Development Time Ratio, or DTR, is the percentage of the overall time spent after first crack, in the final stage of roasting. Depending on the roast level, most roasts will have a DTR of 15-30%. As we discussed in our How to Read a Roast Curve blog post, there are three main stages during the roasting process. We believe the development phase has the largest effect on the perceived flavor of the coffee.
Why do roasters care about DTR?
DTR can be a useful metric when evaluating coffee during cupping. You could have the same green coffee, roasted to the same finish temperature, but the flavor is different because of the overall time spent in the development phase.
As Steve likes to say: “Chemistry is most reactive at heat”. Heat transfer occurs most rapidly during the development phase. When roasting, the outside of the coffee bean is going to be heated first, which is reflective of the final drop temperature. However, it takes time for heat to penetrate the coffee. The internal color / darkness of the coffee is reflected in the DTR, and is what makes the coffee taste more developed. Adjusting the DTR and finish temperature of the coffee will make the most noticeable difference on the cupping table when trying to dial in your roast profiles.
What is the ideal DTR?
Like a lot of things in coffee, there is no straightforward answer to this question. There are simply too many variables to say that “this coffee should be at 20% DTR, while this other coffee should be at 15%”. Each coffee is going to react differently to development, and therefore taste different.
For example, a medium roast coffee from Colombia with an 18% DTR might taste exactly how we want it, but if I have the same DTR target for a coffee from Brazil, it might taste underdeveloped and a little tart. Some coffees just need more time in development than others to reach a similar flavor profile. This could be because of variables such as density, moisture level, or varietal. Ultimately, cupping your coffee regularly is going to show how much you need to adjust your roasts. Tasting too sour? Probably needs more time in development! Tasting too dark and muddy? Speed up the development stage to get to a similar finish temperature while decreasing the DTR.
Every detail matters
For the most part, using DTR should be a metric for adjusting roast profiles after evaluation, not necessarily a generic target for every roast profile. Having a certain “magic number” doesn’t always guarantee that the coffee will be properly developed and stretching out a roast so you can reach a certain percentage won’t magically make the coffee taste better. But once you find a DTR that works well with the rest of the roast profile, and most importantly, tastes good, that is when you want to use this metric to stay consistent on every batch.
