Grounding Green Grading in Sensory Science: Research to Understand Physical Coffee Defects | 25, Issue 24
The SCA’s Publications Manager, LAUREL CARMICHAEL, introduces a Coffee Science Foundation research project on the sensory impact of physical defects in green coffee, undertaken at the Coffee Excellence Center at the Zurich University of Applied Sciences.
Introduction by MIRNA NAGI, CSF and SCA Research Program Manager and PETER GIULIANO, CSF Executive Director and Chief Research Officer
The Specialty Coffee Association’s (SCA) Green Coffee Classification standard describes certain physical attributes of coffees as “defects,” which it defines as “a material property of the green or roasted coffee beans that is broadly seen as negative.” But why are these properties “negative” in the first place? Some defects are discolored or visually unappealing, but more significantly, they are assumed to contribute to negative flavors in the cup.
Recently, during a review of the scientific literature on these defects, we at the Coffee Science Foundation (CSF) realized that there is insufficient research actually linking these physical defects to negative flavors. As part of its commitment to grounding standards in sound science, the SCA provided funding to fill this research gap.
In late 2024, the CSF released a request for proposals, seeking scientists who were interested in studying this crucial relationship between physical coffee defects and flavor. The aim of the project is to investigate “defects” and their impact in the context of modern sensory, chemical, and toxicological science, in order to provide high-quality information to the SCA’s standards development group and inform their upcoming revision of the SCA Green Coffee Classification and the Coffee Value Assessment.
Upon reviewing the proposals, the CSF awarded a grant to the Zurich University of Applied Sciences (ZHAW), who are collaborating with CESURCAFÉ in Colombia on the project “Understanding Physical Defects in Green Coffee: Impact on Sensory, Aroma Formation and Green Bean Composition.”
The following feature, authored by our colleague Laurel Carmichael, gives us a glimpse into the research that is underway and shows how challenging flavor science can be. We're excited about the research and other studies that are happening in parallel—it’s all part of our commitment to making coffee better through science and research.
For over a century, coffee professionals have used their eyes to assess the quality of green coffee.
The process of identifying physical, visible defects in coffee—known as green grading, or physical assessment—is a core part of how coffee lots are separated, differentiated, and priced in the marketplace. Coffee professionals throughout the supply chain are trained to count and remove “imperfect” beans from “quality” or “clean” beans, a process has conventionally been part of assessing whether a coffee is “specialty” or not.
Some of these defects occur during the post-harvest processing of coffee, such as when beans get broken or chipped in a depulper or at the dry mill. Many others—including small holes that insects bore into coffee cherries as they grow on the tree—occur naturally, impacted by the vibrant and sometimes unpredictable ecosystems in which coffee grows. As Camila Khalifé points out in her 2024 talk “Tasting the Standards,” “coffee doesn’t grow without defects.”[1] It’s not uncommon to find a piece of concrete from a drying patio, a kernel of corn (which, yes, might literally pop inside the roaster), or a whole, dried coffee cherry in a bag of green coffee—a testament to the journey coffee takes before it even reaches a roastery.
As well as implementing agricultural practices designed to optimize coffee quality, producers and coffee pickers often perform sorting during harvesting: separating overripe, underripe, or damaged cherries. Before coffee is exported, there’s extensive sorting at wet and dry mills, completed by hand and/or with a series of machines that can separate coffee based on color, size, and density.
Why is so much labor expended on a process that ultimately reduces the volume of coffee sold, or coffee sold at a premium price? Why is it considered worthwhile to remove beans that—even if imperfect—have spent months ripening on a tree before being handpicked and meticulously processed? It’s because it’s generally accepted that these defects have a negative impact on the sensory quality of coffee and, accordingly, reduce coffee’s value. This logic underpins numerous grading systems around the world (see figure 1), including the Green Arabica Coffee Classification System, created by the Specialty Coffee Association of America (SCAA) in 2001 and explained in the 2004 Washed Arabica Green Coffee Defect Guide.[2] Green grading is high-stakes: as noted in the Specialty Coffee Association’s Cupping and Sensory Handbook, “the presence of defects or lack of uniformity are existential issues,” impacting coffee sellers’ ability to attain premium prices, or even to sell their coffee entirely.[3]
Figure 1.
