Coffee: Development Time Ratio — DTR and Roast Quality

Development time ratio (DTR) is one of the most discussed metrics in modern specialty coffee roasting. It quantifies a simple but critical relationship: how much of the total roast time is spent in the development phase — the window after first crack where flavor integration, sugar caramelization, and volatile compound stabilization occur.

The DTR Formula

DTR is calculated as:

DTR (%) = Development Time / Total Roast Time × 100

Development time is measured from the first audible or instrument-detected first crack to the moment the roaster drops (discharges) the batch into the cooling tray.

Worked Example

Parameter	Value
Total roast time	12 minutes 00 seconds
First crack onset	9 minutes 00 seconds
Drop time	12 minutes 00 seconds
Development time	3 minutes (180 seconds)
DTR	3 / 12 = 25%

A second example with a faster, lighter profile:

Parameter	Value
Total roast time	9 minutes 30 seconds
First crack onset	7 minutes 45 seconds
Drop time	9 minutes 30 seconds
Development time	1 minute 45 seconds (105 seconds)
DTR	105 / 570 = 18.4%

DTR by Flavor Outcome

DTR Range	Flavor Risk	Likely Sensory Notes
<15%	Severe underdevelopment	Raw, grassy, astringent, salty
15–18%	Mild underdevelopment	Bready, grain-like, muted sweetness
18–22%	Nordic / bright specialty	Bright acidity, clarity, origin-forward
20–25%	Standard specialty target	Balanced sweetness, developed complexity
25–28%	Extended development	Caramel, softer acidity, fuller body
>28%	Risk of baked/flat	Flat, muted, stale if RoR crashes

Why DTR Alone Is Not Enough

DTR is a ratio, not an absolute time. The same 20% DTR can produce very different cups depending on:

• Total roast time: A 20% DTR in an 8-minute roast = 96 seconds of development. In a 14-minute roast = 168 seconds. Both are 20% DTR, but the flavor outcomes differ significantly.
• Rate of rise (RoR) during development: A declining, smooth RoR during development suggests controlled, even heat application. A stalled or crashing RoR — even with a “correct” DTR number — produces baked, flat flavors.
• “Crash and flick”: A RoR that drops to near zero and then spikes upward before the drop is associated with baked character regardless of DTR.

Nordic Roasting Philosophy and DTR

Scandinavian specialty roasters — particularly those associated with the Nordic Barista Cup and the broader Scandinavian coffee culture — popularized lighter roasting with total roast times of 8–11 minutes and DTRs in the 18–22% range. The goal is to maximize origin transparency: preserving the aromatic esters and organic acids that express terroir, variety, and processing character. This approach requires faster total roast times (avoiding baked character at low DTR) and precise RoR management to avoid stalling.

DTR in Practice

Roasters log DTR for every batch alongside RoR curve, charge weight, drop weight, and Agtron reading. Over time, DTR data paired with cupping scores allows roasters to identify the ideal development window for each origin and processing type. Dense, high-altitude washed coffees may benefit from slightly longer development times to unlock sweetness. Naturals with heavy fruit fermentation may express better with shorter development to preserve brightness.

Interaction with Roaster Type

Drum roasters and fluid-bed roasters handle DTR differently. In a fluid-bed (air) roaster with faster overall roast times, achieving a 20% DTR in a 5-minute batch means only 60 seconds of development. Drum roasters with 10–12 minute profiles have more runway to dial in development. The roaster type must be accounted for when setting DTR targets.