Green coffee bean defects are physical imperfections that develop before coffee reaches the roaster. They form because of agronomic stress, harvesting errors, processing issues, or storage conditions, and they influence both flavour and roast behaviour.
The Specialty Coffee Association classifies defects as either primary or secondary. Primary defects are severe and not tolerated in specialty coffee. Even one primary defect in a 350 g sample disqualifies a lot from being classed as specialty. Secondary defects are less severe and allowed up to a set limit. This system helps buyers and roasters understand quality, risk, and consistency before coffee is roasted.
From here, defects generally fall into three groups: those that damage flavour directly, those that disrupt roasting, and those that do a bit of both.
Defects that damage flavour
Black and sour beans are the clearest indicators of serious quality loss. Full black beans are a primary defect and usually produce burnt or ashy flavours. Partial blacks are a secondary defect and tend to flatten the cup. These beans are brown or black, shrivelled, and often split open, and they usually result from overripe harvesting, excessive fermentation, water stress during cherry development, or damp storage.
Full sour beans are also a primary defect and produce sharp, vinegary acidity. Partial sour beans are less severe but still reduce sweetness and clarity. These defects are caused by delays between picking and depulping, uncontrolled fermentation, or storing coffee with moisture levels that are too high. Prevention depends on ripe harvesting, clean water, controlled fermentation times, and proper drying.
Fungal damage also sits in this group. Beans exposed to high moisture and warm storage conditions can develop mould from fungi such as Aspergillus, Penicillium, or Fusarium. Affected beans show powdery yellow or reddish-brown spots and create musty, mouldy flavours that can spread through a lot if storage is not controlled.
Defects that disrupt roasting
Broken and chipped beans are common secondary defects caused by poorly adjusted depulping or milling equipment. Because they are smaller and have more exposed surface area, they heat faster in the roaster, making even development harder to achieve.
Unhulled beans are another roasting issue. When parchment is not fully removed, the remaining hull can burn during roasting, adding bitterness and astringency while also damaging surrounding beans.
Floaters and bleached beans are low-density beans caused by uneven or rapid drying and poor storage. They roast unpredictably and usually produce flat cups. In SCA grading, five floaters count as one full defect.
Quakers also create roasting challenges. These underripe beans lack the sugars and amino acids needed for proper browning, so they remain pale during roasting and often taste bitter or hollow. They are easier to remove in wet processing but much harder to eliminate in dry-processed coffees, especially where mechanical harvesting is used.
Defects linked to agronomy and climate
Insect damage is a secondary defect caused by pests such as the coffee berry borer, white stem borer, and coffee bean weevil. Beans affected by the coffee berry borer often have small holes and can taste woody, earthy, or sour. Rising temperatures are allowing this pest to survive at higher altitudes and reproduce more frequently, increasing pressure on producers and raising sorting costs.
Potato taste defect (PTD) sits within this group and is most commonly found in coffees from the Great Lakes region of Central Africa, including Rwanda, Burundi, DR Congo, and Uganda. When present, affected beans smell strongly of freshly peeled potatoes once roasted and ground. There is no health risk associated with PTD, but a single affected bean can ruin an entire roast, making it one of the most commercially damaging defects for roasters.
PTD is strongly associated with damage caused by the Antestia bug. While the exact biochemical mechanism is still being studied, mitigation focuses on controlling the pest in the field and removing damaged beans during post-harvest sorting. Even with good processing, a small level of risk remains, which is why additional sorting at the roastery level is often necessary for coffees from these regions.
Withered or wrinkled beans are usually the result of drought stress. When cherries shrivel on the tree, the beans inside develop poorly, leading to low sweetness and weak structure. These beans are small and light and can often be removed by floatation in wet processing, while dry processing relies on density and size sorting.
Why this matters
Defects are not a sign of carelessness. They are part of working with an agricultural product that is exposed to weather, labour constraints, and infrastructure limits. What matters is understanding which defects are present, how they affect flavour and roasting, and where they come from.
For roasters, this makes sorting more purposeful and helps explain why some coffees behave differently in the roaster. For buyers, it adds context to pricing, availability, and quality variation between lots and seasons.
