July 19, 2022

Simplifying green coffee grading


Preserving the quality of green coffee is an essential part of the supply chain. But in order to preserve it, we must first identify it.

This process of evaluating green coffee is known as grading, where coffee beans are judged based on a number of different criteria before they can be exported.

However, there is no universal grading system in place, with a number of different procedures and terminology used across different producing countries.

As a result of this, some coffee professionals have started to call for a more simplified and standardised grading system to create a universal language as far as evaluating green coffee is concerned.

I spoke with two coffee experts to learn how grading works in a number of different countries and how the process could be simplified. Read on to find out what they told me.

You might also like our article explaining coffee grading.

Green coffee beans on a grading screen.

What does grading assess?

Although there is no universal grading system, most of them govern the same core attributes. These include bean size, density, colour, physical appearance, and the number of defects present.

The process of grading generally begins with sifting the green beans using several screens. Each of these screens contains different-sized holes, which allow beans to pass through a certain number of them based on size. This separates them accordingly.

The size of rounded screen holes is usually measured in increments of 1/64 inches. For instance, a screen of size 15 means the holes have a diameter of 15/64 inches, which is around 5.95 mm. Usually, most green coffee beans will sit somewhere between screen sizes 12 and 19.

In the case of slotted screens, there are usually two width measurements (also measured in 1/64 inches), while the length of the slot is usually 3/4 inches (19 mm). The most common slotted screen sizes used for green coffee grading are between 8 and 11.

Gravity tables (or densitograders) can also be used to grade coffee. These grading systems use air to separate the beans according to their density.

One of the more well-known green coffee grading systems is the one used in Kenya. The highest-quality grades in the country’s system are AA and AB, which are determined by screen size. After the density of the beans is evaluated, the coffee then receives another, different grade, such as TT or T.

Chuaga Kinuthia is a coffee technologist at the University of Dedan Kimathi in Kenya. He tells me that grading can be confusing for some coffee professionals.

“[In Kenya], we grade based on size, colour, and density as the primary factors,” he explains. “For this, we use screens, colour sorters, and so on.”

The colour of the beans can be an important factor when assessing overall coffee quality. With washed arabica, for instance, a bluish-green tint is considered the most desirable colour. Any faded colours are generally a sign of ageing or improper storage – therefore indicating that quality might have decreased.

He adds that in Africa alone, there are many different types of green coffee grading systems, and that these all have their own nuances.

“For example, with natural processed coffees, you only grade from screen 15 and above,” he says. “However, washed coffees are graded [using the full range of screen sizes.]”

A coffee producer collects green coffee beans.

Why do countries currently use their own different grading systems?

Kenya is just one example. Around the world, many coffee-growing countries have their own grading system that is unique to that origin. But why is this the case?

Each system aims to create more standardisation around the green coffee produced in each country. 

Bean size and density can vary between countries, so separate grading systems can help to maintain consistency within each respective country. For example, Ethiopia’s grading system accounts for the many wild-growing heirloom varieties, which are often smaller in terms of bean size.

Language barriers also play a big role in these differences. For instance, Brazil and Vietnam are the two largest coffee-producing countries in the world, respectively, and therefore both have significant influence on the international coffee market.

However, any universal grading system developed in Portuguese or Vietnamese would pose a challenge for the many farmers in Spanish-speaking Latin America and Africa.

To add to this, predominantly arabica-producing countries need to use different grading systems than countries that mostly grow robusta. This is because the two species are evaluated using different criteria, mainly based on bean size and different processing methods.

As a result of these differences, some coffee professionals are calling for a grading system that is universally recognised.

Adam S. Carpenter is an AST trainer for the Specialty Coffee Association. He’s also a coffee roaster and a green coffee buyer.

He explains that the differences between grading systems can make training more difficult.

“Most of my students are confused by the many grading systems,” he says. “Trainers can have a hard time understanding and explaining the different systems.”

Different grades of Kenyan coffee in sample roasting trays

Why is green coffee grading so important?

No matter where you sit along the supply chain, it’s universally agreed that identifying and maintaining coffee quality is key.

Grading is an important part of this, as coffee needs to be graded before it can be preserved correctly. Coffee that is more dense or larger (markers traditionally associated with higher quality) naturally require more stringent storage to preserve more of the innate quality.

Alongside this, producers also use grading systems to quickly communicate how high in quality a coffee is when working with buyers. This information easily translates when they take the coffee to auction or work with other trading platforms, for instance.

This information is also key for roasters, who need to know how large or dense a coffee is, as this affects roast time, temperature, and more.

“There is certainly a need for green coffee grading,” Adam says. “The size of the beans is important for roasting and how you might blend the coffee.”

Different grades of green coffee behave differently when roasted, too. This largely depends on bean density and size – coffees grown at higher altitudes are often denser and bigger than coffees grown at lower altitudes.

The bottom line is that the larger a bean is, the longer it takes to roast. However, to achieve a proper, uniform roast profile, you want to make sure that all of your coffee is of the same grade, and that you know what this grade is.

Finally, grading enables producers and farm workers to check for defects, which can have significant effects on overall quality.

For instance, beans that were exposed to prolonged periods of drought or too much sunlight may appear shrivelled or misshapen. Alternatively, green beans that have been infested with pests or diseases may contain holes and jagged edges, with some having visible black striations.

Storage can also play a role in green coffee quality. Beans that have been exposed to excessive humidity or water after drying can appear black. On a more concerning note, green coffee that was processed using unsanitary water can smell unpleasant. These are often known as “stinker” beans.

A producer holds green coffee beans above several large bags of green coffee

Could we simplify or standardise grading?

In light of these complexities, there is a growing need to simplify and standardise green coffee grading systems.

“The biggest challenge for us in developing a unified grading system is accounting for historical practices, and even translations, which both have a big impact on the way you grade coffee,” Adam explains.

Chuaga believes that the coffee sector needs a standard grading system that can then be localised.

“Ethiopia and Brazil, for example, produce a lot of natural processed coffees, which means they focus less on grading [than countries who produce predominantly washed coffees],” he tells me.

Ultimately, simplifying the green coffee grading process could make quality assessment easier for producers and farm workers – especially those who are less experienced or have reduced access to resources and formal training.

Adam believes a universal system could also help roasters.

“Standardising the sizing of beans could be the first step towards achieving a unified grading system,” he says. “Some roasters do not own screens and they don’t understand sizing, therefore the quality of their roasted coffee can suffer if this isn’t properly communicated.

“If coffees have been poorly sorted, and a roaster mixes 19/20 beans with 15/16 beans, the smaller beans will burn,” he adds.

Furthermore, Adam suggests there needs to be a greater distinction between grading and sizing, as well as more standardised language around different grades of green coffee, such as AA, A, and AB grades.

“When you use language in green coffee grading, you can create bias,” he says. “We should communicate the exact sizes of the coffee that is purchased.”

However, Chuaga believes that there are currently too many factors to consider to establish a universal system.

“We should remain with the current systems, but tailor them for fairness,” he concludes. “Ultimately, grading systems should be guided by the market.”

A producer runs their hand through a pile of green coffee beans

Green coffee grading is clearly a key step in the journey from seed to cup. Without it, farmers may struggle to accurately communicate the quality of their coffee, and roasters may struggle to properly roast it.

There are, however, challenges to standardising the system. For now, it seems likely that each individual origin will continue to use their own procedures and terminology. Whether or not this will change in the future remains to be seen.

Enjoyed this? You might also like our article on how to stop green coffee from becoming contaminated.

Photo credits: Peter Gakuo

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