The criteria used for green coffee quality standards by country. Data taken from the National Quality Standards outlined in the 122nd session of the International Coffee Council. *As reported in document PM-29/13, September 2013.[4]
The Current State of Green Grading
Within the Green Arabica Coffee Classification System, defects are categorized into two groups based on their presumed severity: category 1 (primary) and category 2 (secondary) defects. The system assigns defects different weightings, according to how significantly they impact the affected bean and based on an understanding of how they impact a coffee’s overall quality (see figure 2). For example, one “full black bean” (a category 1 defect where the entire bean appears a blackish shade, see figure 3) is equivalent to three “partial black beans” (a category 2 defect). A grader must count five broken, chipped, or cut beans (category 2) before these are considered equivalent to a full category 2 defect. The 2004 cupping protocol—now superseded by Coffee Value Assessment (CVA) standards—stated stringent requirements for specialty grade coffee: “zero category 1 defects” and no more than “five full category 2 defects” in a 350 g sample.[5]
Figure 2.
An excerpt from the CVA Physical Assessment Form (alpha version), marked up to show the categories of defect included in the study. This research excludes four defects from the study—dried cherry, parchment, hull/husk, and foreign matter (items such as wood, nails, corn, or stones that are sometimes found in coffee). Fungus-damaged beans are included for chemical but not sensory analysis, because of the associated health risks.
Despite the importance of green grading in the coffee supply chain and economic system, the sector has little objective and scientifically validated information about the impacts that visible defects have on cup quality, and even less so, the threshold at which they impact what we can taste and smell. With little evidence about their sensory impact, defects are assigned fixed values, formally detracting value from the cup. The importance that we assign to the visual identification of physical defects is complicated further by the fact that some of the most impactful defects on cup quality—sensory defects such as phenolic (including the so-called Rio defect), potato, and mold—are usually not visible to the naked eye.[6]
If we’re doing arithmetic to calculate coffee’s quality, we want to be sure that the logic behind these equations reflects not just what we can see, but what we smell and taste. Are we excluding coffees with great sensory potential because of their physical appearance, the coffee equivalent of judging a book by its cover?
Interrogating the System
With this question in mind, the Coffee Science Foundation (CSF)—a sister organization of the SCA—launched a research project into physical green coffee defects, focusing on their impact on the cup and, importantly, the thresholds at which tasters can perceive them. This research aims to revisit and modernize long-standing standards, quantifying defects not exclusively based on what we can see, but by determining the point (or sensory threshold) at which we can reliably taste a defect. “Current green grading guidelines are based on tradition, rather than on sensory science,” shares Peter Giuliano, head of the CSF. “This research allows us to build more informed standards,” continues Mario Fernández-Alduenda, the SCA’s Technical Officer. “We want to build a system that’s informed, rather than arbitrary.” These aims are also driven by the SCA’s CVA, a paradigm that focuses less on scores as an exclusionary threshold for defining quality and “specialty” and more on the multiple attributes that make coffees valuable in the marketplace.[7]
Figure 3.
Unroasted “full black” defects. Photo submitted by the Coffee Excellence Center.
Researchers at the Coffee Excellence Center at the Zurich University of Applied Sciences (ZHAW) in Switzerland are collaborating with CESURCAFÉ, a coffee research center at the Universidad Surcolombiana in Colombia, to test if the current logic of the SCA green grading protocol is justified. This study, along with research conducted by University of California Davis on defects from Brazil and Guatemala, will inform an update to the CVA Physical Assessment and Form.
“Coffee is a natural product that spends a long time on the tree during cultivation and undergoes extensive post-harvest processing,” shares Dr. Sebastian Optiz, a green coffee expert and the project lead from ZHAW. While conventions punish certain defects very heavily, this weighting isn’t always based on the sensory science of what we can perceive in the cup, Sebastian shares. “Identifying defects isn’t always a black-and-white process.”
