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Editor’s Note: This article first appeared in the Jan/Feb 2020 edition of Roast Magazine under the Column “Technically Speaking” and is reprinted here with permission. You can view and download the article in its original format here. Sections of the article also appeared in Part III of our Green Coffee Analytics series, first published on this blog in 2016.

Technically Speaking

Grading Green Coffee for Physical Defects

By Chris Kornman / Photos by Evan Gilman

A moment resonates with me from my first trip to Brazil. An exporter picks up a pile of green coffee and, upon nothing more than a brief physical inspection, identifies its process and grade, smelling it for freshness. He smiles at me, a roaster, and says, “Know the green first.”

Since that day, I’ve taken a holistic approach to understanding a green coffee’s potential: screen size, density, water content and more. Even a selection with ideal moisture content can be marred by a single defective bean.

Beautifully, there is significant intersection between all of coffee’s defects: sensory spoilers, roast flaws and visible problems in the green. This is, in fact, directly related to why we grade coffee visually and physically before roasting: to catch potential problems before the investment of additional time and energy in the roaster and at the cupping table.

Methods of Grading and Removal

Sorting before export is frequently a collaboration between human and machine labor. Talented individuals who are hand-picking can clean about one 60-kilo bag of coffee per day. A conveyor belt may pass coffee along a table of workers to help keep a steady production pace. Automated dry mill removal typically involves a density table to sort foreign objects and optical color sorters to remove discolored coffee, each machine capable of processing hundreds of bags in a day.

The most familiar systems for counting defects are the Specialty Coffee Association (SCA) and Coffee Quality Institute (CQI) format and the Brazil/New York method. Using 300 grams of green coffee, a grader will pick and categorize different defects.

SCA/CQI divides coffee into two severities: Category 1 (none allowed in CQI 85-plus specialty grade) and Category 2 (up to five allowable in CQI specialty grade, less severe). The Brazil/New York method differentiates between intrinsic defects (like insect damage or shells) and foreign defects (such as sticks and stones).

Annex 7 of Joackim Mutua’s Post Harvest Handling and Processing in African Countries (a publication for the Food and Agriculture Organization of the United Nations) contains the details on both methods, which each have merit. SCA places emphasis on flavor, while Brazil/New York helps to secure contracts and protect equipment.

Defect Cause & Effect

In lieu of reproducing already existing guides like the SCA defect handbook or Jean Nicholas Wintgens’ seminal work, let’s examine a few notable and common defects and their implications for roasters and quality-control agents.

Pre-Processing Defects

The main culprits of damage to coffee while on the tree or immediately after picking are insects, genetic flaws, disease, environmental factors or human error.

Frost damage can blacken the seed, as can insects like the mealy bug or antestia bug, fungi like coffee berry disease, and even nutritional deficiencies, overripe cherries, or delays before pulping. Withered, wrinkled, low-density coffee is often the result of drought. Insects, like the coffee berry borer, will burrow directly into the seed, leaving small, visible holes.

Underripe coffee often curls into a U-shape, and may result in quakers—coffee seeds that don’t properly react during the Maillard stage of roasting. Even at a relatively dark roast, quakers will stand out with their pale orange or khaki appearance, and when ground emit a rancid toasted peanut aroma. (For further reading, see Andi Trindle’s informative article “Detecting Defects” in Roast’s January/February 2008 issue.)

Quakers are identifiable only when the coffee has been roasted. Not all green coffee identified as underripe results in quakers, and vice versa. Both CQI and SCA recognize underripes as an allowable secondary defect, but the SCA defines specialty coffee with zero tolerance for quakers.

Flavor attributes of quakers and underripe coffee differ. A single quaker may taint a sample and create inconsistencies from cup to cup on an evaluation table ranging from harsh peanut to fatty flavors. However, underripes present at the cupping table as astringent, accompanied by a fresh hay-like or grassy flavor.

Development anomalies can take the form of peaberries, shells (sometimes called “elephant ears,” not to be confused with oversized E-grade “elephant” beans), and triangular seeds.

Peaberries trade at premiums in some markets, but other deformed seeds are less desirable. Coffee typically produces two seeds per cherry, and if the peaberry is the single-seed divergence, triangular seeds and shells represent the opposite phenomenon. Triangles are caused by an extra seed, or multiple extra seeds—in extreme cases, I’ve seen images of eight or more seeds per cherry. Each of these can be sorted by properly calibrated density and screen tables at the dry mill.

