What's the Purpose of Using Filter Paper in Espresso? Why Add Filter Paper to Espresso?

Some friends have noticed recently that many coffee shops serve espresso with a piece of white paper in the coffee puck. When this friend visited FrontStreet Coffee, he eagerly asked what the paper really was. Here is what it looks like. See if you can guess its identity.

That is right: it is filter paper, a circular piece of it. As we all know, the main purpose of filter paper is filtration, keeping coffee grounds out, even when it is placed in the portafilter. Besides filtration, though, adding it to espresso preparation serves other purposes, and the specific effects vary depending on where it is placed.

In fact, FrontStreet Coffee has mentioned its function in many earlier articles, just without going into detail. So today, FrontStreet Coffee intends to discuss thoroughly the pros and cons of using filter paper in espresso preparation.

Adding filter paper at the bottom of the portafilter

When we talk about adding filter paper to a portafilter, we usually mean adding it at the bottom. A piece of filter paper at the bottom can both reduce the bitterness of the coffee and increase its cleanliness.

We know that during espresso extraction, many fine particles are pressed out of the portafilter along with the coffee. Because these particles are extremely small, they fall outside the blocking range of the portafilter's filter holes. So when the machine begins high-pressure extraction, these tiny particles flow out with the coffee and into the cup. Fine particles themselves carry a certain bitterness, so their presence heightens the coffee's bitterness while hurting its cleanliness. If we do not want an espresso to be too bitter and want a cleaner taste, we can lay 1-2 circular filter papers flat at the bottom of the portafilter before making espresso. The filter paper then acts as a filter, blocking the fine particles in the portafilter, so the espresso tastes cleaner with reduced bitterness.

It is worth noting, however, that this approach comes with two drawbacks. With fewer fine particles, the coffee loses some body, because fine particles provide both bitterness and body; as they decrease in the cup, the body decreases too. The other drawback is that extraction time shortens and the extraction rate is relatively lower. Because coffee particle sizes are not uniform, some fine particles similar in size to the bottom holes block them during extraction, reducing the channels through which hot water can flow. That extends the extraction time and raises the extraction rate.

Add a layer of filter paper at the bottom of the portafilter, though, and nothing can block the portafilter's filter holes. With the hot water channels restored, the flow rate during extraction naturally increases. Extraction time shortens and the extraction rate falls accordingly. FrontStreet Coffee has run several comparisons of adding filter paper at the bottom of the portafilter versus not adding it. The results showed that espresso without filter paper had an extraction time several seconds longer than espresso with filter paper, and a higher extraction rate as well.

So if we want to reduce coffee bitterness by adding filter paper at the bottom of the portafilter, it is best to adjust the grind size on the premise that filter paper is being used during extraction. Otherwise, the gap between the two will make you adjust your espresso several more times, which is somewhat counterproductive.

Placing filter paper above the coffee puck

If the filter paper is placed on top of the coffee puck, it can reduce channeling and help the coffee extract more evenly.

Because there is a certain distance between the coffee puck and the shower screen, the hot water has some impact when it falls. Although this force will not drive hot water straight through the puck, it can easily lift the coffee particles on the surface, making channels more likely to form and extraction more uneven.

Add a filter paper on top of the puck, though, and it disperses the impact of the water before it contacts the puck, while letting the water spread more evenly through the coffee bed for more even extraction. But if you ask about the actual improvement it brings, it is relatively modest, because channeling is more often caused by operational issues such as improper tamping or too many clumped particles. That is why few people add filter paper to the surface of the puck, and most do it at the bottom of the portafilter.

Adding filter paper on top of the puck extends extraction time, because the hot water has to pass through more material. How long depends on the thickness of the filter paper: the thinner the paper, the less the time is extended, and the thicker the paper, the more it is extended. That is not necessarily a bad thing, because it means we can use less coffee or a coarser grind for extraction. We are no longer bound by the standard of the portafilter, and the amount of coffee used can be more flexible.

FrontStreet Coffee Recommendation

Filter paper at the bottom of the puck trades a little body for a cleaner, less bitter shot, so it rewards a bean whose sweetness shows through even at lower extraction. FrontStreet's Classic Blend, designed for espresso and Moka pot brewing, uses a traditional Mamba-style recipe for a full, mellow cup with outstanding crema, and its butter, nut, spice, and dark chocolate notes hold up clearly whether you make an Americano or a latte.

If you favor a nutty, chocolatey espresso, the FrontStreet Fine Blend combines Brazilian Cerrado with Colombian Huila for the classic taste nearly a decade of FrontStreet shops have served, and the FrontStreet Black Cocoa Blend, a 100% Arabica recipe of Yunnan small-bean coffee and Brazilian Cerrado, brings nut, chocolate, and cream aromas with the ripe plum acidity of Yunnan and a soft, balanced body at a friendly price, adaptable enough for lattes, Americanos, and specialty coffee drinks alike.