Visual Changes in Roasting Offer Important Clues When Used With Other Inputs Too!
Color is only one of the ways to determine degree of roast. By itself, color change is of limited use. When complemented by the audible cues (first and second crack) and the aromas of the roast process, color can be extremely informative.
We will go through the roast process using static pictures and text below.
Color and Texture Changes in the Roast, a Video
But first …here’s a video showing the color changes that occur during roasting:
Degree of Roast, Temperature, Description
The important thing here is to see the transformation the coffee goes through as it roasts and what look, bean size and surface texture, corresponds to the degree of roast. (see note from home roaster George Steinert at the bottom of this post). Roasting is more about exceptions than rules. For more information, check out this page about whole bean color vs. ground coffee color. Note: This coffee was roasted on my Probat 12 kilo so I could take advantage of the sample trier. Ignore the times, and take the temperatures as a ballpark figure.
1. Green unroasted coffee 0:00 – 75F
Pictured is a wet processed, Central American coffee, an accidental blend I had sitting around. Each photo shows a different coffee seed from the batch I roasted so size and shape will vary seed to seed. The second image shows the same coffee/roast level, but many beans. Hopefully the comparison of the two images gives a better idea of color than the single bean image taken by itself.
2. Starting to pale 4:00 – 270F
Drum roasters take a long time to transfer heat to coffee so there is little discernible change in the first few minutes. In an air roaster, coffee gets to this stage much faster because of the efficient heat transference of the rapid moving air stream – the whole warm-up phase can be as fast as two minutes.
3. Early yellow stage 6:00 – 327F
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 1st 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 – 345F
The roast is starting to assume a browner color and a marbled appearance is starting to emerge. No bean expansion yet. The first “toasty” smells (toasted grain, bread) can be detected, and there is less wet, humid air coming off the coffee. Note that some coffees may turn a brighter and more distinct yellow at this time, such as Costa Rican and Mexican coffees.
5. Light brown stage 8:00 – 370F
1st 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 – 393F
Now we are right at the door of 1st 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!) There is a very specific appearance to the roasted coffee at this point. It has marbled accents but has not yet expanded and the crease remains tightly shut.
7. 1st crack begins 9:20 – 401F
At this point, the very first popping sounds of the 1st Crack can be heard. This sound can be similar to popcorn pops (in distinction to the sound of the 2nd Crack, which has a shallower sound, more like a snap). At the point of 1st crack the internal bean temperature would be around 356F.
8. 1st crack under way 10:00 – 415F
As 1st 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.
1st crack is an exothermic reaction; the beans are giving off heat. But then the beans quickly become endothermic, taking on heat. 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 (340-400F internal bean temperature) a roast that loses heat will taste “baked”, perhaps due to the disruption on long-chain polymerization. The melting point of sucrose is 370F and corresponds to this window of temperatures when caramelization begins.
9. 1st crack finishes 10:40 – 426F
This is considered a City Roast. 1st 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 1st crack, and time is allowed for an even bean surface appearance to develop.</span></p> <p><span style=”font-weight: 400;”>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 1st and 2nd 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 2nd crack.
11. Full City roast 11:30 – 444F 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. Look <strong><a href=”https://library.sweetmarias.com//roastedcoffee_grindvssurfacecolor/”>here</a></strong> for the whole bean vs. ground coffee comparison photo of this roast color.</span></p> <p><span style=”font-weight: 400;”>The internal bean temperature for 2nd crack normally is 446F. In practice, 2nd crack is a bit less predictable than 1st crack… why? Possibly due to the fact that 1st crack is the physical expansion of the coffee seed as water and carbon dioxide split and CO2 outgassing starts. 2nd Crack is the physical fracturing of the cellulose matrix of the coffee. This matrix is composed 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 makes sense that the particular cell matrix is different too, and not as universally consistent.
12. Full City+ roast 11:50 – 454F First audible snaps of 2nd crack
The darker side of a Full City roast is Full City +, where the coffee has just barely entered 2nd crack. A few snaps are heard and then the roast is stopped. 2nd crack may continue into the cooling phase – this is called “coasting”. The more effective and rapid your cooling, the better your ability will be to stop the roast at the degree you want. Look here for the whole bean vs. ground coffee comparison photo of this roast color.
Compare the full size images from the Full City roast to Full City+ and I think it’s easy to see a difference. Maybe not easy, but the Full City+ roast is fuller, with more small cracks on the face (or flat side) of the bean.
13. 12:15 – 465F 2nd crack is under way
(This is my darkest roast for espresso. I don’t like to drink ash and charcoal!)
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 carbon-y roast flavors. Nonetheless, 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. Northern Italian style espresso is usually roasted to 440F – 446F internal bean temperature. Southern Italian (Scura) is generally a Light French Roast or a tad darker.
14. 12:40 – 474F
2nd crack is very rapid, nearing its end. Coffee tastes ashy and burnt.
Sugars are heavily caramelized (read as burned) and degraded; the woody bean structure is carbonizing and the seed continues to expand and lose mass. The body of the resulting cup will be thinner because the aromatic compounds, oils, and soluble solids are being burned out of the coffee and rising up to fill your house with smoke. 474F is well beyond any roast I do on the Probat. I will go as high as 465F 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! In the photo you can see a couple small circular divots blown out of the coffee by rapid degassing of the roast during the roast cycle.
15. Fully carbonized 13:00 – 486F
Some call this Italian or Spanish roast, an insult to either culture, IMO.
At this stage, the coffee can be over 25% ash; it is carbonized, dead, charcoal.
16. Imminent fire … 13:30 – 497F
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 cooling 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 than photo #15 due to the randomness of the seeds selected to photograph – coffee does not get smaller at this stage…
Dark Roast Tips from the Master (of bad Christmas sweaters)
If you like the dark roast above, you might also appreciate this coffee guru and his amazing roasting technique.
Agtron Roast Color Discs from the (RIP) SCAA
Here is a representative image I took of the Agtron Roast Color Tiles, and might give you a basic idea of the color scale. These were a set of roast calibration discs for those who could not afford a $4000 color analyzer. They were pretty useful. There is a bit of glare on the left side though (most visible on Agtron 45). Since this is such an approximation and the appearance depends so much on monitor calibration, we are working on a better method of sharing this roast color information. The article that accompanied the disks by Carl Staub was also very useful! This was the best roasting science article in the early days and I read it 1000 times. For real. Thank you Carl!
Also check out the few Roasting Articles here: Coffee Science: Academic Papers and Documents
Why not make your own Roast Calibration Kit?
Here are some images of “degree of roast” sets that I made. The boxes are a clear type used for displaying beads and other craft objects. I bought these from The Container Store as I recall.
A homemade roast color calibration kit I made with bead / crafting containers and samples representing the degree of roast from green to Full City+
The set on the left dedicates one row to washed (wet-process) green coffee and the other to decaf, since decaf roast color can be so hard to gauge.
The set on the right also has wet-process coffee, compared alongside natural (dry-process) coffee. Each stage is the roast time and temperature equivalent. You can see how perceived color varies though.