Roast defects and how to taste them

When coffee tastes bad, most people assume they brewed it wrong. Sometimes they did. Sometimes the coffee was already compromised.

Learning to distinguish the two saves a great deal of pointless adjustment.

Roast defects

Baked. The most common and the most insidious.

Caused by a roast where the rate of temperature rise stalled — the coffee spent too long at too low an energy input, particularly through the Maillard phase.

Tastes like: flat, papery, cardboard, low sweetness, no defined acidity, a dull and short finish. The coffee is not offensive; it is simply absent.

Distinguished from brewing error by the fact that no adjustment to grind, ratio or temperature produces sweetness or definition. Under-extraction is sour; baked is flat.

Underdeveloped. Dropped too soon after first crack, or roasted so fast the interior did not develop.

Tastes like: sour, grassy, vegetal, sharp, with a hollow finish and sometimes a peanut or raw cereal note.

Easy to confuse with under-extraction. The test is to grind finer and brew hotter — if it improves substantially, it was brewing; if it stays grassy and sharp, it was the roast.

Scorched. Excessive conductive heat early, burning the bean surface against the drum.

Visible as dark marks on the flat faces of beans.

Tastes like: burnt, acrid, harsh, with an ashy finish.

Tipped. Excessive heat causing the bean tips — where the seam and the germ are — to burn.

Visible as small dark spots at the ends of beans.

Tastes like: a burnt, slightly bitter note over an otherwise adequate coffee.

Faced or flat-faced. Similar to scorching, with a burnt flat side.

Over-roasted. Taken too far, into or past second crack when that was not intended.

Tastes like: smoky, carbonised, bitter, with no origin character. Visible as an oily surface and very dark colour.

Uneven roast. Beans of noticeably different colours in the same bag.

Caused by inconsistent green coffee, poor agitation, or an overloaded drum.

Tastes like: simultaneously sour and bitter, since some beans are underdeveloped and some over.

Green coffee defects

Faults that were present before roasting.

Ferment. Processing went wrong — cherries sat too long, fermentation was uncontrolled, or drying was too slow.

Tastes like: sharp vinegar, rotten fruit, alcohol, or in mild cases an over-ripe wine note.

Distinguishing a deliberate anaerobic process from a ferment defect is genuinely difficult and is a live argument in specialty coffee.

Phenolic. A medicinal, chemical, iodine-like character.

Associated with certain processing and storage problems. Unmistakable once encountered.

Potato defect. A raw potato aroma in individual beans, associated with bacterial infection following antestia bug damage.

Found in some East African coffee, particularly from Rwanda and Burundi. Affects single beans, so a cup may be affected while the next is not.

Woody or past crop. Green coffee that has aged too long.

Tastes like: flat, papery, woody, with faded acidity and a dry, dusty quality.

Distinguished from baked by the fact that it is present regardless of who roasted it, and by the harvest date if stated.

Mouldy or musty. Poor drying or storage. Damp, earthy, cellar-like.

Rio or rioy. A harsh iodine and medicinal character associated with certain Brazilian coffees, historically caused by particular processing conditions.

Staling

Not a defect exactly, and the most common cause of disappointing coffee.

Tastes like: flat, muted aroma, cardboard, and in darker roasts a rancid greasy quality from oxidised oils.

Distinguished by the roast date and by the bloom — stale coffee barely swells when wetted.

The diagnostic sequence

When a coffee disappoints, work through this.

Check the roast date. Over six weeks old, or no date at all, and staling is the likely answer.

Check the bloom. Little or no swelling confirms it.

Adjust grind in both directions. If neither improves it materially, it is not extraction.

Look at the beans. Uneven colour, tipping, scorching and excessive oil are all visible.

Brew it a different way. A coffee that is bad in every method is the coffee. One that is bad in one method is probably technique.

Taste it cool. Defects that hide behind heat become obvious at room temperature, which is why cuppers taste as the cup cools.

What to do about it

Tell the roaster. Good ones want to know, and a specific description — flat and papery, or sour and grassy — is more useful than a complaint.

Reputable roasters will generally replace a defective bag.

And if a roaster's coffee consistently shows the same fault, that is information about the roaster rather than about a single bag.