Shells, perhaps the largest deformation nuisance for the roaster, are the result of false polyembryony—multiple embryos developed within a single seed, where an abnormal dual embryo develops instead of two independent embryos. Both shells and triangular seeds tend to over-roast quickly, imparting minor “off” flavors of carbon.

Processing Defects

Defects that occur during processing are generally the most commonly encountered, and they are usually the result of some combination of human and mechanical errors.

Chipping and breaking can happen both at the pulper or during parchment hulling. Pulper-damaged coffee will frequently discolor slightly at the point of impact and can infrequently lead to a bacterial infection. Broken and severely chipped beans may lead to minor roast defects. However, slight nicks from the pulper or the huller are rarely cause for concern.

Reddish-hued silver skin correlates most frequently to prolonged contact with the fruit, as happens in dry- or honey-processing styles, and is rarely a sensory concern (other than potentially darker-than-usual chaff during roasting). Rarely, it may be related to dirty water in fermentation tanks if the coffee is fully washed, which is usually more problematic sensorially.

Hulling errors that leave pods (dried coffee cherries) or parchment stuck to the seed are a more serious concern, sometimes harboring fungal infections and never roasting properly.

A few severe defects are endemic to fully washed coffees. The washing channels are a first step in grading coffees, ideally separating identifiable “floaters” (which can include underripe seeds, drought-affected coffee and other developmental anomalies) from “sinkers” (high-density coffee of good quality). Fermenting coffee can also result in “stinkers”—coffee that has over-fermented and emits an acetic or briny aroma and flavor due to dirty tanks or unstirred pockets in the slurry.

No matter the processing type, however, post-harvest drying is the most critical stage in quality preservation and management, and errors during this hypersensitive process are magnified in the cup.

Underdried coffee will be soft, pliable, and often darker-green in appearance, while coffee that has had interruptions in drying may appear blotchy. Frequently, these coffees will taste musty, swampy or vegetal—at least until they begin to (rapidly) age, fade in color, and taste like cardboard and burlap. Traditionally these aren’t visual defects that are sorted out by people or machines, despite the severe impact on flavor.

Mold, fungi, bacteria and other infections may occur as the wet coffee dries, particularly if it is not turned frequently to promote even drying and good airflow. It is possible to spot some fungus visually, while some microbially damaged coffee remains latent until roasting, like the potato defect. If an infection is particularly bad, brownish spores may appear on the surface of the seeds. Black lights can also be used to spot fungus damage, but black lights may return false positives for less consequential anomalies.

Mechanical dryers can efficiently turn and dry large quantities of coffee. A maximum operating temperature of 45–50 degrees C/113–122 degrees F is a widely acknowledged safe limit. Many studies suggest 35–40 degrees C/95–104 degrees F might be the best range, as higher operating temperatures have been shown to damage cellular structure of the coffee and negatively impact cup quality. An untended, miscalibrated, or rushed dryer can easily overdry and even begin to roast the coffee, resulting in coffee that appears pale brown in color and tastes woody and bitter.

Storage Defects

Even if a coffee makes it all the way from picking to shipping with no problems, there are still risks for defects. Beetles are notorious pests, chewing their way through dried coffee and reproducing prolifically. Storage insect damage holes tend to be much larger than those from insect damage that occurs in the field. Coffee infested with beetles, or with moths or worms that eat the jute bags, must be quarantined and purged by freezing or oxygen removal.

Poor conditions are the other main culprit of storage-damaged coffee. Hot and humid conditions can exacerbate aging. In wet conditions, coffee may re-absorb water and fade in appearance as well as flavor. Black spots can occur if coffee has been poorly dried and pockets of water within the bean cause rot, and mold can develop on the surface of the coffee as well. Stable storage conditions will help prevent these defects. Dried coffee, whether in parchment or green, prefers environments near 65 degrees F/18 degrees C with moderately low (40 to 60 percent) relative humidity.

As with many of the risks across the supply chain, preventative measures are significantly more effective than after-the-fact fixes. It’s not rocket science—good practices and attention to detail are crucial. A well-oiled feedback loop completes the picture: growers, processors and millers who communicate well with their exporters, importers and roasters will have a better understanding of expectations, and of where the gaps in their armor exist and how they can be remedied.