Rethinking green coffee grading: From what we see to what we taste

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For more than a century, coffee professionals have relied on their eyes to judge the quality of green coffee. Visual inspection, counting defects, sorting by color, shape, and uniformity, has been the cornerstone of green grading.

Recent research led by CESURCAFÉ in Colombia, in collaboration with the Coffee Excellence Center at the Zurich University of Applied Sciences (ZHAW), is asking a fundamental question: Do physical defects really correlate with sensory defects? Their project, “Understanding Physical Defects in Green Coffee – Impact on Sensory, Aroma Formation and Green Bean Composition”, aims to determine whether long-held assumptions about bean appearance hold up when tested through chemistry and controlled sensory evaluation.

When "defects" aren’t always defective

History reminds us that coffee’s perception of “defect” is not static. Peaberries, those small, rounded beans that occur when a cherry produces one seed instead of two, were once rejected as inferior. Today, many roasters and consumers celebrate their unique flavor concentration.

As processing methods diversify and climate change alters how coffee cherries develop and dry, the very idea of what a “perfect” coffee looks like is evolving. Insect activity, variable rainfall, and microbial influences during fermentation all leave visual marks on beans—but not all marks translate into negative sensory experiences.

“Identifying defects isn’t always a black-and-white process,” notes Camila Khalifé during her talk “Tasting the Standards” at the 2024 Roast Summit. “We’re realizing that some of what we call defects are simply expressions of a coffee’s journey.”

Science steps in: From sight to smell

To go beyond appearances, the CESURCAFÉ–ZHAW team is using state-of-the-art analytical chemistry. One of their key techniques is solvent-assisted flavor evaporation (S.A.F.E.), which extracts volatile aromatic compounds from coffee samples without damaging them. These volatiles—the molecules that give coffee its aroma—are then diluted and evaluated by trained sensory panels.

In a further step, researchers apply gas chromatography coupled to olfactometry (GC–O). Here, separated aroma compounds pass through a “sniffing port,” allowing scientists to literally smell individual compounds as the machine identifies their chemistry. This combination of sensory and analytical data bridges what the nose perceives with what the lab measures.

For example, “sour” beans have long been flagged as defective, but the precise mix of volatile compounds responsible for that perception was largely unknown. With GC–O, researchers can finally match odor impressions with molecular identity, a crucial step in distinguishing harmless flavor variation from true quality loss.

Testing the thresholds

Perhaps the most striking finding so far is one of scale: in controlled tests, panels of expert tasters could only identify a defect’s influence when defective beans made up around 7.5% or more of a batch. Below that threshold, even professionals struggled to perceive a difference.

For everyday consumers drinking espresso or filter coffee, this suggests that small proportions of physical defects may be sensory invisible. If that’s true, the industry’s long-standing penalty system for visual defects could be overstating their real impact on cup quality and potentially undervaluing coffees with strong sensory potential but imperfect appearance.

Defining “defect” in a new era

The Specialty Coffee Association’s (SCA) current green grading system divides defects into Category 1 and 2, each assigned a weighted score. But as Mirna Nagi and Peter Giuliano of the Coffee Science Foundation emphasize, there is insufficient research directly linking these physical defects to negative flavors.

The emerging view is that defect definitions must be reframed through modern sensory, chemical, and even toxicological science, not just visual inspection. After all, value in coffee is more than flavor: it includes traceability, sustainability, and the story each producer tells through their work.

From Lab to Farm: Implications for Producers

At Capilla del Rosario, we see this new wave of research not as an academic abstraction, but as an opportunity to rethink quality from the ground up. The studies led by the Coffee Excellence Center (ZHAW) and CESURCAFÉ in Huila—where researchers hand-sorted 12 defect types, roasted and cupped them, and analyzed volatiles using S.A.F.E. and GC–O—have shown how scientific rigor can reveal what truly matters to cup quality.

Inspired by this model, we’re designing our own scaled version of the experiment:

Defect sorting: We’ll manually isolate and classify defective beans from our harvests, documenting each by variety, process, and environmental conditions.

Controlled cupping: Using a consistent roast curve, we’ll cup samples with increasing defect ratios (0–10%) to identify sensory thresholds relevant to our coffees.

Volatile analysis: In collaboration with local research partners specializing in food chemistry and volatile analysis, we plan to conduct simplified SPME–GC–MS studies to connect what our cuppers perceive on the table with measurable changes in coffee aroma compounds.

Open data: Results will be shared with our roasting and research partners, contributing to a broader, evidence-based understanding of how visual defects actually affect flavor.

Our goal is to develop a Capilla del Rosario Green Grading Framework, a system rooted in science, adapted to our land, and grounded in sensory reality. Because if coffee science is teaching us that not every imperfection is a flaw, it’s time to start tasting beyond appearances.

Rethinking green coffee grading: From what we see to what we taste

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