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II. The Bean Temperature Progression Shalt Always Decelerate
In every batch, the bean-temperature’s rate of rise (ROR) initially increases at a rapid rate and then decreases as the coffee roasts. That’s the natural result of putting room-temperature beans into a hot roaster. The machine operator’s goal should be to produce an always-declining ROR. Should the ROR increase during a roast (other than the illusory increase in ROR during the first 2-3 minutes of a roast), development will suffer and some of the coffee’s potential sweetness will be sacrificed.
If the ROR is constant or horizontal, even for just 1 minute, it will also destroy sweetness and create “flat” flavors reminiscent of paper, cardboard, dry cereal, or straw. Every time I’ve tasted this flaw in a coffee and had the opportunity to view its roast data, the ROR had flatlined.
If the ROR declines at a moderate, steady pace and then drops precipitously, it compromises development, and unless the roast is discharged immediately, baked flavors develop. Baked flavors are similar to, but more extreme than, the flat, dull flavors created by a constant ROR. Should a roast stall, meaning the bean temperature stops rising (i.e., the ROR is 0 or has a negative value), baked flavors will dominate and sweetness will all but disappear. Researchers have not, to my knowledge, established the chemistry of baked coffee flavors.
The top graph shows four roast profiles and the bottom graph illustrates their respective ROR curves. The long, flat section (l) of the green ROR curve indicates the coffee will taste flat and lack sweetness. The increases, or "flicks," (2 and 3) in the blue and red ROR curves indicate those roasts did not develop as much as they could have at their respective roast degrees. The yellow profile and its corresponding ROR curve have no obvious problems. (Graphs graciously provided by Cropster.)
The leading theory is that a stall in a roast causes developing sugar chains to “cross-link,” which decreases sweetness and creates baked flavors.
For clarification, the following profiles illustrate various ROR patterns.
Experienced roasters know the ROR has a natural tendency to change course at particular moments. Perhaps the most challenging stretch of a roast occurs during first crack. To achieve a smoothly decreasing ROR, the operator must anticipate and adjust for each of the following common situations:
The ROR often flatlines at some point during the minute or two before first crack.
The ROR tends to drop sharply during first crack due to evaporative cooling. After first crack, the ROR tends to turn upward rapidly.**
A typical roast curve’s profile and ROR at around first crack, adapted from a Cropster-generated screenshot. An ROR curve often flattens before first crack, plummets as evaporation peaks, and rebounds as first crack ends. Most roasters are so used to this pattern that they are not aware it damages coffee flavor.
III. First Crack Shall Begin at 75% to 80% of Total Roast Time
Experience has taught me that the roast time from the onset of first crack*** to the end of a roast should make up 20%-25% of total roast time. Put another way, the “development-time ratio” (DTR) should be between 20%-25% of total roast time. I’m confident that the optimal ratio is actually in a much narrower range, and the ratio should vary slightly depending on roast degree desired, but I don’t have enough data yet to back up those beliefs.
If the DTR is greater than 20%-25% of total roast time, the coffee will probably taste flat. If the DTR is less than 20%-25% of total roast time, development will likely be insufficient.
Most roasters seem to adjust a roast’s development time separately from the rest of the roast curve, but such an approach will often lead to baked flavors or underdevelopment. Instead of focusing on development time, I recommend that roasters adjust the last phase of a roast curve to ensure it is proportional to the entire roast curve. I hope roasters will find this suggested ratio useful and that the conversation among roasters shifts from “development time” to “development time ratio” or a similar phrase.
First crack should ideally begin in the shaded zone.
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