Timing the Drying phase

 Drying time in coffee roasting—the phase from charge to the yellowing/browning point (typically 100–160°C / 212–320°F)—is determined by the need to evaporate 10–12% moisture from the green beans. This stage usually lasts 4–7 minutes. 

Key factors influencing this duration include:

1. Bean Physical Properties

Density: Denser beans (e.g., high-altitude, strictly hard bean) are harder for heat to penetrate and require more energy and longer drying times.

Moisture Content: Higher moisture content requires a longer drying phase to evaporate water without creating uneven moisture pockets, which can lead to burnt exteriors and raw interiors.

Processing Method: Natural process beans often have higher sugar content and can dry faster, sometimes needing lower initial heat to avoid scorching compared to washed beans. 

2. Roaster Technology and Control

Roaster Type: Fluid-bed (air) roasters often dry faster than drum roasters due to more efficient convective heat transfer.

Batch Size: Larger batches hold more total moisture and have higher thermal mass, requiring more energy and longer time to heat evenly.

Charge Temperature: The initial temperature of the roaster affects how fast the beans turn color. A higher charge temperature generally results in a shorter drying phase, while a lower charge temp extends it.

Airflow Settings: Higher airflow increases the rate of evaporation, speeding up the drying phase. 

3. Environmental and External Factors

Ambient Temperature: A colder roasting environment will increase heat loss from the machine, requiring more time to reach the yellowing point.

Humidity: Higher humidity in the air can slow down the roasting process by making it harder for the beans' moisture to escape. 

Optimal Drying Strategy

Avoid Rushing: Rushing the drying phase can lead to uneven, underdeveloped, or "baked" coffee.

The "Yellowing" Target: A common goal is to reach the yellow/tan color shift around the 5-minute mark.

Aroma Cues: The stage ends when the aroma changes from a grassy/hay-like smell to a sweeter, bread-like aroma.

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Typically I like to see a ROR of about 18-25

It will be higher at the beginning as you are introducing more heat very quickly

Typically the time from "charge" to "dry end" = 50% of the total roast time

Browning = 30%

Development = 20%

These ratios though not exact give you a good idea if your roast is progressing in a timely manner