Seeking Defective Coffee
Instead of rigorously removing defects, this research involves deliberately sourcing them. To achieve this, the team from CESURCAFÉ has been gathering and manually selecting defective beans from coffee farms, collection points, and processing mills in southern Huila, Colombia. The specific defects are taken from pasilla lots—coffees with high defect counts typically not destined for export. Led by Professor Nelson Gutiérrez Guzmán and Nicolás Tovar Jacobo, the team dedicated weeks of meticulous attention to gathering and classifying 500 g of each of the 12 targeted defects from both the category 1 and category 2 list of the Arabica Green Grading system (see figure 4). These defects are sourced exclusively from arabica lots, primarily consisting of Castillo, Colombia, and Caturra cultivars.
Figure 4.
Professor Nelson Gutiérrez Guzmán (left) and Nicolás Tovar Jacobo sort pasilla coffee into 500 g samples of 12 different defects. Photo supplied by CESURCAFÉ.
The team at the Coffee Excellence Center then roasts both the defective and non-defective coffees to a “medium” degree. Sebastian explains that the roast profile was developed based on a clean reference lot (sourced from the same region, a similar blend of cultivars, and considered “specialty”), with the aim of optimizing its sensory qualities. To simulate real-world roasting conditions, this clean coffee roast profile is then replicated for each of the defective lots. Subsequently, these carefully sourced and roasted defects are used to “spike” the “clean” reference lot.
Sensory Thresholds and Perception
This process of spiking a clean coffee with certain ratios of defects and completing sensory analysis is the core of the research. Research is conducted with a trained panel of coffee professionals in two steps: descriptive assessment and triangulation (the triangle test). First, the team conducts a descriptive analysis, testing how the defects manifest in the cup at different concentrations. To record their results, the panel uses a customized version of the CVA Descriptive Form. The initial descriptive assessment exercise ensures that the panelists are calibrated to identify the defects, ensuring that they know what they’re looking for when they later identify the thresholds for sensory perception. The defects, says Samo Smrke, acting head of the Coffee Excellence Center, vary subtly at different concentrations and even temperatures, meaning that it’s important to build a “character profile” for each of the key defects. Beyond calibration, this process builds more precise descriptors—based on the SCA Flavor Wheel and Lexicon—for the wider sector.
Once the defects have been characterized, the next step is to determine the threshold at which they’re perceptible. The panel conducts weekly triangulation tests—a common style of discriminative sensory test, where cuppers attempt to identify the “odd one out” of sets of three cups.[8] For these triangulation experiments, the team spikes the defect-free reference lot with different ratios of defects, ranging from 1 g of defects per 60 g of clean coffee (approximately 1.67%) to 30 g of defects per 60 g (50%). These ratios are the same as the cuppers learn to describe and identify the defect at during descriptive calibration. They then group one "defect spiked" cup alongside two "clean cups" in triangles—a format similar to the Cup Tasters Championship—with defect ratios presented at random along the table.
The goal for the cuppers is to consistently identify the cup containing the defects, if any, that they learned to identify during the descriptive analysis. When they can reliably do so, this establishes the sensory threshold—the specific ratio of defective to clean coffee at which the defect becomes reliably perceptible. As a hypothetical example: if panelists can’t consistently identify a defect at a 1:60 g or 3:60 g ratio but consistently identify it at 5:60 g (or 8.3%), then 5:60 g is established as that defect’s sensory threshold. The implications for the wider sector are significant: if a panel of coffee tasters can only identify a defect at a threshold of 7.5% in a controlled experiment, this indicates that consumers are highly unlikely to notice a flavor impact below this threshold, especially in a café or home-brewing context.
The Chemistry of Green Defects
A coffee assessor’s nose, Sebastian notes, can be the most sensitive tool for analyzing coffee’s aroma. However, conducting chemical analysis on physical defects allows the research team to build an additional layer of information, turning what we can smell into data—in this case breaking down defects into a series of chemical compounds. The team at the Coffee Excellence Research Center uses a process called gas chromatography (GC) to separate individual compounds, and a technique called mass spectrometry to identify these compounds.[9]
A further step, known as gas chromatography coupled to olfactometry (GC-O), combines sensory and chemical analysis. Once the compounds are separated by GC, their odor is shot out of an olfaction port, giving the researchers a chance to smell the individual odor compounds in concentrated form. Sour beans, for example, have a number of fruity and sour-smelling compounds, but the coffee sector and scientists don’t always know what they are. Using GC-O, the researchers can identify compounds by smell while the machine identifies their chemistry, allowing them to positively identify the causes of the aroma.
