DTR in Coffee Roasting: How to Calculate and Analyze Development Time

In coffee roasting technique, DTR (Development Time Ratio) is the percentage of development time after First Crack relative to the total batch time. This metric helps rosters track the final stage progression, compare roast profiles, and adjust the balance between the bean's origin characteristics and flavors developed during roasting.

However, DTR is not a direct measure of the level of chemical development within the bean. A batch with a 20% DTR is not necessarily better developed than one with 17% or 23%. For accurate assessment, it is necessary to combine data on time, temperature curve, Rate of Rise (RoR), roast color, and cupping results.

1. What is DTR in coffee roasting?

DTR stands for Development Time Ratio. In a roast profile, development time is typically calculated from the start of First Crack to the point when beans are discharged from the roaster.

First Crack is the stage where coffee beans emit a cracking sound due to physical changes inside the bean, including increased gas and steam pressure during heating. After this point, chemical reactions continue, and the thermal profile influences aroma, acidity, sweetness, bitterness, and overall roast characteristics.

DTR represents the proportion of time spent in the development phase relative to the entire roast batch. It is a relative metric, unlike Development Time, which is an absolute duration measured in seconds or minutes.

2. DTR Calculation Formula

The formula for calculating the development ratio is as follows:

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

Where:

• Development Time: The duration from First Crack to bean discharge.
• Total Roast Time: The duration from the conventional start of the roast profile to bean discharge.
• DTR: The percentage of total roast time allocated to the development phase.

Example of DTR calculation in a roast profile

Assume a roast batch has the following parameters:

• Total roast time: 10 minutes, equivalent to 600 seconds.
• First Crack starts at minute 8.
• Bean discharge time: minute 10.
• Development time: 2 minutes, equivalent to 120 seconds.

Applying the formula:

DTR = (120 / 600) × 100 = 20%

Thus, 20% of the total batch time is allocated to the phase from First Crack to bean discharge.

This does not mean the coffee beans have reached a fixed level of chemical development. Two roasts with the same 20% DTR can still differ in temperature, RoR, heat transfer, and sensory attributes of the final product.

3. Is a 20–25% DTR the ideal range?

The 20–25% DTR range is frequently cited in specialty coffee roasting practice. Scott Rao explained the basis for this recommendation in his 2016 article Development Time Ratio.

According to Rao, the 20–25% range was selected based on experience evaluating numerous roast profiles, where the majority of those he rated as excellent fell within this range. However, he also emphasizes that this is an observed correlation, not evidence that every roast must achieve a 20–25% DTR.

This reference range is suitable for starting experiments, particularly with light to medium roast profiles on drum roasters. The target DTR may vary depending on batch size, machine capacity, green bean characteristics, roast level, and brewing purpose.

Important principle: Do not increase or decrease development time solely to hit a specific DTR figure if doing so causes the temperature curve to become uncontrollable or degrades cup quality.

4. How does development time affect flavor?

The phase after First Crack is a crucial part of flavor formation. Chemical reactions continue under the influence of heat, while aromatic compounds are formed, transformed, or broken down. Therefore, changing the development time can alter sensory attributes even when the final roast color is equivalent.

4.1. When development time is short

In some roast profiles, a short development phase can better preserve fruity characteristics, acidity, and the bright sensation of the beans. However, if the roasting process has not achieved appropriate balance, the coffee may exhibit grassy, raw bean notes, sharp sourness, or an unbalanced aftertaste.

One cannot conclude that a roast is underdeveloped based solely on a low DTR. It is necessary to further examine the temperature curve, roast level, bean structure, and sensory evaluation during cupping.

4.2. When development time is longer

Extending the time after First Crack can shift the sensory balance by reducing the prominence of fruity flavors and increasing roasted, nutty, chocolate, or bitter notes under certain conditions. The degree of change also depends on temperature, rate of rise, and final roast level.

Excessively long development time with inappropriate thermal energy can cause flavors to become flat, reduce brightness, or introduce cardboard and dry straw characteristics often described as baking. However, a high DTR alone is insufficient to diagnose baking defects.

4.3. The role of chemical reactions

Maillard reactions and other reactions involving the metabolism of sugars, amino acids, and various precursors begin before First Crack and continue during the development phase. One should not assume these reactions only start when DTR reaches 20%.

Similarly, caramelization is not a switch that turns on at a specific DTR ratio. Flavor formation is the result of multiple reactions dependent on temperature, time, and ingredient composition.

5. Does a low DTR equate to underdevelopment?

No. Underdevelopment, or lack of development, is an assessment of the roasting outcome, not a label for a fixed DTR range.

A roast with a low DTR can still produce a balanced cup if the thermal profile and roasting conditions are appropriate. Conversely, a roast with a 20–25% DTR may still be underdeveloped or have defects if the thermal progression is unsuitable for the bean type and batch size.

