Benedict Smith speaks with coffee expert Denis Sabou to find out more about why adding a paper filter to your portafilter basket could be the key to a better cup of coffee.

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A few years ago, coffee consultant and author Scott Rao put up a post on his Instagram account that drew widespread intrigue.

“The cat is out of the bag,” he wrote. “I’ve been working on a trick to greatly increase extraction, while decreasing channelling.”

The trick, it turned out, was simple. On the recommendation of a friend, Rao had started experimenting with using AeroPress filter papers both above and below espresso pucks.

He found that not only did each filter paper increase extraction, but the top filter mitigated channelling – a situation in which water finds a specific narrow path through the puck of coffee instead of flowing through the entire bed evenly.

This discovery led a number of baristas and coffee experts to carry out their own set of experiments, with many reporting similar results. Among them was Norway-based Denis Sabou.

“[When I tried it] I noticed higher extraction with more even flow out of the basket,” he explains. “I also got less side-channelling.”

This was exciting news. Channelling and under extraction are two problems commonly affecting baristas. If a simple AeroPress filter paper (or two) could prevent it from happening, then it could open the door to more consistent and enjoyable cups of coffee.

But does the science back it up?

How do paper filters affect extraction and channelling?

When pulling an espresso shot, it can be easy to under- or over-extract the coffee.

In short, an under-extracted coffee can be identified by a lack of flavour, whereas an over-extracted coffee is one that tastes bitter, dry, and generic.

While ways of measuring extraction can vary, a good indication is the total dissolved solids (TDS).

According to experiments by the Real Prometheus, a YouTube channel focused on espresso, a standard shot without filter papers produced a TDS of 9.49, a rinsed filter on the bottom of the puck produced a TDS of 9.89, and a rinsed filter with a dry filter on top produced a TDS of 10.03.

It’s believed that the reason for the higher extraction, albeit only a small increase in this case, is that the filter paper on the bottom prevents the fine coffee particles from clogging the holes in the basket.

In theory, this should allow baristas to grind finer than they typically would, while maintaining a high flow – thus leading to even higher extraction. Indeed, in Rao’s Instagram post, he reports that his extraction yield (the percentage of dissolved solids in the water) had jumped 1.5-2% to 25%.

Meanwhile, the filter paper on the top is shown to reduce channelling. “The higher extraction yield causes less clogging and better water dispersion in the puck,” Denis explains.

This is because the paper filter on top helps create an even bed of water before it makes contact with the coffee. Rather than carving out a channel, the water hits the paper and acts as a buffer between the puck and the full force of pressure.

Are paper filters necessary?

Since Rao popularised the concept of using paper filters with espresso, many within the coffee industry have advocated its benefits.

In addition to the impact on extraction and channelling, some even claim that it can be beneficial to health. This is based on the idea that cafestol, a molecule present in coffee beans, can raise cholesterol levels.

Cafestol molecules are typically found in higher amounts when made using unfiltered methods, such as with a French press. In filtered coffee drinks, on the other hand, the paper prevents the majority of the cafestol from getting into the cup.

However, Denis points out that for those who prefer lighter roasts, this shouldn’t be too much of a concern. “It reduces oils,” he explains, “but when you drink light beans you don’t really care too much about oils anyway.”

There’s also the taste factor. While an even extraction and lack of channelling will help reduce the risk of unwanted characteristics appearing in the cup, filter papers don’t necessarily guarantee a “perfect” shot of espresso.

Former World Barista Champion James Hoffmann tried one of Rao’s filter paper espressos and commented in a video shortly after: “It wasn’t the best espresso of my life. It was good, but it wasn’t great. The texture was light and delicate, which was quite shocking in many ways. It had tons of acidity and it was sweet. It did have some astringency which I didn’t love and I wanted to get rid of.”

That said, other factors were also at play, including a grinder which was reportedly producing “undue astringency”. In addition, Rao was purposefully pulling shots with high extraction – around 28% – to showcase the capabilities of the espresso machine.

Denis says that using filter papers can also change the mouthfeel of the coffee. “The mouthfeel is different because the filters might filtrate some of the coffee’s oils or components,” he says.

This all leads to the question of whether baristas should be using paper filters. Although around the time of Rao’s post the concept seemed exciting, a number of people in the industry think that it may not be worth it. 

What’s more, using single-use paper filters can add to the environmental impact of a business at a time when consumers are becoming increasingly discerning about which brands they buy from.

That’s without considering the financial cost involved in keeping a regular stock of filters – money that could probably be better spent on some better quality equipment.