Coffee Taints and Defects: How to Identify Off-Flavours in the Cup

Every coffee professional and serious home brewer eventually encounters off-flavours in coffee — the cup that tastes of cardboard, of mould, of overripe fruit, of chemicals. Some off-flavours are defects that originate in the green bean (insect damage, over-fermentation, mould); others are taints that develop during processing, storage or brewing. The Specialty Coffee Association has catalogued the major defects and taints, and learning to identify them is one of the most useful cupping skills.

Identifying off-flavours is not just about avoiding bad coffee; it is also about understanding what went wrong. A phenole taint (medicine, iodine) often indicates insect damage to the cherry or bean during growing. A ferment taint (overripe fruit, wine gone bad) usually indicates over-fermentation during processing. A mould taint (musty, damp) indicates poor storage or drying. Each off-flavour tells a story about how the coffee was produced or handled, and reading that story is part of becoming a skilled cupper.

The principle behind coffee taints and defects

Coffee defects and taints have specific chemical origins, and each one tells you something about the production, processing, storage or preparation of the coffee. Identifying them requires a calibrated palate, a vocabulary to describe what you perceive, and an understanding of the underlying causes. The first step is recognising that an off-flavour exists; the second is identifying what it is; the third is deciding whether the cup is still acceptable or should be discarded.

An origin name is a starting point, not a flavour guarantee. Variety, altitude, how the cherry was processed, how far it was roasted and how you brew it all move the cup further than the country on the bag does. Treat regional character as a tendency worth knowing rather than a promise the producer made.

What most changes the result

Primary defects

These are the most serious and affect a single bean or small group of beans. They include black beans (over-fermented), sour beans (under-fermented), insect-damaged beans (borer holes), and fungus-damaged beans (mould). Primary defects are typically identified by visual inspection of the green bean before cupping.

Secondary defects

These affect the cup quality but are less visually obvious. They include over-fermented cups (wine, sour, chemical), under-fermented cups (grass, vegetable), and improper drying (musty, mould).

Taints

These are off-flavours that develop during processing or storage. They include chemical taint (from contamination during wet processing), phenole taint (from insect damage or certain fungi), and musty taint (from improper storage).

Staling

This is a separate category of off-flavour that develops over time after roasting. Stale coffee tastes of cardboard, paper, or sawdust. The chemistry involves the breakdown of aroma compounds and the oxidation of oils.

Common off-flavours and their meanings

Onion/garlic = fermentation issue or insect damage; cardboard/paper = staleness; rubber = over-fermentation or specific processing error; medicine/iodine = phenole taint from fungi or insects; musty/mouldy = improper drying or storage; chemical = contamination during processing.

A repeatable starting method

1. Cup suspicious coffees alongside a known-good reference. Off-flavours are easier to identify when you have a clean baseline.
2. Smell the dry grounds first, then the wet grounds. Some off-flavours are most apparent in the aroma.
3. Taste at multiple temperatures. Some off-flavours (especially ferment) are more apparent as the cup cools.
4. Use the SCA defect categories to label what you perceive. The vocabulary helps you communicate and remember.
5. Keep a defect log. Over time, you will see patterns that help you identify which producers and processing methods are more or less prone to specific issues.

Keep the bag in front of you while you taste: producer, region, process, roast date. Origin character is only legible once you can tell it apart from roast level and freshness, and that means recording all three.

Common mistakes

• Confusing intentional flavour with defects. Some "funky" naturals and anaerobic processed coffees have fermentation character that resembles ferment defects but is intentional and desirable. The line between intentional fermentation flavour and defective fermentation is a matter of degree.
• Throwing out good coffee because of a single off-flavour. Some lots have minor taints that do not ruin the cup. Evaluate the whole cup, not just the dominant note.
• Assuming staling is acceptable. A cardboard or paper taint is a sign that the coffee is past its prime. While stale coffee is not harmful, it is significantly less enjoyable.
• Trusting visual inspection alone. Many off-flavours are not visible in the green bean. Cup the coffee to know.

How to choose your next adjustment

Develop your off-flavour vocabulary through deliberate practice. Cup a range of coffees, including some intentionally flawed samples if you can find them, and label what you perceive. The skill takes time to build but pays off in better purchasing and brewing decisions.

One caveat on caffeine: it varies with species, dose and serving size rather than with origin or roast colour. A dark roast is not a stronger cup, and a famous region is not a stronger one either.

Related on CupCura

• Tea Storage & Airtight Canisters
• The coffee catalogue
• Flavour

Frequently asked questions

What should I change first with coffee taints and defects?

Primary defects. These are the most serious and affect a single bean or small group of beans. They include black beans (over-fermented), sour beans (under-fermented), insect-damaged beans (borer holes), and fungus-damaged beans (mould). Primary defects are typically identified by visual inspection of the green bean before cupping.

What is the most common mistake?

Confusing intentional flavour with defects. Some "funky" naturals and anaerobic processed coffees have fermentation character that resembles ferment defects but is intentional and desirable. The line between intentional fermentation flavour and defective fermentation is a matter of degree. It is worth ruling that out before changing anything else, because it makes every other adjustment harder to read.

Does common off-flavours and their meanings matter?

Onion/garlic = fermentation issue or insect damage; cardboard/paper = staleness; rubber = over-fermentation or specific processing error; medicine/iodine = phenole taint from fungi or insects; musty/mouldy = improper drying or storage; chemical = contamination during processing.

Can I trust the region printed on the bag?

As a tendency, yes; as a guarantee, no. Two lots from the same region can differ more than two lots from different continents once processing and roast are taken into account.

Sources

• SCA Arabica Green Coffee Defect Handbook — official defect classification; Food Chemistry 2013; 138(2–3): 1374–1380 — chemical basis of coffee off-flavours.