In the world of coffee we characterize the degree of roast using a common form of terminology, which we will discuss in this next section.
Degree of Roast, Temperature, Description
This coffee was roasted on Sweet Maria’s Probat 12 kilogram so they could take advantage of the sample trier. Ignore the times, and take the temperatures as a ballpark figure.
The important thing is here is to see the transformation the coffee goes through as it roasts and what look, color, bean size and surface texture, corresponds to the degree of roast. Roasting is more about exceptions than rules.
1. Green unroasted coffee 0:00 – 24 Celsius
As the name states, green coffee, is raw coffee that has yet to be roasted. It can vary in shades depending upon how it is processed (more on that in the processing coffee section). But all else aside it is coffee that has not been roasted.
2. Starting to pale 4:00 – 132 Celsius
This is the initial stages of the endothermic process as the beans begin to take in heat the color of the beans begin to develop a yellow hue. At this stage the coffee will smell like grass or even hay.
3. Early yellow stage 6:00 – 164 Celsius
At this point the coffee is still losing water in the form of steam and no physical expansion of the bean has taken place. The coffee has a very humid, hay-like smell at this point. All of these warm-up stages leading up to first crack are part of an endothermic process, as the coffee takes on heat, leading to the first audible roast reaction, the exothermic 1st crack.
4. Yellow-Tan stage 6:30 – 174 Celsius
The roast is starting to assume a browner color, and a marbling appearance is starting to emerge. No bean expansion yet. The first “toasty” smells (toasted grain, bread) can be detected, and a bit less wet, humid air coming off the coffee. Note that some coffees turn a brighter and more distinct yellow at this time, such as Costa Rican and Mexican coffees.
5. Light Brown stage 8:00 – 188 Celsius
First crack is drawing near at this point. Some bean expansion is visible, as the central crack in the coffee has opened slightly. The coffee releases some silverskin or chaff.
6. Brown Stage 9:00 – 200 Celsius
Now we are right at the door of first crack. The coffee has browned considerably, which is partly due to browning reactions from sugars, but largely due to another browning reaction called the Maillard Reaction (which also is responsible for browning of cooked beef.
7. 1st crack begins 9:20 – 205 Celsius
At this point, the very first popping sounds of the First Crack can be heard. This sound can be similar to popcorn pops (in distinction to the sound of the Second Crack, which has a shallower sound, more like a snap). At the point of first crack the internal bean temperature would be around 180 Celsius.
8. 1st crack under way 10:00 – 213 Celsius
As first crack continues the coffee still appears mottled and uneven in color. The coffee starts expanding in size and shows visible cracks. The amount of chaff in the crease of the seed is noticeably less.
First crack is an exothermic reaction; the beans are giving off heat. But then the beans quickly become endothermic, meaning that a roaster that is not adding enough heat to the process will stall the roast at this point; this is not a good thing. Once caramelization begins (170-205 Celsius internal bean temperature) a roast that looses heat will taste “baked”, perhaps due to the disruption on long-chain polymerization. The melting point of sucrose is 188 Celsius and corresponds to this window of temperatures when caramelization begins.
9. 1st crack finishes 10:40 – 219 Celsius
This is considered a City Roast. First crack is done and the roast is stopped.
Notice the bean surface is smoother somewhat from expansion but still has darker marks in the coffee, like a finely etched pattern. The edges of the seed are still fairly hard. At this point the coffee starts giving off carbon dioxide.
10. City+ roast 11:05 224 Celsius
City+ means the coffee has cleared first crack, and time is allowed for an even bean surface appearance to develop.
There are only very small changes between the #9 picture above and this one, notably the edges of the bean are a bit softer. The whole stage between the first and second crack is a short period (15 to 30 seconds) where a lot is happening chemically to the beans. The coffee gains heat once again until its woody cellulose matrix, the bean structure itself, begins to fracture that is, the Second Crack.
11. Full City roast 11:30 – 229 Celsius On the verge of 2nd crack
This image represents a Full City roast; the coffee is on the verge of 2nd crack. This might be hard to judge the first few times you roast; after a while, you will have a feel for it. The beans have a slight sheen of oil and the edges are softer.
The internal bean temperature for second crack normally is 230 Celsius. But in fact second crack is a bit less predictable than first crack, in our experience. Why? It could be due to the fact that first crack is the physical expansion of the coffee seed as water and carbon dioxide split and CO2 outgassing starts. Second Crack is the physical fracturing of the cellulose matrix of the coffee. This matrix is both organized cellulose that reacts readily to heat, and not-so-organized cellulose that does not. Since every coffee is physically different in size and density due to the cultivar, origin, altitude, etc., it make sense that the particular cell matrix is different too, and not as universally consistent.
12. Full City+ roast 11:50 – 234 Celsius
First audible snaps of 2nd crack
The darker side of a Full City roast is Full City+, where the coffee has barely entered 2nd crack. A few snaps are heard, and the roast is then stopped. Second crack may continue into the cooling phase – this is called “coasting”. The more effective and rapid you are cooling – the better your ability to stop the roast at the degree you want.
Compare the full size images from the Full City roast and this one, and I think it is easy to see a difference. Well, maybe not easy, but the Full City+ roast is a bit fuller, more small cracks on the face (or flat side) of the bean.
13. Vienna – Light French roast 12:15 – 240 Celsius
2nd crack is under way
The Vienna stage (also called Continental) to Light French stage is where you begin to find Origin Character eclipsed by Roast Character. A dark or heavy roast is at odds with buying coffee for its distinct origin qualities. Dark roast coffees tend to taste more like each other – as the differences due to distinct origins are obscured by the carbony roast flavors. Nontheless, some coffees are excellent at this stage (our Puro Scuro blend is engineered for this roast range).
By the way, espresso is not a roast. But Northern Italian style espresso is usually roasted to 227 – 230 Celsius internal bean temperature. Southern Italian (Scura) is generally a Light French Roast or a tad darker.
14. Full French roast 12:40 – 474 f
2nd crack is very rapid, nearing its end.
Sugars are heavily caramelized (read as burned) and are degraded; the woody bean structure is carbonizing and the seed continues to expand and loose mass. The body of the resulting cup will be thinner/lighter as the aromatic compounds, oils, and soluble solids are being burned out of the coffee and rising up to fill your house with smoke. 245 Celsius is well beyond any roast Sweet Maria’s does on their Probat. They will go as high as 240 Celsius on a couple blends, and that’s their limit.
Notice how fast and dramatic the change is from the previous photo – all which happened in less than 30 seconds.
15. Fully carbonized 13:00 – 252 Celsius
Some call this Italian or Spanish roast, an insult to either!
At this stage, the coffee can be over 25% ash; it is carbonized, dead, charcoal.
16. Imminent fire – 13:30 – 258 Celsius
This bean is right at the verge of fire – in fact you can actually start a fire with a large batch once you dump the coffee out of the roast drum into the cool tray. The sudden rush of oxygen might be the needed ingredient for cafe del fuego. Kids, grab your marshmallows! Hope you like ’em smokey!
Needless to say, this roast level is full-on carbon and you can write your name with a coffee bean. The bean size here is smaller that photo 15 due to the randomness of the seeds selected to photograph – coffee does not get smaller at this stage…