Development Time Ratio Is the Wrong Kind of Number

Roasting, In Practice · Part 3

Development time ratio is not a weak metric. It is the wrong kind of number. Reaction rates depend on absolute temperature and duration, and a ratio of two durations carries neither.

01 What the number claims to describe

The definition is simple.

Take the time from first crack to the end of the roast, divide it by the total roast time, and express it as a percentage. Somewhere between fifteen and twenty-five percent gets quoted as the target range.

The claim behind it is that this ratio tracks flavour development.

Too low and the coffee is underdeveloped, too high and it is baked. The number is treated as a quality gauge that transfers between coffees, machines and batch sizes.

It does not transfer, and the reason is not a lack of evidence.

The problem is structural. A ratio of two durations cannot describe a process governed by temperature and time, because dividing one by the other throws both away.

02 The chemistry says something different

Reaction rate is set by absolute temperature.

The Arrhenius relationship states that the rate constant of a reaction rises exponentially with temperature. Small changes in temperature produce large changes in rate, and the relationship is to the temperature itself, not to any proportion.

How far a reaction goes is rate multiplied by time.

What ends up in the bean is the accumulation of that rate over the duration it was held there. Two numbers are required: what temperature, and for how long. A percentage supplies neither.

The same ratio describes very different roasts.

A twelve-minute roast with two minutes of development and an eighteen-minute roast with three minutes of development share the same figure. The beans in the second spent half again as long in the hottest part of the process. Chemically these are not comparable, and no ratio will tell them apart.

Batch size breaks it further.

Twelve kilograms and twenty-eight kilograms in the same drum reach the same ratio by entirely different routes, because the mass being heated is different.

03 The starting point is not even fixed

First crack is marked by a person listening.

One roaster marks the first audible pop. Another waits for a steady run of them. A third marks it when the sound fills the room. The gap between those three judgements can be thirty seconds or more on the same batch.

Thirty seconds rewrites the number.

On a twelve-minute roast, half a minute of difference moves the ratio by several percentage points, enough to cross from one side of the recommended range to the other. The coffee has not changed. Only the person holding the timer has.

The drum is loud and the crack is not a single event.

Beans crack over a spread of time because they do not all reach the same temperature at the same moment. Treating that spread as a point, then building a ratio on it, compounds the error rather than measuring anything.

04 Record these instead

Keep the quantities the chemistry actually depends on.

They are no harder to write down than a percentage.

Record this	Why it matters	What the ratio does with it
Temperature at drop	Sets how fast reactions were running at the end	Discards it entirely
Minutes above a chosen temperature	Approximates accumulated reaction	Replaces it with a proportion
Charge weight and charge temperature	Determines how heat moves into the mass	Ignores both
Weight loss and final colour	Measures the result rather than the path	Unrelated to it

The ratio adds nothing these numbers do not already cover.

Whatever you believed it was telling you is carried by temperature, time and weight loss, and those mean the same thing on any machine and in any roastery.

Taste the coffee.

Underdeveloped and baked are real faults with real causes, and both are found by drinking the result and looking at temperature and time. Neither is diagnosed by a percentage.

First Penguin Coffee Company Roasting since 2010 and teaching roasting since 2016, from our production facility in Korea. Our coffees are listed under Beans in the top menu. Enquiries: 010-3444-1721

Next: judging green coffee, and which of its numbers are worth reading.