roast curve is how a roaster turns a batch they liked into a batch they can repeat. It plots bean temperature against time and marks the moments that decide the cup.
This is a measurement guide, not a how-to for roasting itself. You can roast a good batch by ear, watching color and listening for the cracks. The curve and the numbers below are for when you want to measure what you did and reproduce it on purpose.
Read the shapes first and the numbers second. On a roaster every absolute figure moves with the machine, the batch size, and the bean, so the relationships between milestones tell you more than any single value.
Before you read this
One thing to hold onto throughout. There is no universal roast curve. A drum roaster, a fluid-bed roaster, and a popcorn-style popper produce different temperature readings for the same bean, and batch size and room temperature shift them again. The targets below are reference frameworks read against one specific setup. They are not numbers you transfer from another machine to yours.
The curve and its milestones
The roast curve is a single line: bean temperature on the vertical axis, time on the horizontal. A bean probe sits in the mass of beans and reports their temperature as the roast runs. The line itself is less interesting than the points marked along it, the milestones where something physical changes in the bean.
Charge
Green beans drop into the hot drum. The probe reads high for a moment, then falls as cold beans absorb heat.
Turning point
Bean temperature bottoms out and starts to climb. Usually within the first minute or so, machine dependent.
Drying and yellowing
Water boils off. The bean smells grassy then bready and shifts from green to yellow.
Browning
Maillard reactions and caramelization darken the bean and build aroma and sweetness.
First crack
An audible pop as the bean expands and releases steam and gas. From here the coffee is developed enough to drink.
Development
The stretch after first crack where you decide the roast level. Short and bright, or longer and rounder.
Drop
You release the beans to cool. The drop time and temperature set the end of the roast.
Charge and turning point are about heat transfer, not flavor yet. The cold charge pulls the drum temperature down, the beans bottom out at the turning point, then they begin absorbing more heat than they shed and the temperature climbs for the rest of the roast. Everything from drying through drop is where the cup is built.
Rate of rise: the shape that matters most
Rate of rise, or RoR, is how fast bean temperature is climbing at any moment, measured in degrees per minute or per thirty seconds. It is the canonical roast lever in the Scott Rao roasting literature, because it describes the momentum of the roast rather than a single snapshot. You read RoR as a curve of its own, plotted under the temperature line.
The thing to read is the shape. A healthy roast generally has a RoR that declines smoothly from the turning point to the drop. The beans climb fast early, when they are cold and the heat gradient is steep, then climb more slowly as they approach drum temperature. A smooth downward RoR is the signature most roasters chase.
Two faults show up as kinks in that line. A crash is when RoR drops suddenly, often near first crack as the bean briefly absorbs a burst of energy. The result is flat, baked-tasting coffee. A flick is the opposite, when RoR ticks back up after it should keep falling, usually just after first crack, which leaves the roast uneven. Reading RoR is mostly watching for a smooth decline and reacting to a crash or a flick before it sets the flavor.
Development time ratio
Development time ratio, or DTR, measures how much of the total roast happened after first crack. It puts a number on the part of the roast where the level is decided, which makes two roasts comparable even when their total times differ.
- Formula
- DTR = (drop_time minus first_crack_time) / drop_time x 100(all times measured from charge, in seconds)
- Typical range
- about 18 to 22 percent(for light-to-medium roasts; a guideline, not a universal target)
- Worked example
- first crack 8:00, drop 10:00 gives 20 percent((600 minus 480) / 600 x 100 = 20)
How DTR is calculated
Walk the example through. First crack lands at 8:00, which is 480 seconds. You drop at 10:00, which is 600 seconds. The development stretch is 600 minus 480, or 120 seconds. Divide 120 by the total 600 and you get 0.20, so the DTR is 20 percent, comfortably inside the typical band.
Read DTR as a guideline, not a law. The useful range depends on the roast style, the bean density, and the machine. A faster roaster reaches the same color with a different ratio, and some styles deliberately sit outside the common band. Use it to compare your own roasts on your own machine, not to grade a roast against a fixed number.
Agtron: color you can repeat
Agtron is a measured roast-color number from an instrument that reads how much light the ground coffee reflects. Lower numbers are darker. It runs roughly from around 95 at very light to around 45 at very dark, and it exists to make roast level objective instead of a judgment call about how brown a bean looks.
- Around 95
- very light(higher number means more reflected light, lighter roast)
- Around 45
- very dark(lower number means darker roast)
- What it is for
- an objective, repeatable color reading(replaces eyeballing how brown a bean looks)
Reading the Agtron scale
Tying the four together
These four are how a roaster turns a good accident into a repeatable roast. Together they let you write down what happened: the heat, its momentum, the development stretch, the final color. Write that down and you can return to it.
Common questions
- What is a roast curve?
- A roast curve is a plot of bean temperature against time, with milestones marked along it: charge, turning point, drying, browning, first crack, development, and drop. It lets a roaster see what the heat did and reproduce a roast they liked. The shape of the curve matters more than any single temperature, which varies by machine.
- What is a good development time ratio?
- DTR is the share of the roast after first crack, calculated as (drop time minus first crack time) divided by drop time, times 100. For light-to-medium roasts it commonly sits around 18 to 22 percent. Treat that as a guideline rather than a target, because the useful range shifts with the roast style, the bean, and the machine.
- Does the right roast color mean the coffee is well roasted?
- No. Agtron color tells you how dark a bean is, not whether it is fully developed. A bean can read the correct color and still be underdeveloped if it spent too little time after first crack. Color is one tool; the curve shape and development ratio tell you how you reached that color.