What roasting does to a bean
A green bean is about 10 to 12 percent water. As the bean heats, its sugars and amino acids react and turn it brown. Steam builds up inside until it pops the cell walls, like popcorn. That pop is first crack. Roast on and the brittle bean snaps again, more softly: second crack. Over the whole roast, the bean roughly doubles in size.
How light roast tastes
With a light roast, the bean itself does most of the talking. Where it was grown shows through in the cup. Light roasts also hold on to more of the bean's chlorogenic acids. In one study, a light roast kept about half of them. It also lets its gas out slowly after roasting. Many roasters rest a light roast for a few days before brewing it.
How dark roast tastes
The roast flavour takes over past second crack. Medium-dark roasts lean to chocolate and nuts. Further on, smoke and bitterness come forward. In one study, a French roast kept almost none of the bean's chlorogenic acids. Body fills out up to about medium-dark. Roast much further and the coffee can taste baked. The oil on the surface also goes stale faster.
| Light | Medium | Medium-dark | Dark | |
|---|---|---|---|---|
| Roast ends at | About 196-205 °C, around first crack | About 210-220 °C, before second crack | About 225-230 °C, start of second crack | About 225-250 °C, into or past second crack |
| Colour and surface | Light brown, matte, no oil | Medium brown, dry | Dark brown, some oil spots | Shiny, near black, oily |
| Typical taste | Bright, fruity, floral | Balanced, sweeter, caramel and nuts | Heavier, bittersweet, chocolate | Low acidity, bitter, smoky |
| Often used for | Pour over and other filter | Filter or espresso | Espresso, French press | Espresso |
Caffeine, by weight and by scoop
Dark roast does not have more caffeine. By weight, the two are nearly the same. If anything, light roast is slightly higher. The scoop is where the difference sneaks in. Dark beans are bigger and lighter. So a scoop of dark holds less coffee. Less coffee means a little less caffeine. If you care about the number, weigh your beans. Studies disagree on exact milligrams per roast.
How to choose a bag
Names on the bag are not standard. One roaster's medium is another's medium-dark. Words like City or French mean different things in different shops. Even the colour of the beans is only a rough guide. Light roast usually shines in a slow filter brew like pour over. Espresso usually suits medium or dark. A light roast in espresso can come out sour. For a French press, medium-dark works well.