Coffee roasting is about exposing green coffee beans to an increasing temperature for a specific amount of time.
The graphical representation of the beam temperature at any given time during the roast is what we call a roasting profile.
Most roasting profiles look very similar, ... sometimes even identical.
This is a consequence of how all green coffee beans behave during a roast.
As soon as we drop green coffee beans inside of our roaster's drum, which is set to a preset temperature, we notice the temperature drops drastically as the beans absorb heat.
After this, the beans will start reaching an equilibrium represented by a steadily growing temperature.
During this period we usually raise the gas & decrease airflow
After the first crack the beans will start radiating their own heat. So the airflow should be maxed out and the gas set to a minimum.
This is normal behavior and there is little we could do to change it.
The only thing we can control is a profile slope.
The slope represents the roast speed and is expressed in degrees per min.
It is widely known as the Rate of Rise.
A steep slope as a consequence of a high Rate Of Rise.
This is known to grant bright acidity to the beans.
Medium slope triggers sweetness, Aroma and fragrance.
A low slope is characterized by chocolate and nutty flavor notes .
Sometimes the differences on a roasting profile are so small they are hard to see or they overlap
with each other.
For this reason there isn't a specific curve for the Rate of Rise.
It is expressed in degrees per minute.
During the beginning of the roast the ROR line usually disappears indicating the values are
negative or the temperature is lowering instead of rising.
After the Turning Point the rate of race becomes visible in the chart.
The normal tendency of the rate of rice is to be high at the beginning of the roast
and get lower as the roast progresses.
As a rule of thumb... a rate of rice lower than 10 is slow .
Between 10 and 15, its medium.
About 15 is a high rate of rise.
DRYING PHASE Targets
Start with a high ROR during the drying phase.... average of 15 or 16/min.
The aim is to dry the beans efficiently. Be cautious, don't go too fast as too much heat can scorch the bean surface and not dry the inside of the bean completely.
The ROR will commonly get into the mid 20's as it approaches the turning point.
It often peaks at the TP, (24-25/min) then starts to drop till it reaches dry-end.
I try to get to a ROR of about 15 at dry-end.
MAILLARD Targets
I like to start this phase at around 15/min and aim to drop it around to 5/min at 1st crack.
Aim for a ROR-level around 10°C per minute in the middle Maillard phase of the roast and around 5°C/min from when First Crack starts.
The ROR slope should get less steep during this phase.
DEVELOPMENT PHASE
It is common to aim for a lower ROR at First Crack and during the development phase.
Plan for the Exothermic Shift after yellowing (~160°C).
Remember that beans release their own heat. Reduce burner settings early to avoid ROR spikes.
Keeping a close eye on the rate of rise is important because it doesn't only
measure the quality of the rocks. It also measures your consistency and dexterity
as a roaster.
A smooth descending line indicates an uneventful roast and skillful roaster.
A choppy line indicates a dirty or maybe a defective temperature sensor, heat leakage or an
insecure operator.
Sudden changes in rate of Rise going upwards are called "flicks" and going downwards "crashes"
Flicks may result in Burnt, scorched, Rubbery and Toast notes while
Crashes may cause flat & dull notes .
Keep in mind: ambient humidity, green bean density, and roaster type (drum vs. fluid bed) all influence how ROR behaves. Eg: air roasters generally don't have a turning point.
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