tags: [] - coffee/roasting - coffee/roasting/defects aliases: - Scorching - Scorched coffee - Bean scorching

Scorched Roasts¶

Overview¶

Scorched roasts (or scorching) is a roast defect in which the outer surface of coffee beans is locally overheated and charred before the interior of the bean has had time to absorb heat evenly, producing darkened patches or stripes on the flat faces of the bean surface. Scorching is caused by too-high charge temperatures or excessively slow drum rotation that allows beans to remain in sustained contact with the very hot drum surface, concentrating conductive heat on one face while the opposite surface and bean interior remain cooler. The resulting cup is harsh, acrid, and smoky, with an unpleasant bitterness that cannot be corrected through brewing adjustment.

Mechanism¶

When green beans are charged into a drum at an excessively high temperature, or when the drum rotates too slowly to cascade the beans regularly:

Beans resting against the drum wall or flights receive intense, sustained conductive heat input on their contact surface
The contact surface temperature rises far faster than the bean's interior can equilibrate
At the contact surface, temperatures sufficient for rapid caramelisation and pyrolysis are reached while the bean body remains in the drying phase
Dark, carbonised patches develop on the face in contact with the drum; the opposite face and interior develop more slowly

Scorching is distributed across the flat faces of beans — the largest surface area in contact with drum walls — rather than at the tips (which is the pattern of tipping). Both defects can appear simultaneously in severely overcharged batches.

Visual Identification¶

Scorched beans are identifiable after roasting by:

Dark patches or irregular dark stripes on the flat sides of the bean, against a lighter background body colour
The darkened areas have a flat, matte, or slightly pitted texture rather than the uniform gloss of a normally developed bean
In severe cases, the bean appears two-toned — dark on one face, normally developed or lighter on the other

Sensory Impact¶

Scorching contributes acrid, smoky, and harsh off-flavours:

Aroma: Charred, smoky, ash-like; the normal roast aromatics are obscured
Flavour: Harsh bitterness, acrid, burnt wood or rubber notes
Aftertaste: Persistent harsh bitterness and dryness

Even a small proportion of scorched beans in a batch can detectably affect the cup because the charred surface material contributes disproportionate bitterness and off-flavour compounds when ground and extracted.

Prevention¶

Correct charge temperature: The primary control; charge temperature should be calibrated to avoid excessive temperature differential between drum surface and bean at charge
Adequate drum rotation speed: Sufficient rotation prevents individual beans from sustained contact with the drum wall; faster rotation distributes conductive exposure across all bean surfaces
Drum pre-heating equilibration: Ensuring the drum is at a stable, even temperature throughout before charging, rather than charged with residual hot spots from previous batches or burner cycling
Batch size matching: Running batches significantly below the roaster's designed capacity can leave proportionally more beans in prolonged contact with drum surfaces; minimum batch sizes should be observed

Key Facts¶

Scorching: localised charring of bean flat faces caused by excessive conductive heat at charge or inadequate drum rotation
Distinguished from tipping (which affects bean tips specifically)
Causes: too-high charge temperature; too-slow drum rotation; excessive temperature differential at charge
Visual: dark patches on flat faces; bean appears two-toned
Sensory: harsh, acrid, smoky bitterness; off-flavour compounds detectable even in small proportions
Prevention: appropriate charge temperature calibration, adequate drum rotation speed

References¶

Changelog¶

Date	Change
2026-04-27	Note created

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