When underdevelopment is suspected, check the following simultaneously:

• Sensory: Are there grassy, raw bean smells, sharp sourness, or an unbalanced aftertaste?
• RoR Curve: Does the rate of rise progress logically during the development phase, or do crashes, flicks, or abnormal declines occur?
• Roast Color: Do the surface and ground colors match the roast target? Are there signs of uneven roasting?
• Raw Material Conditions: Do moisture content, density, bean size, and processing method affect the rate of heat transfer?
• Repeated evaluation: Are cupping results consistent across multiple controlled roast batches?

6. Does a high DTR equate to baking?

Baking is a sensory term used to describe coffee with flat, lifeless flavors, sometimes resembling cardboard or dry straw. This phenomenon must be distinguished from a roast with a long development time that still achieves good flavor.

To assess the risk of baking, one must consider the entire thermal trajectory, particularly the Rate of Rise (RoR) during and after First Crack. If heat transfer to the beans diminishes excessively or the profile is prolonged without generating the desired flavor development, the quality of the cup may be compromised.

Therefore, one should not set a rigid threshold such as a DTR above 25% or 30% and conclude that the roast is baked. Thermal data must be cross-referenced with roast color and sensory results.

7. How to analyze DTR alongside the RoR curve

RoR (Rate of Rise) indicates the rate of temperature change over time, typically expressed in °C/minute in roasting monitoring software. While DTR shows what percentage of the total time is occupied by the development phase, RoR helps observe the dynamics of the heating rate during that phase.

When analyzing roast profiles, pay attention to:

• RoR before First Crack: Helps understand the energy state of the batch as it enters the development phase.
• RoR around First Crack: Observe the decline or fluctuation in the rate of temperature rise. Some fluctuations may be related to the release of moisture and changes in heat transfer.
• RoR after First Crack: Monitor energy dynamics during the development phase, avoiding adjustments made solely to force the DTR to a target number.
• RoR at drop time: Record this for comparison between batches using the same bean type and roast target.

There is no single RoR curve suitable for all machines, batch sizes, and coffee types. Equipment characteristics, probe placement, temperature measurement lag, and profile repeatability must be considered before drawing conclusions.

8. How to optimize DTR according to brewing goals

8.1. Roasting for Filter methods

For coffee intended for Pour-over or other filtration methods, roasters can experiment with development time to maintain the clarity of fruity and floral notes, acidity, and sweetness. The goal is to create a balanced cup, not to achieve a low DTR at all costs.

8.2. Roasting for Espresso

For Espresso, one must evaluate the balance between acidity, sweetness, bitterness, body, and extractability. Some profiles with longer development times may produce a more pronounced roasted flavor, but the result also depends on the roast level, solubility, and extraction recipe.

It cannot be stated that Espresso always requires a higher DTR than Filter. Flavor goals, bean type, and brewing method are the basis for deciding the appropriate profile.

8.3. Adjusting according to bean characteristics

Coffee beans differ in moisture content, density, size, structure, and processing method, and may react differently to the same roasting program. Therefore, a single development ratio should not be applied to all bean lots.

An effective approach is to build a reference profile for each bean type, make controlled changes to certain variables, and then compare roast color and cupping results.

9. Practical process for optimizing DTR

1. Define the goal: Select the desired sensory attributes, such as fruity acidity, sweetness, body, or roasted notes.
2. Record the baseline profile: Log total time, First Crack time, drop time, temperature, RoR, and batch information.
3. Calculate DTR: Use the same convention for recording First Crack and calculating time for all comparison batches.
4. Controlled changes: Adjust the profile in small steps, avoiding simultaneous changes to too many parameters, which makes it difficult to identify the cause.
5. Compare roast color: Record bean or ground color using a consistent measurement method.
6. Cupping: Evaluate aroma, acidity, sweetness, bitterness, body, aftertaste, and defects.
7. Select an appropriate profile: Prioritize profiles that produce the desired cup quality and have good repeatability, rather than selecting based solely on DTR.

10. Conclusion: DTR is an analytical tool, not a perfect roasting formula

DTR helps roasters quantify the proportion of time after First Crack and compare roast profiles consistently. Around 20–25% is a useful reference point in some specialty coffee roasting practices, but it is not a mandatory standard for every bean type or roast level.

To properly understand bean development, DTR must be combined with RoR, temperature, roast color, raw material characteristics, and cupping. Only when these data are evaluated together can roasters determine which profile truly produces flavors aligned with the product goal.

References

1. Rao, S. (2016). Development Time Ratio.\n Read Scott Rao's original article.
2. Alstrup, J., Petersen, M. A., Larsen, F. H., & Münchow, M. (2020).\n The Effect of Roast Development Time Modulations on the Sensory Profile and Chemical Composition of the Coffee Brew as Measured by NMR and DHS-GC–MS.\n Beverages, 6(4), 70.\n https://doi.org/10.3390/beverages6040070.
3. Münchow, M., Alstrup, J., Steen, I., & Giacalone, D. (2020).\n Roasting Conditions and Coffee Flavor: A Multi-Study Empirical Investigation.\n Beverages, 6(2), 29.\n https://doi.org/10.3390/beverages6020029.

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