Every home roaster produces defective roasts — the trick is recognizing what went wrong so you can fix it next time. Here's a practical guide to the most common defects, what causes them, and how they taste.
Underdevelopment
What it looks like: Pale color, sometimes with a grassy or hay-like tinge. Uneven color across the batch — some beans lighter than others.
What it tastes like: Sour, grassy, vegetal, astringent. Thin body. The acidity is sharp and unpleasant (not the bright, clean acidity of a good light roast).
What went wrong: Roast ended too early — before the sugars had time to caramelize and the Maillard reactions completed. Often happens when trying to roast too light or when heat was too low during development.
Fix: Extend development time. Aim for at least 15-20% DTR (Development Time Ratio) for light roasts, more for medium and dark.
Baking
What it looks like: The beans look normal — correct color for the target roast level. Visually hard to detect.
What it tastes like: Flat, bread-like, papery. No sweetness, no brightness. The coffee tastes "dead." This is the most insidious defect because it's invisible.
What went wrong: The Rate of Rise (RoR) stalled or went flat during the roast. The beans cooked slowly without momentum, drying out the sugars instead of caramelizing them. Often caused by reducing heat too aggressively before or during first crack.
Fix: Maintain a declining but positive RoR through the entire roast. Don't cut heat — reduce it gradually.
Scorching
What it looks like: Dark spots or burn marks on the flat side of the bean (the side that contacts the drum or chamber wall). The rest of the bean may look fine.
What it tastes like: Smoky, burnt, acrid. Can taste like burnt toast even in an otherwise light or medium roast.
What went wrong: Charge temperature was too high, or the bean mass was too small for the heat applied. The surface of the beans burned before the interior had time to develop.
Fix: Lower your charge temperature. On air roasters, make sure you're not underfilling the chamber — too few beans means each one gets blasted with too much direct heat.
Tipping
What it looks like: Dark or charred points on the tips or edges of the beans.
What it tastes like: Similar to scorching — smoky, bitter, acrid notes that don't belong at the roast level.
What went wrong: Too much heat applied too early, especially during the drying phase. The thin edges of the bean cook faster than the dense center.
Fix: Start with moderate heat and ramp up gradually. On a Fresh Roast, this means not starting at heat level 9 right from charge.
Overdevelopment
What it looks like: Darker than intended. Oily surface on beans you meant to roast medium. Very even color (unlike underdevelopment).
What it tastes like: Bitter, ashy, carbonic. Thin body despite the dark color. The origin character is gone — all you taste is roast.
What went wrong: Roasted too long past first crack (or into/past second crack unintentionally). On air roasters, the gap between City+ and French can be 15-20 seconds.
Fix: Drop earlier. Set a target development time and stick to it. On fast-roasting air roasters, be ready to dump the moment you hit your mark.
Have you dealt with a defect not listed here? Share what happened and how you fixed it.