What is coffee roasting?
Coffee roasting transforms green seeds into aromatic, flavorful beans by heating them through distinct chemical stages that increase solubility for better extraction.
- Roasting makes coffee soluble — green coffee seeds produce only grassy water without heat transformation
- Five roasting stages exist: drying/yellowing, Maillard reaction, first crack, second crack, and carbonization
- Roast level dramatically affects flavor — lighter roasts emphasize acidity and fruit notes, darker roasts develop chocolate and carbon flavors
- Modern roasters use spectrophotometers (like Agtron) to measure roast color and ensure batch-to-batch consistency
As we have written about in past articles, water is an amazing solvent. Maybe the most amazing solvent. However, it isn't amazing enough to break down a green coffee seed into anything better than grassy tasting water.
That's why we roast coffee.
A very, very brief history of food processing
Despite what you may have heard from fear-mongering, minivan-driving PTA board members, processed foods are not inherently evil. They actually may be responsible for homo sapiens developing large brains and evolving from our animal brethren. Sure, sure. There are many types of food processing. But cooking is the earliest and most prevalent form. About 1.8 million years ago, those wonderful half-monkeys threw a Mammoth on a spit and realized that not only did cooking make the food taste better, but also made it easier to eat.
Fast forward to the 15th century when some Turkish dudes cooked coffee seeds and macerated them in hot water. Modern coffee was born.
What is coffee roasting?
Coffee roasting is the process of heating the coffee cherry seeds to augment aroma and flavor and ultimately increase solubility. Why do we keep talking about solubility? Because it is the essential element of coffee brewing. The rate of solubility of compounds in a specific roast is the key to achieving the right extraction through temperature, time and grind size. It is the "unifying theory" of coffee geeks.
In order to get to solubility, this coffee is roasted, typically in a commercial roaster that is, according to Ed Kauffman (head of roasting at Joe Coffee Company) "a cross between a pizza oven and a clothes dryer. It's a big, metal cylindrical machine that turns like a clothes dryer, but it has flames underneath." However, coffee can also be roasted in a popcorn popper or a cast iron pan. Having just returned from the coffee show in Seattle, I would say that the majority of the innovation was occurring in the home or small batch roasting space.
Below is a coffee tasting wheel. On the right side, you will see a progression of flavor compounds that get developed as the roast progresses from lighter to darker - with light roasts showing more of the acidic qualities (citrus, malic acid, and apple flavors), medium roasts developing nut and chocolate qualities, and then darker roasts eventually turning to carbon.
What happens when coffee is roasted?
These areas of roast profiles occur along a temperature spectrum that will be unique to each of the beans by varietal, region, and altitude. However, all coffee will eventually go through five distinct stages:
- Drying or Yellowing: a crucial phase, according to former Water Avenue roaster (and Clive employee) James Holk. "This is the phase that will determine the overall batch time, since it is driven by the initial moisture content of the beans. It basically sets the foundation for all of the other stages, because it will be indicative of how hard or soft, fast or slow your roast is driven into first crack and beyond."
- Maillard Reaction: the first "browning" of the coffee. This reaction is the catalyst for the formation of many of the 1,000 volatile chemical compounds (i.e., compounds that are easily evaporated in the air, and therefore contribute to the aroma of coffee) that are created during the coffee roasting process. This process is most closely associated with the aroma of baking bread, as well. Both equally delicious.
- First Crack: while the early roasting phase is all about heat acting upon the coffee bean from the outside in (endothermic), "first crack" is the initial part of the exothermic reactions, where pressure from moisture evaporation and heat has built up inside the bean and begins to break the bean down from the inside out. It is an audible sound, like popcorn popping. However, it usually produces no cracks or explosions like it's buttery brethren.
- Second Crack: while no longer "cool" for mustachioed millennials, darker roasted coffee will sometimes be roasted through a second crack, which is nearing the full breakdown of the coffee.
- Carbonization: full breakdown. Fire hazard.
Roast Levels
While roasters of yore would roast coffee based on both the sounds and smells of the above phases, today's roasters have to be far more sophisticated in order to produce repeatability between batches. Not only do most commercial roasters come equipped with automation to repeat a specific roast profile, but almost all have a "light" that allows these artisans to specifically measure roast development.
Below, you will see a chart of coffees from a light roast, all the way through very darkly-roasted. There are lots of potential technologies that will allow roasters to understand these "colors", but the 800-pound gorilla is Agtron. It's a spectrophotometer that measures the relative light absorption of the surface of the roasted bean. Companies like Coffee Review use it in their reviews as a guide to allow consumers to understand what level of roast they prefer, rather than using the word medium-dark, which could encompass a whole range of development levels, depending on the roaster. In fact, they use two Agtron scores, one for the whole roasted bean and one for the ground bean. The combination is often most telling because it can be an indicator of roasting skill and consistency.
| Roast Color | Bean Surface | Agtron Numbers | Common Names | Look |
| Light brown | Dry | 80-70 | Light, Cinnamon |  |
| Medium brown | Dry | 70-50 | Medium, American |  |
| Medium-dark | Dry to tiny patches of oil | 50-40 | Full-city, Espresso |  |
| Dark brown | Shiny | 40-35 | French, Espresso |  |
| Very dark brown | Very shiny | 35-30 | Italian, Dark French |  |
| Black-brown |
Very shiny
|
30-25 | Dark French |  |
Chart courtesy of coffeereview.com
Mistobox claims that they use an algorithm that can match an individuals taste to a specific score, and send them only those roasts. Check out their article on coffee roasting!
In summary - Fire. Robots. Lasers!
What does it mean to me?
As we mentioned above, solubility plays the most crucial role in coffee extraction. As a roast becomes more developed, it becomes more soluble, as the compounds begin breaking down further, allowing them to more quickly bond with the water molecules. Whether this solubility is a good thing or a bad thing depends on the skill of the roaster. Either way, a darker roasted bean will need a lot less time to extract than a lighter roasted choice. So, we recommend dosing down a dark roasted coffee and limiting the extraction time.
We've made a detailed video that explains how to do just that!
Sounds delicious, now where can I get good coffee?
Our partners at Mistobox offer over 500 coffees. Filter by roast, origin or taste. Schedule a coffee subscription now so that your coffee arrives automatically when you need it.
Frequently Asked Questions
Why do we roast coffee instead of brewing green coffee beans?
Green coffee seeds are essentially insoluble in water and produce only grassy-tasting liquid. Roasting transforms the chemical structure of coffee seeds, creating over 1,000 volatile aromatic compounds and increasing solubility so water can extract the flavors we associate with great coffee.
What happens during first crack and second crack in coffee roasting?
First crack occurs when internal pressure from moisture and heat causes the bean to crack audibly (like popcorn). This marks the transition from light to medium roast. Second crack happens at higher temperatures as the bean structure breaks down further, producing darker roasts with more caramelized, smoky flavors.
How do roasters measure and control roast levels consistently?
Professional roasters use spectrophotometers like Agtron that measure light absorption on bean surfaces, assigning numerical scores (80+ for light roasts, 25-30 for very dark). This technology ensures repeatability between batches and helps consumers identify their preferred roast level more precisely than vague terms like 'medium-dark.'
What is the Maillard reaction in coffee roasting?
The Maillard reaction is the first browning phase of coffee roasting that creates hundreds of volatile aromatic compounds. It's the same chemical process that produces the smell of baking bread and is responsible for developing many of coffee's complex flavor notes during the roasting process.