Why Roast Defects Happen
Roast defects occur when heat application during roasting is uneven, too rapid, or inappropriate for the bean's density and moisture content. Unlike green bean defects (physical damage or disease), roast defects are caused by roaster technique — and most are avoidable once you understand their causes. The three most common home roasting defects are tipping, scorching, and baking.
Tipping
Appearance: The tips (pointed ends) of beans turn dark brown or black while the rest of the bean is lighter. Cause: Too much heat too early in the roast (during the drying phase), before the bean has absorbed enough heat uniformly. The tips, being thinnest, char before the rest of the bean is ready. Cup impact: Astringent, papery, or slightly burnt notes concentrated in the finish. Fix: Lower the initial heat charge or extend the drying phase. In air roasters: start at lower heat setting for the first 3–4 minutes before increasing.
Scorching
Appearance: Dark patches on the flat face of beans (where they contact the drum surface or heated element) while other areas are lighter. Cause: Direct contact between beans and a very hot surface during the early roast when beans still have high moisture content. More common in drum roasters with too-high initial drum temperature. Cup impact: Acrid, smoky, burnt flavor concentrated in the affected roast areas. Fix: Preheat the drum to a lower temperature before charging (adding) beans. Ensure good drum rotation and bean agitation.
Baking
Appearance: Beans look visually normal for their roast level — baking is detected by taste, not appearance. Cause: Too-low roast temperature (especially stalling — allowing temperature to plateau) during the later stages of the roast. The beans "bake" rather than developing flavor compounds through the Maillard reaction. Cup impact: Flat, bread-like, or cardboard-like flavor. Lacks the sweetness and complexity expected at the roast level. Reduced acidity and thin body. Fix: Maintain steady temperature rise through the roast — avoid plateaus or drops in temperature after the drying phase. Roast time should be appropriate to batch size and roaster capacity (don't rush, but don't stall).
Reading Your Roasted Beans
Inspect roasted beans against a white background or in direct light. Even color across the batch indicates even heat application. Color variation within individual beans (lighter on one face, darker on the other) indicates uneven drum rotation or uneven airflow. Checking your roasted beans before you brew trains you to see the connection between visual defects and flavor problems in the cup.
Infrared Thermometer (Roast Monitoring)
$A non-contact infrared thermometer lets you track bean temperature during air roasting without opening the roaster — useful for correlating temperature to roast stage.
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Tipping: too much heat too early (dark tips). Scorching: hot surface contact early in the roast (dark patches). Baking: temperature stall producing flat flavor (no visual indicator). All three are technique defects, not bean defects — adjusting heat application eliminates each one.