Even with the best equipment and technology, roasting coffee is still a delicate process that requires experience and precision. When things go wrong, identifying the issue and making the right adjustments can save an entire batch. Here’s a deep dive into common roasting mistakes, their causes, and how to fix them.

1. Scorching: Burnt Spots on Beans

Symptoms: Beans have dark, burnt patches, especially on one side, leading to an ashy, bitter taste.

Causes:

Charge temperature too high → Beans get excessive heat shock upon entering the drum.

Slow drum speed → Beans sit in direct contact with the hot drum surface for too long.

Insufficient airflow → Heat accumulates unevenly, causing some beans to overheat.

Fixes:

✔ Lower the charge temperature slightly, especially for smaller batches.
✔ Increase drum speed to keep beans moving and avoid prolonged surface contact.
✔ Improve airflow to ensure heat is distributed evenly.

2. Tipping: Burnt Edges on Beans

Symptoms: The tips of the beans appear darker than the rest, often resulting in an unpleasant burnt-sugar taste.

Causes:

Too much heat applied too early → The bean structure weakens, causing the tips to overheat.

High initial drum temperature with rapid heat application → Beans can’t absorb heat evenly.

Fast RoR (Rate of Rise) spike in the early stages → Leads to thermal stress.

Fixes:

✔ Lower initial heat application and introduce heat more gradually.
✔ Slow down the drying phase to allow even internal heat distribution.
✔ Monitor RoR carefully—avoid aggressive heat jumps early on.

3. Baking: Flat, Lifeless Coffee

Symptoms: Coffee tastes dull, lacking complexity, sweetness, and acidity.

Causes:

Roasting too slowly after first crack → Beans lose aromatic compounds without proper development.

Low Rate of Rise near first crack → The roast stalls, resulting in a flavorless profile.

Excessively long total roast time → Causes over-drying rather than proper development.

Fixes:

✔ Avoid “stalling” the roast—maintain a steady heat flow.
✔ Adjust RoR to ensure enough momentum into the first crack without letting it drop too quickly.
✔ Optimize airflow—too little airflow can extend the roast unnecessarily.

4. Underdevelopment: Grassy, Sour, or Woody Notes

Symptoms: Coffee has a green, hay-like, or peanutty taste with sharp acidity.

Causes:

Too short of a roast time → Beans haven’t gone through proper chemical transformations.

Not enough development time after first crack → Acidity is too pronounced, and body is weak.

Airflow too high → Heat is pulled away too fast, preventing proper roasting.

Fixes:

✔ Extend post-first crack development time (typically 15-25% of total roast time).
✔ Ensure a gradual but steady rise in temperature throughout the roast.
✔ Slightly decrease airflow to allow better internal heat transfer.

5. Overdevelopment: Muted Acidity, Smoky or Charred Flavors

Symptoms: Coffee lacks complexity and has a flat, overcooked taste—almost like a dark roast but without the boldness.

Causes:

Too long of a development time after first crack → Sugars break down too much, muting acidity.

Excessive heat in the later stages → Causes unnecessary caramelization, making the coffee taste dull.

Not enough airflow → Traps smoke, leading to unwanted flavors.

Fixes:

✔ Shorten post-first crack development time while maintaining proper heat balance.
✔ Increase airflow in the later stages to avoid trapping smokey flavors.
✔ Monitor bean temperature closely to avoid going too deep into the roast curve.


6. Uneven Roasting: Some Beans Appear Lighter or Darker Than Others

Symptoms: The batch has a mix of light and dark beans, leading to inconsistent flavors.

Causes:

Uneven heat distribution in the drum → Some beans get more direct heat than others.

Overloading the drum → Beans don’t have enough space to move freely.

Inconsistent bean size or moisture content → Some beans absorb heat faster than others.

Fixes:

✔ Ensure proper drum capacity—don’t overload the machine.
✔ Increase agitation (drum speed or airflow) to ensure even heat exposure.
✔ Sort beans by size and moisture content before roasting for consistency.

7. Charring: Beans Look Over-Roasted on the Outside but Taste Underdeveloped Inside

Symptoms: The exterior is very dark, but the interior remains raw, causing a mix of bitterness and sourness.

Causes:

High heat applied too quickly → The outside burns before heat penetrates the core.

Too fast of a roast overall → Beans don’t get enough time for internal development.

Insufficient drum rotation speed → Beans spend too much time in contact with hot surfaces.

Fixes:

✔ Reduce charge temperature and allow a steadier heat build-up.
✔ Extend roast time slightly to allow heat to penetrate the core.
✔ Increase drum speed for better movement.

Bonus: Fine-Tuning Roasts for Different Brew Methods

Once you’ve fixed roasting mistakes, you can optimize your roast profile for specific brew methods: