Scott Rao’s post earlier this year on “baked coffee” caused a lot of stir in both the professional and home roasting community. His idea that steep ROR (Rate-of-Rise) crash during first crack causes baked flavors has given me a lot of insights into my own home roasting. Prior to reading his post, I had struggled often with an off-flavor that I couldn’t quite put my hands (or rather tongue) on. Scott puts it well that baked coffee seems “hollower” than well rounded, juicy coffee. What Scott doesn’t address, however, is how to avoid this baked flavor.
I’m by no means an expert (and have still definitely baked some batches), but I’ve recently had some insights into how to avoid the ROR crash at first crack to achieve a more linear ROR curve. First, I’ll show you an example a very baked coffee. Please note that I’m roasting with a Huky 500, which is a small home roaster with quite sensitive probes. Ignore some of the oscillations in the ROR early in the roast, as this is just noise (which I’ve been struggling to get rid of).
This was a light-roasted Rwanda that ended up very baked. The ROR crash, which occurred approximately 30 seconds after first crack, was preceded by a significant flattening of the ROR curve. During this part of the roast I was turning down the gas, but not fast enough. I didn’t log it, but I started turning down from max gas pressure of 5kPa around 350 degrees. Typically I will start ramping down the gas at 300 degrees, but based on the dry time I could see that my roast was going to extend too long, so I was attempting to “speed up” the roast by adding more heat during the Malliard phase.
My theory is that this additional heat during the Malliard phase causing a very explosive first crack. During the first crack the beans release steam, which can cause a decrease in the ROR due to high latent heat of vaporization. They key that I’ve found to resolving this ROR crash is to charge with more energy so that gas can be turned down A LOT during the Malliard phase. The heat inputs that you make between 325 and 375 have a DRASTIC impact on the reaction of the beans at first crack. On my roaster, I’ve found that 1) charging at a hotter temperature, 2) adding more heat during the drying phase, and 3) charging with smaller volumes of coffee, allow me to have enough heat going into the Malliard phase to avoid having to push the gas too hard.
Here’s another example of a baked roast, this time in a darker roast, where you can see very similar results. In this case I took the roast to Full City to hide some of the baked flavors. It was not successful.
Between these two roasts, you can see that one of the key indicators is the shape of the MET curve (in green). The MET curve is fairly flat, with a very slight downward slope at first crack. This is important, because on my roaster the MET curve is much more responsive than the BT or ET curves. The MET curve tends to respond with ~10 seconds of making a gas change, whereas the ROR curve may take 30 seconds (or more).
While still not perfect, the below curve shows a much more steadily declining ROR curve, which is preceded by a steadily declining MET curve. In this case I charged very hot (500 degrees) and started turning down the gas aggressively during the Malliard phase.
In this roast I’ll admit that I don’t like the flattening of the ROR curve during the final 30 seconds, but unfortunately I was not able to continue to turn the gas down because I didn’t have enough energy to hit the final BT that I was shooting for. In this case, I should have started with even MORE energy so that the ROR didn’t peter out at the end of the roast. For my little Huky, this would probably mean charging at a lower weight. I believe that this was a 410 gram charge, but I should have tried a 350 gram charge.
The final question is WHY the shape of these curves result in the flavors that they do. I don’t have a good answer, but my speculation is that it involves the way that the sugars develop and caramelize during first crack. That’s a post for another day.