tags: [] - coffee/tasting - coffee/quality-control aliases: - Coffee defect identification - Off-flavour recognition - Cup fault detection

Defect Recognition¶

Tags: #coffee/tasting #coffee/quality-control Aliases: Coffee defect identification, Off-flavour recognition, Cup fault detection Related: Quality Control MOC | Defect Categorisation | Defect Recognition Training | Espresso MOC | Milk Texturing Status: ✅ Complete

Overview¶

Defect recognition is the ability to identify off-flavours and quality problems in the cup and trace them to their source. At the foundational barista level, the goal is to reliably identify the most common defects encountered in daily café work — primarily sourness from under-extraction, bitterness from over-extraction, and milk texture faults — and link each defect to its correctable cause. The sequence of practice is: recognise → name → trace → correct. For the formal sensory training programme, reference standards, and defect categories across the full supply chain, see Defect Recognition Training and Defect Categorisation.

Primary Espresso Defects¶

Sourness (Under-Extraction)¶

Cup character: Sharp, harsh, vinegary acidity with absent sweetness; thin body; short, unpleasant finish.

Common causes: - Grind too coarse → water flows too quickly → insufficient extraction - Dose too low → water passes through too easily - Yield too high → long shot time but sparse actual extraction - Water temperature too low - Very fresh coffee (active CO₂ degassing repels water)

Corrective approach: A finer grind is the primary adjustment. If grind change does not resolve the defect, dose, yield, and machine temperature are investigated. Exceptionally fresh coffee may require additional resting time.

Bitterness and Astringency (Over-Extraction)¶

Cup character: Harsh, drying bitterness at the back of the throat; lingering astringency; heavy but rough body; absent sweetness.

Common causes: - Grind too fine → water moves too slowly → over-extraction - Dose too high relative to yield - Channelling → water finds a path of least resistance and over-extracts that section - Very dark roast (inherent bitterness that cannot be fully removed by extraction adjustment) - Water temperature too high

Corrective approach: A coarser grind is the primary adjustment. Where channelling is suspected, distribution and tamping technique are examined.

Channelling¶

Cup character: Often sour and bitter simultaneously — under-extracted in the channelled section, normal or over-extracted elsewhere.

Visual indicator: An irregular flow from the basket during extraction; streaming from one side rather than even, centred flow.

Causes: Uneven distribution; poor or angled tamping; damaged or worn basket; irregular dose.

Corrective approach: Consistent distribution technique, verification of tamping angle, and basket inspection for damage.

Stale Coffee¶

Cup character: Flat, papery, cardboard, or musty notes; absent sweetness and brightness; empty aftertaste.

Cause: Coffee past its optimal post-roast period (typically two to four weeks from roast date for espresso, though this varies by coffee and storage conditions). CO₂ has fully degassed; aromatic compounds have oxidised.

Corrective approach: No grind adjustment resolves staleness. Fresher stock is required. Roast date should be checked and stock rotation reviewed if staleness recurs.

Rancid Oils and Dirty Equipment¶

Cup character: Rubbery, rancid, or intensely bitter notes; sour-milky or cheesy aftertaste in milk drinks; sometimes described as ashtray.

Cause: Accumulation of oxidised coffee oils in the group head, portafilter basket, portafilter collar, or steam wand. These transfer to every subsequent shot and drink.

Corrective approach: Immediate thorough cleaning — backflushing, cleaning the portafilter, scrubbing the basket, purging the steam wand — is required. The machine should not continue service until cleaning is complete.

Primary Milk Defects¶

Over-Steamed or Scalded Milk¶

Cup character: Flat, cooked sweetness; reduced milk flavour; can taste unpleasant or slightly sour.

Cause: Milk taken above 70°C; lactose breaks down and delicate proteins are heat-damaged.

Corrective approach: The affected milk is discarded. Fresh cold milk is used for a new steam cycle. Previously steamed milk must not be re-steamed.

Coarse Foam with Large Bubbles¶

Cup character: Dry, airy, inconsistent texture; bubbles burst on the tongue; the drink separates into foam and liquid layers.

Cause: Excessive air incorporation; insufficient rolling to break down bubbles; steam wand positioned too close to the surface throughout steaming.

Corrective approach: Increased rolling time during steaming and repositioning the steam wand to create a vortex through the rolling phase.

Under-Steamed Milk¶

Cup character: Thin, watery, cold texture; little body or microfoam.

Cause: Insufficient steaming time; steam pressure issue; incorrect wand positioning.

Corrective approach: Fresh cold milk and a correctly executed steam cycle. Previously steamed milk should not be re-steamed.

Fermentation and Environmental Defects¶

Less common in a well-managed café but worth recognising:

Fermented or winey: Harsh, vinegar-like sourness beyond what extraction can explain. May indicate a green coffee defect (over-fermentation during processing) or contaminated storage. Not correctable at the bar — flagged to management for supplier review.

Earthy or potato defect: Common in some East African coffees. A raw potato or starchy note in the cup, caused by Antestia bug damage to the green coffee; unpredictable even in high-quality lots. Cannot be corrected at bar.

Chemical or rubbery: May indicate degrading group head gaskets or cleaning chemical contamination. The machine should be taken out of service for investigation.

Defect Investigation Sequence¶

A systematic approach to defect investigation:

Confirm the defect — retaste; ask a colleague to taste independently to rule out palate fatigue
Name the defect precisely — sour, bitter, flat, rancid, coarse foam, etc.
Identify the most likely cause — use the cause patterns above
Apply the primary correction — usually grind adjustment for espresso faults
Retaste to verify — confirm whether the defect has been resolved
If unresolved after two grind adjustments — investigate equipment condition, coffee age, and cleaning history

Key Facts¶

The primary espresso defects are sourness (under-extraction), bitterness/astringency (over-extraction), channelling, stale coffee, and rancid equipment oils
Grind adjustment is the primary corrective tool for both under- and over-extraction
Milk defects include scalded milk (above 70°C), coarse foam (excess air), and under-steamed milk — each requires fresh cold milk to correct
Fermentation and environmental defects originate upstream (green coffee or processing) and cannot be corrected at the bar
The investigation sequence — confirm, name, trace, correct, verify — prevents blind grind adjustments from masking underlying equipment or sourcing problems

References¶

Changelog¶

Date	Change
2026-05-02	Compliance review: full rewrite — added frontmatter and metadata block; removed `../` wikilinks (Extraction Tasting, Extraction Recognition, Espresso Dialling, Milk Quality Assessment); removed `05_PUBLISHING/Homepage/Coffeepedia` footer; converted second-person correction instructions to third-person corrective approach descriptions; added Key Facts, References, Changelog, copyright

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