An Updated Pictorial Guide to the Roast Process

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. The roast temperatures given are going to differ from any roaster you might use, based on how the coffee is probed for temperature and the type of device used. Mine is a calibrated thermocouple on an Extech dual input thermometer, run through the front of the roaster, probing the turning bean mass. The roast times noted are not of consequence either… I was doing a faster roast in this case anyway, a bad roast profile overall (don’t emulate it), of an unintentional green coffee blend we had made here a while back. 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 every coffee type is different! Dry-processed coffees are not sorted in the wet-mill process, it is done entirely by hand, visually. So the coffee roasts more uneven from seed to seed. Even color is NOT an indicator of roast quality, or coffee quality! 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…

Note: The above image is not the exact same beans pictured below. If it helps. the gray strips on either side of this image are a photographic 18% gray card, Click on any image for the larger version)

This is an Organic coffee, a accidental blend I have had sitting around a few months. Sadly, if we mess up coffee here, there is nothing I can do with it, even though it is fine coffee involved. I wanted to find a charity I could roast our mistake blends for… Anyway, each photo here are different coffee seeds from the batch I roasted so size and shape will vary seed to seed. I actually cannot remember what exact coffees were in this mistake blend. I can say by appearance it is not super high-grown coffee, and I suspect a portion of the blend is actually Kona.

Odd looking seeds – the nearer might be a Kona Typica and the farther is a rounded shape, perhaps the traditional Bourbon cultivar or Mundo Novo. As you have figured out, I am not pulling samples at regular intervals (no sense in that). I am pulling them when the coffee has reached an identifiable and significant visual stage. Bigger drum roasters take a long time to transfer heat to coffee so there is little change in the first few minutes. Also, keep in mind I wasn’t trying to do a good roast on this batch, just trying to make it turn brown so I could take pictures…

At this point the coffee is still losing water in the form of steam and no physical expansion of the bean has taken place. In an air roaster coffee gets to this stage so much faster because of the efficient heat transference of the rapid moving air stream. 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.

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.

First crack is drawing near at this point. Some bean expansion is visible as the central crack in the coffee has opened slightly. This allows the coffee to release some of the silverskin in the form of chaff that has been attached to the bean in the folds of the crack.

Now we are right at the door of first crack. In a short time 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!)

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. Note that the temperatures I give are from a thermocouple probing the coffee. That is a different measure from a probe measuring the roast chamber “environment temperature” (probing the air space in the drum), or probing the internal bean temperature. The later is the most accurate measure, but is very hard to do. At the point of first crack the internal bean temperature would be around 356 f.

As first crack continues the coffee still appears mottled and uneven in color, as the coffee first starts its expansion in size that is marked by the cracking of the seed. Moisture is being liberated from the interior of the coffee and as it expands the crease in the seed usually opens enough to allow much of the remaining chaff to be released. Since first crack is an exothermic reaction, the beans are giving off heat in first crack, but the 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.

This is considered a City Roast, a bean appearance where the surface has smoothed somewhat from expansion but still has darker marks in the coffee, like a finely etched pattern. At this point the coffee has expanded due to the outgassing of First Crack, marking the point where water and carbon dioxide go their separate ways.

The stage between the first and second crack is a short period of further endothermy as the coffee gains heat once again until it reaches the point where its woody cellulose matrix, the bean structure itself, begins to fracture … that is, the Second Crack. You notice a darkening in a very short period of time and small change in temperature between the #9 picture above and this one.

This image represents a lighter Full City roast. At this point, the coffee is on the verge of 2nd crack. The internal bean temperature that second crack normally occurs at is 446 degrees farenheit. But in fact second crack is a little less predictable than first crack, in my experience. Why? It could be explained as this: first crack is the physical expansion of the coffee seed as water and carbon dioxide split and CO-2 outgassing occurs. Second Crack is the physical fracturing of the cellular matrix of the coffee. This matrix is wood, also called cellulose, and consists of 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 might make sense that the particular cell matrix is different too, and not as universally consistent in reactiveness as H-2O and CO-2.

This image represents the darker side of a Full City roast, where the coffee has barely entered 2nd crack, and 10 seconds of snaps are heard, and the roast is then stopped. Compare the full size images from the lighter Full City roast and this one, and I think it is easier to see a difference. Well, maybe I shouldn’t say it is easy, since the main cue that would distinguish the difference between the two is audible, not visual.

The Vienna stage (also called Continental) to Light French stage is where you begin to find Origin Character eclipsed by Roast Character. If you buy coffee for its distinct origin qualities, it makes sense that heavy roasting is at odds with revealing the full effect of the differences we can sense in coffee due to distinct origins. 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, 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.

At this stage, the coffee can be over 25% ash; it is carbonized, dead, charcoal.

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 feed of oxygen might be the needed ingredient for cafe del fuego. Kids, grab your marshmallows! In a smaller roaster it is a bit harder to get ignition because there just isn’t enough fuel. In a really large 2-3 bag roaster, you should have called the fire department before you roast to 497 – you will have a fire. Needless to say, this roast level is full-on carbon and you can write your name with a coffee bean. Note that 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…

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
Having lived my entire life at or near sea level (Chicago, Boston, Ohio, Oakland), I may be chromosomally incapable of understanding the effects of high altitude or cooking or roasting. But I did want to make a note to pass on what seems to be the collective wisdom on high altitude home roasting, which is that in general you will see the roast happen more quickly, at a lower temperature. This is most true in small convection roasters, like a popper, or Fresh Roast or i-Roast, but also occurs in conduction roasting. So expect to adjust roast times accordingly. If you can control the temperature on the roaster at all, adjust target temperatures downward by 20 to 30 degrees.