An Updated Visual Guide to the Roast Process
Color is just one of the ways to determine degree of roast. By itself, it is of limited use. When complemented by the audible cues (first and second crack) and the aromas of the roast process, it is extremely informative . Here is a You Tube video showing the color changes that occur during roasting -
There is always a balance between the "roast character" and the "origin character" of the coffee in any roast. In darker roasts, the origin character of the coffee becomes secondary to the flavors of the roast. I subscribe to the idea that the best roast is the one that maximizes all the "origin character" ... one that reveals the unique origin character of the coffee. This is usually associated with a lighter "City" roast. At some point between French and fire, it really doesn't matter much what the "origin character" of the coffee was, all coffees will tend to taste the same (i.e. ashy and bitter). It is up to you, the home roaster, to navigate the variables of roasting to create the coffee you truly enjoy. This may be a darker roast than I recommend but stand your ground, and brave the smoke! The experience of finding the combination that suits your senses only deepens the appreciation for the great variety of coffee available, and the enjoyment of the diverse cup qualities available...
The Sounds of First and Second Crack now have their own page.
Degree of Roast, Temperature, Description
This coffee was roasted on my Probat 12 kilo so I could take advantage of the sample trier, which allows me to pull out and save a sample from the batch whenever I want. 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. ***(see note from home roaster George Steinert below).
This coffee should give you a good, general idea of roast appearance but different coffees can present differently in the roast.
Roasting is something you learn by doing it. You can't pin it down to a set of numbers. Roasting is more about exceptions than rules. I have this page about bean color vs. ground coffee color that might be helpful. So go get to it...
Picture of Roast
1. Green unroasted coffee 0:00 - 75 f
This is a wet processed, Central American coffee, a accidental blend I have had sitting around. Each photo here are different coffee seeds from the batch I roasted so size and shape will vary seed to seed.
2. Starting to pale 4:00 - 270 f
Drum roasters take a long time to transfer heat to coffee so there is little change in the first few minutes. In an air roaster coffee gets to this stage so much faster because of the efficient heat transference of the rapid moving air stream, so the whole warm-up phase can be as fast as two minutes.
Odd looking seeds - the near one might be a Kona Typica and the farther one perhaps the traditional Bourbon cultivar or Mundo Novo. Click on image for large photo, or here for multi-bean photo
3. Early yellow stage 6:00 - 327 f
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 - 345 f
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 - 370 f
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 - 393 f
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 - 401 f
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 356 f.
8. 1st crack under way 10:00 - 415 f
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 ...not a good thing. Once caramelization begins (340-400 degrees 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 370 f and corresponds to this window of temperatures when caramelization begins.
9. 1st crack finishes 10:40 - 426 f
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 435f
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 - 444 f
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 are have a slight sheen of oil and the edges are softer.
The internal bean temperature for second crack normally is 446 degrees farenheit. But in fact second crack is a bit less predictable than first crack, in my 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 celllose 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 - 454 f
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 your 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.
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 440 - 446 internal bean temperature. Southern Italian (Scura) is generally a Light French Roast or a tad darker.
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. 474 is well beyond any roast I do on the Probat. I will go as high as 465 on a couple blends, and that's my limit.
Notice how fast and dramatic the change is from the previous photo - all that happened in less than 30 seconds!
15. Fully carbonized 13:00 - 486 f
At this stage, the coffee can be over 25% ash; it is carbonized, dead, charcoal.
16. Imminent fire ... 13:30 - 497 f
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...
***Note from George Steinert, home roaster: "I see a lot of new users of the iRoast2 struggling to make sense of roast levels. I have been using the iR2 since February. I got the best success when I added the digital thermometer with thermocouple probe (both from SM) and condensed the well-documented "degree of roast" information on the SM web site into a one page chart (below). By using the temps on the chart (as measured with the thermocouple planted in the bean mass) as a "guide" and when combining that with actual experience of hearing the cracks (when they can be heard through the fan noise), the color, the smell, and the time (when using a consistent load of green beans), I have had very predictable results. I continue to use the chart as my baseline of understanding when roasting with the RK drum which I started using in June. I reference the start and end of first crack to tell me how the roast is progressing. After a while, the aroma becomes a factor. About 60-90 seconds before first crack begins, I swear I can tell it's coming because of a characteristic aroma. Primarily I use the sounds of the crack(s) and secondarily, the time. With the iRoast2, the temp from the thermocouple gives a pretty reliable indication of where they are in the roasting process...given some flexibility regarding start and end of first crack depending on the type of beans. With the iRoast, you also have the helpful cues of color and seeing what's happening on the surface of the bean. Users should note that First Crack is not going to necessarily begin when your thermocouple hits 401 degrees F nor end when it hits 426 F but after you've done a few dozen roasts you will get the feel for how to overlay the progress of the roast onto the temperatures you are reading.
George Steinert's Degree of Roast/Temperature chart:
Please note that both Tom and George emphasize that temperature alone will NOT determine degree of roast. Each roaster is different and different beans roast slightly differently as well. All the information on this page is to be take together to help determine degree of roast - no one element (appearance, sound, temperature, etc) can determine degree of roast. Most importantly - TASTE THE COFFEE - and see what that tells you about how it roasted.
Note about High Altitude Roasting
Here is a representative image I took of the Agtron Roast Color Tiles, and might give you a basica idea of the color scale. There is a bit of glare on the left side though (most visibile on Agrton 45). Since this is such an approximation and the appearance depends so much on monitor calibration, etc, I am not going to put a ton of work into this ... I am working on a better method of sharing these roast colors information.