Coffee roasting basics: A complete guide to screen sizes

Peter Lancashire
December 13, 2021
green bean screen size

When it comes to producing a good cup of coffee, consistency is essential. If the quality of one cup is vastly different from another, it can discourage customers from making repeat purchases.

Consistency relies on a number of factors, not least the skill of the roaster. However, one of the biggest influences is screen sizing.

Used in several coffee-producing countries, screen sizing traditionally involves sifting green beans through progressively smaller holes on metal sheets.

The beans are then categorised based on their size, typically in the range of 8/64 to 20/64 of an inch. In Africa, beans with a screen size of 18 are certified as AA, 16 as AB, and 14 as C.

For specialty roasters, understanding screen sizes is important as it will ultimately inform their approach to a roast. It can also help them choose high-quality beans that will match the preferences of their target audience.

To find out more about screen sizing I spoke with the owner of MABÓ Coffee, Bogdan Georgescu.

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green bean screen size
Screen size can help inform a roaster’s approach to their beans

What is screen size?

Once ripe coffee cherries have been picked, they begin a long journey to prepare them for roasting.

Known as “processing”, it involves the transformation of the coffee from a fruit to a green bean that’s ready to be roasted.

Typically, the final stage of processing is screen sizing. Although there are several approaches to screen sizing, the most common is using large, multi-tiered tables that sit atop each other with a slight downward slope.

As the coffee enters from above, the tables vibrate and the beans fall through progressively smaller holes, almost like a sieve. The more the coffee is passed through, the more precise the measurement.

Once screen sizing is complete, the batches are separated and labelled according to their size. These labels vary depending on the origin. For example, the grading system in East Africa typically uses letters A, B, C, and T, with AA (17-19 screen size) considered the highest quality beans.

Colombia, on the other hand, uses the terms “Supremo” and “Excelso”, while Central American coffees are often labelled EP (European Prep) and American Prep.

Screen sizing is important because the size of beans often point towards a variable density in the coffee, which plays a role in how a coffee tastes.

Therefore, by carefully separating the different bean sizes, farmers can offer a range of coffee to potential buyers, all coming from the same crop.

However, it isn’t always necessary. Bogdan tells me that farmers who predominantly work with the same variety of coffee tend to find their crops are already fairly uniform in size. This means they will usually bypass screen sizing when processing their beans.

“All Catuai beans from Brazil, for example, are pretty much the same size,” he says. “Therefore, the whole production from a certain farm will have roughly the same size, making it unnecessary to use screen sizing.”

roasting coffee
Cup quality is not necessarily linked to screen size

Does size affect quality and flavour?

The extent to which screen size affects the overall quality of coffee beans tends to divide opinion. While some link larger beans to better quality, others claim that other factors such as terroir, bean density, and processing methods have greater influence.

Indeed, in a Royal Coffee blog post titled ‘Green coffee analytics: screen size’, Chris Kornman notes that it is often not as simple as saying “bigger is always better”.

He writes there are “claims that higher elevations produce smaller seeds with more concentrated nutrients and higher density, and therefore higher quality”, but that some regions which struggle to produce large-sized coffee are widely known for producing some of the best coffees in the world.

High-altitude coffee-growing regions in Colombia, Southern Rwanda, and Ethiopia, for example, tend to produce relatively small seed sizes but with exceptionally high-quality cup profiles.

Bogdan points out that the same is true of peaberry coffee.

“Cup quality is not related to screen size,” he tells me. “But due to the grading systems from different countries, consumers tend to believe bigger beans are much better than small ones.

“The best example is from Kenya, where AA beans are often sold at a higher price than PB (peaberry), despite the fact peaberries are usually better than bigger beans.”

green coffee beans
It’s important for roasters to place consistency at the heart of their operations

The importance of consistency

While the size of coffee beans does not necessarily indicate quality, screen sizing can help improve the overall cup profile thanks to the greater uniformity that results.

In his experience, Bogdan says he has found that having crops of beans all sitting within a uniform size gives roasters an easier time when profiling a coffee.

“One advantage is clear when roasting beans of the same screen size: the heat applied will have a more consistent way of penetrating the beans and the result will be very consistent throughout the whole batch of coffee.

“If we roast in the same batch with very different screen sizes, we will see at the end of the roast that we have some underdeveloped beans and some very dark ones.

“It’s as simple as that: consistent bean size will give us a consistent roast,” he adds.

Working with a consistent product allows roasters to finely tune a roast profile and unlock the full potential of a coffee’s flavour, batch after batch.

High-barrier bags are essential to preserving the freshness and quality of your coffee

Although screen sizes may not determine the overall quality of a coffee bean, they can help improve the consistency of beans offered to buyers. This, in turn, enables roasters to understand what they are investing in and to create a repeatable experience for consumers.

Screen sizes are also a useful tool for farmers who grow crops of aren’t different varieties, while creating greater diversity that will appeal to a broad range of customers.

At MTPak Coffee, we understand how important it is to preserve the quality and freshness of your roasted coffee. Our sustainable high-barrier coffee bags offer protection against external factors, while keeping the coffee intact between roastery and consumer.

We offer a range of recyclable, compostable, and biodegradable options that can be fully customised to your needs, including degassing valves and resealable zippers.

For information on our sustainable coffee bags, contact our team.

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