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Sorting and screen size: how green coffee is graded

Before coffee reaches the roastery, and then the cup, the green bean goes through a process of assessment and grading that few people think about. It is precisely at this stage that it is decided whether the coffee will be deemed specialty or commodity, and how much it will be worth. Sorting and grading green coffee is a world of screens of particular sizes, of weight and volume, of counting defects and assessing quality. It is a seemingly technical, dull stage, but fundamental for the whole coffee industry. In this post you will learn how green coffee is graded: what the screen system is, how it is sorted by density, how defects are counted and what makes coffee specialty. It is a journey into the stage that happens between the farm and the roastery, and decides the value and quality of the coffee. Let us start with what grading green coffee even is.

What grading green coffee is

Grading green coffee is the assessment of raw, unroasted coffee in terms of its physical traits and quality. The size of the bean, the number of defects, density, moisture, and sometimes also the quality in the cup, through sample roasting and cupping, are examined. The goal is to objectively determine the quality and grade of the coffee, which translates into its price and purpose. It is a key stage in the coffee trade: it lets buyers and sellers speak a common language of quality. Without grading it would be impossible to fairly price and compare coffees from different countries. Grading is the bridge between the producer and the roastery, a tool of standardisation in global trade. It is not a subjective opinion, but a system based on measurable traits. Understanding how green coffee is assessed opens the eyes to how much happens before the coffee is even roasted. So let us get to know the most important criteria, starting with the size of the bean.

The screen system - bean size

The best-known criterion of grading is the size of the bean, measured by means of screens. They are metal plates with round holes of a particular diameter, measured in 1/64 of an inch. Green coffee is passed through a stack of such screens, each of which has holes of a different size, which allows the beans to be sorted by size. The most common screen sizes (that is the diameters of the holes) fall in the range from 10 to 20. Typical specialty coffee falls usually in the range from screen 14 to 18. Larger beans are often, though not always, associated with higher quality, which is why size is sometimes an element of the coffee designation. Importantly, uniformity of size has a practical meaning: beans of similar size roast evenly, while a mix of large and small roasts unevenly. The screen system is the foundation of grading, a simple but effective method of sorting. That is why bags of coffee often bear a screen number. Bean size is the first, most visible indicator of the quality of the raw material.

Density sorting

The second important criterion is the density of the bean. Density sorting separates less dense beans, often defective, from denser ones, usually of better quality. Density is determined on the basis of the weight and volume of the beans. This trait is key, because it affects the roasting process and the final quality of the coffee. Denser beans usually come from higher altitudes, where slower ripening of the fruit gives a more compact bean rich in sugars - that is why density is sometimes an indirect indicator of quality. Density sorting usually takes place on special densimetric tables, which use vibration and airflow to separate light beans from heavy ones. Less dense, light beans are often unripe or damaged, so removing them raises the quality of the lot. Density also directly affects roasting: denser beans require a different approach than light ones. Density sorting is another screen of quality, separating full-value beans from worse ones. It is an important element of the road to good coffee.

Counting defects

The third, key criterion is the number of defects in the bean. A sample of green coffee, usually 300 or 350 grams depending on the protocol, is hand-sorted under controlled lighting, and all defective beans are identified, categorised and counted. Defects are divided into primary and secondary. Primary defects, such as full black beans, full sour beans or foreign matter like stones and sticks, have a serious, negative impact on the quality of the cup. Secondary defects, like partial black, insect-damaged, broken or shells, are less severe individually, but add up. The number and type of defects directly decide the grade of the coffee. It is a labour-intensive but indispensable process, requiring a practised eye. Counting defects is the most direct indicator of the cleanliness and care of coffee processing. The fewer defects, the higher the grade and price. That is why producers take such care over selection at every stage. Counting defects is the heart of the quality grading of green coffee.

What makes coffee specialty

All these criteria combine in the assessment of whether the coffee deserves the name of specialty. According to a protocol recognised in the industry, specialty-grade coffee must have zero primary defects and no more than five secondary defects in a 350-gram sample. It is a very rigorous requirement. Added to this is the assessment in the cup: specialty coffee is beans that in cupping reach a score of 80 or above on a 100-point scale. This means that specialty is not a marketing slogan, but a concrete, measurable grade of quality, based on the number of defects and sensory assessment. Coffee that does not meet these criteria falls into lower categories. That is why specialty costs more: behind every bag stands rigorous selection and assessment. Understanding these thresholds helps appreciate what specialty coffee really means. It is not only flavour, but also hard grading criteria. We write more about this in our post on what specialty coffee is.

What is it all for

One might ask: what is such a complicated grading system for? There are several reasons. First, standardisation: grading lets buyers and sellers from different countries speak a common language of quality, without having to inspect every lot. Second, fair pricing: better coffee deserves a higher price, and grading makes this possible objectively. Third, quality control: removing defective beans raises the quality of the cup and protects reputation. Fourth, predictability for the roastery: knowing the grade and size of the bean, the roaster knows what to expect and how to roast. Without grading the coffee trade would be chaotic and unfair. It is a system that orders the global market and rewards quality. Grading is the invisible infrastructure of the coffee industry, thanks to which good coffee can be appreciated and fairly priced. It is a technical stage, but of huge importance for the whole chain - from farm to cup.

Grading criteria in a table

Let us set the main criteria of assessing green coffee side by side:

The table shows that grading green coffee is a multi-criteria assessment. Screen size ensures uniformity and even roasting, density betrays altitude and ripeness, the number of defects speaks of cleanliness, and cupping assesses quality in the cup. Together these criteria give a full picture of the quality of the raw material. Specialty coffee must meet rigorous thresholds in all these dimensions. That is why grading is so important - it objectifies quality, which otherwise would be a matter of opinion.

Why it is worth knowing this

Understanding the grading of green coffee changes the way we look at coffee. First, it shows how much work and selection stands behind a good cup, even before roasting. Second, it explains why specialty costs more: behind the price stand hard quality criteria, not only marketing. Third, it helps read coffee descriptions: a screen number or defect grade is concrete information about the quality of the raw material. Fourth, it lets you appreciate the work of graders and producers, who take care over selection at every stage. A conscious coffee lover knows that quality begins long before the roastery, in the laborious sorting and counting of defects. Next time, seeing a screen number or grade designation on a bag, it is worth knowing what they mean. It is knowledge that deepens respect for the craft and helps better understand what we are paying for. Grading is the key to understanding the world of specialty coffee and its value.

The key points in a nutshell

Before coffee reaches the roastery, the green bean is sorted and graded by measurable criteria. Size is measured with screens of holes in 1/64 of an inch, and specialty falls usually in the range of screens 14-18 and requires uniformity for even roasting. Density, determined from weight and volume, betrays the altitude of cultivation and ripeness. Defects are counted by hand in a sample of 300-350 grams, divided into primary and secondary. Specialty coffee must have zero primary defects, at most five secondary, and a cupping score of 80 or above. It is an objective system that standardises quality and pricing in global trade. Want to compare coffees of different grades and record your impressions? Keep tasting notes in the GustoNote app. See also our posts on specialty coffee and on coffee cupping.