Development time ratio: the number that explains most flavor problems
If we had to pick one number to watch on every roast, it would be development time ratio, or DTR. It sounds technical. It's just this: how long the coffee spent between the start of first crack and the drop, divided by the total roast time.
Say you dropped at 11:00 and first crack began at 8:30. Development was 2:30, which is 150 seconds out of 660, so DTR is about 23%.
Why it works so well
Two coffees can drop at exactly the same temperature and taste completely different. One got there fast and the inside of the bean never caught up with the outside. The other got there with enough time after first crack for the heat to work through. The ratio is a rough measure of how much of that finishing time you gave it.
Too little development and you tend to get sharp, grassy, sour coffee, sometimes with a papery or peanut-like edge. Too much and the sweetness fades into something flat, woody or just bitter. Somewhere in between is the part you want.
Where most people land
Most roasters end up somewhere between 15% and 25%, with lighter roasts toward the bottom of that range and espresso-style roasts toward the top. Treat those as a place to start looking, not a target. Your machine, your batch size and your coffee all move it.
Dense, high-grown coffees often want a bit more time than soft, low-grown ones. A natural process coffee can scorch if you push the finish too hard. The ratio gives you something to compare, but the cup decides.
How to use it
Pick a coffee you roast often. Roast it a few times, changing only the time after first crack, by thirty seconds or so each step. Cup them side by side, blind if you can get a friend to pour. Write down which one you liked and its DTR.
After a handful of batches you'll have a number for that coffee that you can aim for, and a way to tell when a batch went wrong for a reason other than the finish. That's the point: DTR doesn't make the coffee good, it just lets you stop guessing which variable you touched.
Things that trip people up
The ratio depends on catching first crack consistently. Mark the first pops, not the peak, every time, or your numbers will drift.
A short roast and a long roast can have the same DTR and taste different. If your total time is wildly outside the usual range for your machine, fix that first. We talk about it in the guide on roasts that take too long.
And don't chase a number you read online. Your 20% and someone else's 20% were measured on different probes, in different drums, on different coffees.