How Coffee Roasting Works
From Green Seed to Brown Bean
Green coffee beans smell like grass and taste of almost nothing. Roasting turns them into the brown, aromatic beans you buy, creating hundreds of flavor compounds in about 10 to 15 minutes. How far a roaster takes the beans, from light to dark, decides much of what your coffee tastes like, often as much as where it was grown.
This article explains what happens in the roaster, how roast levels are named, and what “espresso roast” means. For each level in detail, see light , medium and dark roast.
What Happens in the Roaster
Most roasters use a rotating drum heated from below. The beans tumble through hot air and go through clear stages:
- Drying (first few minutes): water in the beans evaporates. They turn from green to pale yellow and smell like hay or bread.
- Browning: sugars and amino acids react (the Maillard reaction, the same one that browns toast and seared steak) and sugars begin to caramelize. Color deepens to tan and brown; sweet, nutty aromas appear.
- First crack, at roughly 385–400 °F (196–205 °C): steam and gas build up inside the beans until they pop, like popcorn but quieter. From here on, the coffee is drinkable. Light roasts end shortly after this.
- Development: the time after first crack. Acidity softens, sweetness and body grow. Medium roasts end in this window.
- Second crack, at roughly 435–445 °F (224–230 °C): a faster, crackling sound as the bean’s structure starts to break down and oils rise to the surface. Dark roasts end here or beyond.
Temperatures vary between machines and thermometers, so roasters judge mainly by sound, color, smell and time.
Then the beans are dropped into a cooling tray and stirred with air to stop the roast quickly.
Roast Levels and Their Names
| Level | Common names | When it ends | Surface | Flavor |
|---|---|---|---|---|
| Light | Cinnamon, City, “blonde” | Shortly after first crack | Dry, light brown | Bright, fruity, floral, tea-like |
| Medium | City+, Full City, “breakfast” | Between first and second crack | Dry, medium brown | Balanced, sweet, caramel, nuts, chocolate |
| Medium-dark | Full City+, Vienna | Start of second crack | A few oil spots | Bittersweet chocolate, spice, heavier |
| Dark | French, Italian | Into or past second crack | Oily, very dark | Smoky, roasty, bitter, low acidity |
There is no official standard. One roaster’s “medium” can be another’s “light”, so read the tasting notes as well as the roast label.
Weight, Size and Caffeine
Beans lose water and mass as they roast, roughly 15% for a light roast to about 30% for a very dark one, and they swell and become less dense.
That explains a common myth. Dark roast is not stronger in caffeine. Caffeine survives roasting almost intact, so a dark bean has about as much as a light one. Because dark beans are lighter and bigger, a scoop of dark roast holds fewer beans and slightly less caffeine; weighed in grams, the two are close. The “strong” taste of dark roast is bitterness, not caffeine.
Tip: Weigh your coffee instead of scooping it. Your brew stays consistent whatever the roast level.
What Is “Espresso Roast”?
“Espresso roast” is not a roast level. It is a roaster saying: this is the roast we think tastes best as espresso. There is no standard behind it.
Traditionally that meant medium-dark to dark: lower acidity, more body and chocolate, which stand up to milk. Many specialty roasters now sell lighter espresso roasts, closer to medium, and some sell an omni roast meant to work for both espresso and filter.
You can brew espresso with any roast, and filter with an espresso roast. The label just tells you what the roaster aimed for.
Which Roast Should You Buy?
- You like fruit, flowers and bright acidity, black coffee: light roast.
- You want sweet, balanced, chocolatey coffee that works black or with milk: medium.
- You love bold, smoky, low-acid coffee, or drink it with lots of milk: medium-dark to dark.
If you are not sure, start with a medium roast, then try one step lighter and one step darker. Within three bags you will know where you sit.