As an avid coffee drinker, I set out to learn all the ins and outs of coffee roasting so that I could share them.
In simple terms, coffee roasting uses heat to turn raw green coffee beans to a form more suitable for brewing and drinking.
Read on to learn where coffee comes from and all the details on the coffee roasting process. I’ll discuss the roasting process, equipment used to roast, resulting roast types, and what happens once roasting is complete.
Where Does Coffee Come From?
The form of coffee we know and love is a lot different than what it looks like in nature. Coffee is actually the seed from a coffee cherry which grows on a small tree.
As a brief overview, once the cherries are ripe they’re harvested. The skin and flesh are removed to reveal the seed, aka the coffee bean. The beans are then dried using various methods and shipped out to roasters as raw green beans.
What is the Process of Coffee Roasting?
The coffee roasting process essentially is the same general concept as cooking many other food items – you add heat to the raw green beans for a certain amount of time and then allow them cool before brewing.
Sounds simple, right? Not so fast.
Roasting coffee is an art as much as it is a science. Two batches of beans roasted in the exact same manner can produce different tasting coffee. This is because many factors affect the roasting process and ultimately the flavor. These factors include the species of coffee bean, processing method, moisture content, density of the beans, roasting environment, roasting equipment, etc.
To complicate the situation even further, coffee beans have anywhere from 850 to over 1,000 flavor and aroma compounds. These compounds develop through various chemical reactions as the beans are roasted through the different temperature ranges. Being able to successfully capture the desired flavors (on a consistent basis) that make a delicious cup of coffee is the essence of being an expert roaster.
Later we’ll discuss the equipment used for coffee roasting. For now, what’s important to know is that while heating the beans, they must be constantly stirred or tumbled to allow them to heat (and develop) evenly.
Let’s take a look at the details of each step of roasting.
- Drying – The beans will begin to dry as the temperature is increased above room temperature. The moisture content in the beans will decrease as the water exits the beans as steam. Over the course of the entire roast, beans can lose about 10-25% of their water. During drying, the beans will start to become a yellow color.
- Browning – Around 320°F the beans will start to turn brown as they undergo the Maillard Reaction. This reaction occurs in many different foods and is a reaction between the amino acids and sugars in the beans. This step creates very appealing aromas, such as that of fresh bread. All reactions occurring in the coffee beans up to this point are endothermic (heat absorbing).
- First Crack – The coffee will continue heating beyond the initial browning step. The reactions that occur in this step are exothermic (heat releasing). Around 385°F, the First Crack of the beans will happen. This sounds much like the popping of popcorn. At this point, the beans are considered a light roast.
- Development – After the First Crack, the coffee beans will continue heating and reacting. During this step, time is critical in determining the roast. An additional roasting of only 30-60 seconds can change the flavor! Oil can start to develop on the surface of the bean. Medium and some medium-dark roasts develop during this step.
- Second Crack – Around 435-437°F, the beans undergo a Second Crack. At this point the beans are well-roasted and dark. The surface of the beans will be noticeably oily. Medium-dark and dark roasts develop during this step.
- Carbonization (Burning) – If the roasting continues and/or the temperature is increased above 482°F, the beans will start to burn and the batch can be ruined.
These steps all occur over time and at different temperature ranges (see the next section for details on different coffee roasts). Graphing the temperature of the beans over time through these steps creates what is known as a roast profile.
Roast profiles are used by roasters to help perfect a roast. The shape of the profile looks like a checkmark. Tweaking the above roasting steps can affect the entire roast.
What are the Coffee Roast Types?
With all the different roast names out there it can be a bit confusing. As mentioned earlier, there are unroasted beans and then four general types of roasts, but beyond that there isn’t much standardization.
The roast types are based on color. Roasters use different methods to determine the roast. Three main methods are using the naked eye, spectrophotometer (such as one from the company Agtron), or temperature monitors placed inside the roasting beans. Combinations of these methods and others are used by roasters.
The naked eye and a spectrophotometer both use the appearance of the beans to determine the roast. Experience is needed to accurately determine roasts by the naked eye, so a spectrophotometer can be a better, more precise tool depending on the roaster. There are general ranges of Agtron values to go by when using that equipment.
