee.

New England: A term not as frequently used as the others, though this roast is apparently common in the eastern United States. It's a little darker than the cinnamon roast, but without the grainy flavour. New England roast will still have some sour tones to it.

American, Light: Medium light brown beans. This roast is the norm for eastern USA. This roast (and sometimes cinnamon as well) is the most often used for cupping or professional tasting.

City, Medium: The colour is darker still, more of a medium brown (think chocolate). This roast is common in the western parts of the USA. This roast is a good choice to taste the differences between varietals.

Full City: Medium dark brown beans. The beans will start to show some oily drops on the surface with this roast. Full City will have caramel or chocolate undertones.

French, Espresso: Beans are starting to get dark brown, and French roasted beans are shiny with oil. There is less acidity, but with burned undertones. This roast is often used when making Espresso. Many people think this is the darkest roast available, but that's not true.

Italian, Dark French: Similar to regular French, but more darker and oilier looking, and with a stronger flavour.

Spanish: Darkest roast of all. Colour is nearly black, beans are extremely oily.

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