The team is also using a solvent-assisted flavor evaporation (S.A.F.E.) method to gently extract volatile aromatic compounds from coffees.[10] They can concentrate these volatiles and then conduct sensory tests on them at different dilutions, mirroring how they might present in coffee.
Beyond identifying compounds associated with certain sensory characteristics, chemical analysis allows the team to test for toxins, such as ochratoxin, a mycotoxin commonly associated with fungus-damaged coffee. This research will help give insights into the possible health risks (if any) associated with physical defects in coffee.
Revising the System for the Sector
The goal of this research is to ensure that green grading systems are rooted in rigorous, contemporary science. The findings, alongside research from the University of California Davis Coffee Center, will directly inform a revision of the SCA’s CVA Physical Assessment Standard in the coming years. This critical revision, Nicolás from CESURCAFÉ shares, could lead to “a more accurate and transparent system for evaluating coffee, based more on sensory features than just the visual count of defects.”
History shows us that perceptions of physical defects can evolve; peaberries—small, round beans that occur when a coffee cherry forms one seed instead of two—were once deemed defects but are now often prized for their unique qualities. The sector is changing: new processing methods are reshaping our idea of what coffee beans should look and taste like, and climate change is making it harder to protect coffee from insects and unpredictable weather during ripening. The way we assess green coffee must evolve to reflect this, helping all actors in the value chain form clearer understandings of each coffee’s sensory attributes. ◊
LAUREL CARMICHAEL is a former roaster and green coffee buyer, and the current Publications Manager at the SCA.
References
[1] Camila Khalifé, “Tasting the Standards: A Comprehensive View of Green Coffee Defects,” Roast Magazine Roast Summit, 2024, https://www. youtube.com/watch?v=feSfLNGXYiI.
[2] Specialty Coffee Association of America, Arabica Green Coffee Defect Handbook (Specialty Coffee Association of America, 2004).
[3] Mario R. Fernández-Alduenda and Peter Giuliano, Coffee Sensory and Cupping Handbook (Specialty Coffee Association, 2021), p. 111.
[4] International Coffee Council “National Quality Standards,” 122nd Session of the International Coffee Council, London, United Kingdom, September 17–21, 2018, August 23, 2018.
[5] The SCA proposes a definition of specialty coffee that celebrates attributes that add perceived value in a market. A lack, or minimal presence, of physical defects is just one of many attributes that could help to define a coffee as “specialty.” Specialty Coffee Association (SCA), Towards a New Definition of Specialty Coffee
(2021), sca.coffee/sca-news/just-released-new-sca-white-paper-towards-a-definition-of-specialty-coffee.
[6] The causes and chemical compounds responsible for these defects have been widely studied, as well as their impact on sensory characteristics. Because they impact a cupper’s impression of quality, phenolic, mold, and potato defects are recorded in the Affective Assessment component of the SCA’s CVA.
[7] Towards a New Definition of Specialty Coffee.
[8] The team organizes this experiment according to standards on sensitivity of taste (ISO 3972:2011) and the triangle test (ISO 4120:2021), established by the International Organization for Standarization.
[9] Thermo Fischer Scientific, Gas Chromatography Mass Spectrometry (GC-MS) Information, accessed on 29.06.2025 at https://www.thermofisher.com/ch/en/home/industrial/mass-spectrometry/mass-spectrometry-learning-center/gas-chromatography-mass-spectrometry-gc-ms-information.html?.
[10] Solven-assisted flavor evaporation methods are time-consuming and rarely used in the coffee sector. These methods allow the researchers to detect volatiles at higher sensitivity.
We hope you are as excited as we are about the release of 25, Issue 24. This issue of 25 is made possible with the contributions of specialty coffee businesses who support the activities of the Specialty Coffee Association through its underwriting and sponsorship programs. Learn more about our underwriters here.
