by Scott Rao

Over the past decade as a consultant, I’ve had the opportunity to cup thousands of roast batches from more than two dozen machines, and to compare those batches’ roast data to their cupping results. Upon focusing on only the most stellar, memorable batches, a few patterns became clear: One was that first crack* began at between 75%–80% in all the great batches. Put another way, “development time” was between 20%–25% of total roast time.

For six years I’ve been waiting to taste a delicious, sweet, well-developed coffee from a roast batch in which first crack began outside of that range. I have yet to find it. This ideal “development-time ratio” (DTR) has been valid for all roast degrees and roast times I’ve experienced. To be fair, I don’t often cup roasts dropped well before the end of first crack or well after the onset of second crack, so I won’t assume the ratio is valid for those roast levels.

(related: A Quantitative Description of the Sounds of Cracks During Roasting)

Roasters have historically focused on “development time” (defined as the time from the onset of first crack until the end of a roast) and discussed it in isolation. For example, roasters have frequently asked me to taste a coffee and then said “development time was three minutes.” To me, that piece of roast data is relatively meaningless out of the context of the total roast time.

I recommend roasters focus on the DTR rather than nominal development time. When roasters manipulate development time without considering it in the context of the total roast time, they often create baked flavors and destroy sweetness.

While achieving a 20%–25% DTR doesn’t guarantee a brilliant roast, it seems to be a prerequisite for a successful roast. Think of it as a marker of balance: if a roast begins very fast and ends slowly or vice versa, the DTR will be outside of the 20%–25% range. I’ve had roasters protest that they roast successfully outside of that range, but none of them have ever, to my knowledge, verified full roast development both organoleptically and objectively — for example, with a refractometer. On the other hand, in my sample of thousands of batches tested for development by smell, taste, and refractometer, every successful roast’s DTR was within that range.

*I mark the beginning of first crack as the moment I hear more than one or two isolated cracks. If you measure the beginning of first crack differently, your results may differ.