Green Coffee Defects and Quakers: How to Read Them and What They Do in the Cup

What counts as a green coffee defect?

A green coffee defect is any physical flaw in a raw bean or its surroundings that can be spotted by eye and is known to cost something in the cup or in consistency. Some are plain damage (a cracked bean), some are processing mistakes (leftover parchment), and some are signs of spoilage (black or sour beans). The Specialty Coffee Association groups them into primary and secondary types.

The defects themselves are not the news; the arithmetic is. Graders do not simply count flawed beans. They convert each flaw into a share of a "full defect" and add everything up, which is why a tray with 20 lightly insect-nibbled beans can pass while one holding a single full black bean cannot.

The 350 g tray: how a lot gets judged

The standard test spreads 350 g of green coffee on a table and asks the grader to pick out every flaw. To count as specialty, a lot needs zero primary defects and at most five full defects from the secondary list, plus no quakers in the roasted sample and a cupping score of 80 or higher (see our SCA score guide).

Try it with a made-up tray. Six partly black beans make 2 full defects, ten slightly insect-damaged beans make 1, and ten withered beans make 2. That is five: right on the line, and a single full black bean on top would sink the lot because it is primary.

One note on currency. On the SCA's own site, standard SCA-110 (grading, specifications and test methods for traded green coffee) is still listed under standards in development. In practice the trade keeps using the defect handbook grid described here.

Primary defects: the six deal-breakers

Primary defects are the flaws serious enough to lower a lot's grade by themselves, and the specialty grade allows none of them. Five count as one full defect per bean; severe insect damage takes five beans.

• Full black: a bean blackened right through. Reported causes include frost, fungus, extreme overripeness, moisture damage and fermentation errors.
• Full sour: caused by too long a wait between picking and pulping, fermentation that ran on, or storage at too high a moisture content. Badly affected lots can show the "stinker" character, an acetic or briny note.
• Dried cherry or pod: a hulling error that leaves the dried fruit and parchment on the bean.
• Fungus damaged: mould growth, often while the coffee dries. Some cases stay hidden until the roast.
• Foreign matter: stones, sticks, even machine parts gathered along the way.
• Severe insect damage: more than three separate holes, with the coffee berry borer (broca) the main culprit.

Secondary defects: ten smaller flaws

Secondary defects hurt less per bean, so the grid is more forgiving, but they are where careful sorting and drying show up in a lot's character.

• Partial black and partial sour: the same problems as above, caught earlier.
• Parchment and hull or husk: leftover skin or fruit from poorly set hulling equipment, which also adds a fire risk in the roaster.
• Floater: a low-density bean, sometimes immature, usually removed by flotation in washed processing.
• Immature: smaller than its neighbours and often curled at the tips.
• Withered: shrivelled, typically after drought.
• Shell: the "elephant ear", two halves nested together that roast unevenly.
• Broken, chipped or cut: damage from pulpers or hullers.
• Slight insect damage: up to three holes.

Quakers: the bean that hides until the roast

A quaker is an unripe bean that fails to brown during roasting and ends up pale beige beside its darker neighbours. It is usually undetectable in green coffee, and it lacks the sugars the Maillard reactions need, which is why it never gets the colour or sweetness of a mature bean.

The classic explanation is that someone picked a green cherry. That happens, but it is not the whole story: plant nutrition, drought stress and diseases such as leaf rust also produce quakers, which is how carefully hand-picked lots end up with some anyway. Sorting at the farm and the dry mill cuts the number without reaching zero.

Every quaker is an immature bean, but not every immature bean becomes a quaker. The green-bean defect called "immature" is judged raw; the quaker is judged after roasting, which is why the SCA tests for it separately.

What the research says about the cup

The most useful number comes from a 2021 Food Chemistry paper by Rabelo and colleagues. They added quakers of three colour levels, at seven concentrations, to natural specialty coffee. The palest ones (Agtron 82.8 or above) damaged the sensory profile from 7 quakers in a cup of 65 beans onward, around 11 percent.

That reads two ways. A stray quaker or two will not wreck a cup, which sits awkwardly beside the SCA's zero rule and is exactly the gap the authors set out to test. And above the threshold the effect is real: it traces back to volatile compounds the roast builds from precursors specific to an immature bean. The astringency and lack of sweetness tasters report are chemistry, not a roasting slip (see the Agtron scale).

For black, sour and mouldy beans, a 2024 proton NMR study in Current Research in Food Science (Hu and colleagues) found less sugar and lipid than in sound beans, and more acetate, the form of acetic acid. Less to caramelise and vinegar in the background fits what cuppers complain about.

A caution: the quaker study covers natural specialty coffees and one tasting protocol. I would not stretch it to washed coffees or other thresholds.

What to do with all this as a drinker

No bag prints a defect count, and a score of 84 does not tell you how many beans were rejected on the way to the bag. What reassures is a lot scored 80 or more (our guide to reading a specialty coffee label helps) from a roaster that cups what it buys.

At home, the only workable sort is on roasted beans: tip a handful onto a white plate, pick out the pale beige ones and bin them before grinding. Trying it on green beans is a good way to strain your eyes.

And before blaming defects, check the brew. A dry or bitter cup more often comes from grind size, temperature or contact time than from a quaker; the guide to metallic and astringent coffee walks through it. Do not mix up a green defect with a roasting fault such as baked, either: one happens before the roaster, the other in it.

Frequently asked questions

Sources

1. Specialty Coffee Association, Arabica Green Coffee Defect Handbook (primary and secondary categories, full defect equivalents, specialty grade limits). https://store.sca.coffee/
2. Specialty Coffee Association, Standards (SCA-110 green coffee grading, in development). https://sca.coffee/research/coffee-standards
3. Rabelo M.H.S. et al., Impacts of quaker beans over sensory characteristics and volatile composition of specialty natural coffees, Food Chemistry 342, 2021 (Agtron 82.8, 7 quakers in 65 beans). https://doi.org/10.1016/j.foodchem.2020.128304
4. Hu et al., Characterization of defective coffee beans and blends differentiation based on 1H qNMR technique, Current Research in Food Science 9, 2024. https://doi.org/10.1016/j.crfs.2024.100870
5. Royal Coffee, Green Coffee Glossary: Defects & Afflictions (definitions and causes of defects, hole counts for insect damage). https://royalcoffee.com/green-coffee-glossary-defects-afflictions/
6. Barista Hustle, The End of Quakers? (unripe seed, lack of sugars, failure to brown). https://www.baristahustle.com/the-end-of-quakers/