Let’s dive into the details of each roast.
| Roast Type | Temperature Range | Agtron Value Range | Bean & Coffee Characteristics | Roast Names |
|---|---|---|---|---|
| Unroasted | <329°F (drying) | N/A | •Raw (room temperature) or dried (up to 329°F) •Grassy aroma | •Green |
| Light | 385-401°F | 80-70 | •Highest acidity •Highest caffeine content •Surface of bean is dry (no oil) •Light-bodied coffee •Light flavors (toasted grain, grassy) •The bean’s origin flavors are recognizable | •Light •Light City •Half City •Cinnamon •New England |
| Medium | 402-428°F | 70-50 | •Lower acidity •Slightly less caffeine content •Surface of bean is mostly dry with little to no oil •Medium-bodied coffee •Balanced flavor •Bean’s origin flavors still present •Popular roast in America | •Medium •City •Regular •American •Breakfast •House |
| Medium-Dark | 429-446°F | 50-40 | •Even less acidity •Even less caffeine content •Surface of bean is slightly oily •Full-bodied coffee •Rich flavor (caramel, little spicy) •Bean's origin flavors mostly gone | •Full City •Vienna •Light French |
| Dark | 447-482°F | 40-25 | •Little to no acidity •Lowest caffeine content •Surface of bean is very oily •Full-bodied coffee, but thinner-bodied at higher temperatures •Bittersweet, smoky, and burnt flavors •Bean's origin flavors are gone •Popular roast in Europe | •High •Continental •New Orleans •European •Viennese •Italian •French •Turkish •Spanish |
Temperature ranges and Agtron values are approximate and can vary between roasters. If the beans are roasted above the 482°F upper limit, they’ll start to burn (specifically their oil and sugar content).
One other important point to note is that many people think that the darker the coffee, the more caffeine it has…while the opposite is actually true! The darker the roast, the less caffeine it has.
What Equipment is Used to Roast?
The two most popular types of machines used by coffee roasters are drum and hot-air (aka fluidized bed) roasting machines. These are used by both commercial and home roasters.
Drum roasters are heated rotating horizontal drums. The heat source is either indirect (the drum itself is heated which then heats the beans) or direct (an open flame heats the drum and is actually in contact with the beans inside the drum). As the drum rotates, the beans roast as they continuously tumble inside.
Hot-air roasters contain the beans in a chamber which has a perforated plate/screen on the bottom. The machine pushes hot air through the chamber to keep the beans suspended inside the chamber. The beans roast as the hot air passes by them.
Roasting is slower with a drum roaster as opposed to a hot-air roaster. This is due to the different types of heat transfer between the two roasters. Both utilize heat transfer by convection (heat from the air to the beans), but the drum roaster also utilizes conduction (heat from the drum to the beans).
Because the methods of heat transfer are different between the two machines, they’re able to produce different roast varieties.
For home roasting equipment, there are three typical types of equipment/methods: stove-top, popcorn machine, and home roasting machine (typically a hot-air type roasting machine).
Keep in mind if you decide to do home roasting that there is potential for smoke or other emissions to be produced when roasting. Make sure your roasting area is well ventilated and you do your homework on any risks for your chosen method.
What Happens After Roasting?
Once roasting is complete, the work isn’t quite finished. The beans are completely cooled since they continue to roast as long as their temperature is elevated.
The beans can be stored once they reach room temperature. They can release carbon dioxide for multiple days after roasting. The release of gas can cause beans to almost double in size.
The beans have to either sit open to the air or be stored in bags with pressure relief valves to allow the carbon dioxide to escape.
Since oxygen and light make the beans go stale, foil-lined bags with pressure relief valves are the preferred storage method. Learn more by checking out our article on how to store ground coffee.
What to Do Next?
Hopefully this gives you a good rundown of the coffee roasting process and the different roasts available. When you’re drinking your next cup, see if you can taste the flavors listed for your roast!
If you’re looking to be adventurous, we recommend picking a few roast varieties that sound appealing and trying them out. You might be pleasantly surprised with a roast you never considered before!
If you decide you want to roast at home, you’ll be able to explore all kinds of different varieties. Just be sure to roast safely, as you can burn the beans if they’re roasted too long.
