Chemex Filter Paper Physics: What Thick Paper Filters Really Remove

The Chemex was designed by chemist Dr. Peter Schlumbohm and has been made since 1941; the company describes its Bonded filters as based on laboratory paper (CHEMEX). This page separates what the research shows about paper filtration from the marketing, and turns it into practical brewing choices. For how strength and extraction yield work in any brewer, read coffee extraction science first.

What a Paper Filter Actually Does

A paper filter is a mat of cellulose fibres. Coffee liquid passes through the gaps between fibres; anything larger than those gaps, such as fine coffee particles and oil droplets, is held back in the paper or in the bed of grounds sitting on it. Two things matter for the cup: how much the paper lets through, and how fast liquid can drain through it.

It keeps oils out of the cup

In a study comparing brewing methods, coffee filtered through paper contained less than 7 mg of lipids per 150 ml cup. Coffee passed through a metal screen contained about 50 mg, and boiled, unfiltered coffee and espresso reached 60–160 mg. Interestingly, most of the oil did not end up on the paper: during paper filtration the lipids stayed mainly in the spent grounds, with the brew holding 0.4% and the filter paper 9.4% of the lipids recovered (Ratnayake et al., 1993).

That is why paper-filtered coffee has a lighter, cleaner body than French press or metal-filter coffee: there is very little suspended oil or sediment to coat the tongue. Whether that is “better” is a matter of taste; some drinkers prefer the heavier body of unfiltered methods.

It keeps cafestol and kahweol low

Cafestol and kahweol are diterpenes in coffee oil that raise blood cholesterol. A 1995 study measured them across brew methods and found that each 10 mg of cafestol consumed per day raises serum cholesterol by about 5 mg/dL. Drip-filtered, percolated and instant coffees contained negligible amounts (Urgert et al., 1995).

Source: Urgert et al., 1995, Journal of Agricultural and Food Chemistry.

But the grounds do most of the work

It is often said that the paper “traps” cafestol. Researchers have tested where it actually goes. Of the cafestol in the roasted coffee, 87.45% stayed in the spent grounds, 12.41% was retained by the paper filter and only 0.15% reached the brew. They concluded that most cafestol is held back by the coffee itself, because hot water extracts little of the lipid fraction (Rendón et al., 2017).

In a follow-up, the same team brewed one coffee through paper filters from six countries. Cafestol in the brews ranged from 1.62 to 2.98 mg/L, the highest levels came from filters with micro-perforations, and filters from different countries had similar retention. Paper porosity and the particle size of the ground coffee were the deciding factors (Rendón et al., 2018). In plain terms: a dense, unperforated paper and a sensible grind keep the cup low in diterpenes.

Adsorption or filtration?

Older articles, including an earlier version of this page, credited “adsorption” (molecules sticking to the fibre surface) for the clean cup. The studies above point mainly to physical filtration and retention by the coffee bed, with no preferential capture of particular lipid types by the paper. Ratnayake and colleagues found the lipids in the brew, the paper and the grounds had the same profile, which is what you would expect if the paper is filtering rather than selectively binding compounds.

Paper Density and Flow Rate

Dense paper resists flow. In a pour-over, the water has to pass through the coffee bed and then the paper, and both slow it down. A slower drain means longer contact time, which increases extraction. That is why the same grind that works in a thin-paper dripper can choke a Chemex and taste over-extracted, and why Chemex brewers usually grind coarser.

The Chemex carafe has a moulded spout with a groove that lets air escape from the lower chamber as coffee fills it. Folded Chemex filters have a thicker, multi-layer side that should sit over that spout, which supports the paper and helps keep the air channel open.

Practical Chemex Method

1. Place the filter with the multi-layer side towards the spout.
2. Rinse with hot water to wash out loose fibres and preheat the glass, then pour the rinse water away (the filter can stay in place).
3. Choose your ratio and weigh both coffee and water. Our brew ratio calculator works out the numbers; CHEMEX also publishes its own brew guide.
4. Grind medium-coarse, coarser than for most cone drippers, and adjust from there by taste and drawdown time.
5. Bloom: wet all the grounds with a small amount of water and wait briefly so trapped gas escapes and the bed wets evenly.
6. Pour in stages, keeping the stream on the coffee bed. Water poured onto bare paper can run down the side without passing through the coffee.
7. Judge by taste, then adjust one thing. Bitter or drying: grind coarser. Sour or thin: grind finer. Keep everything else the same so you know what changed.

For how the Chemex compares with the V60, French press, AeroPress and other methods, see our guide to every home brewing method. Water matters as much as paper; our water chemistry guide covers it.

Frequently Asked Questions

Do Chemex filters remove cafestol?

Paper-filtered coffee contains only negligible amounts of the cholesterol-raising diterpenes cafestol and kahweol. Research suggests most of the cafestol stays in the spent coffee grounds, with the paper holding back a smaller share, and the brew itself carrying very little. Fine perforations in a filter and very fine grinding can let more through.

Why does Chemex coffee taste so clean?

Paper filtration keeps fine particles and almost all of the coffee oils out of the cup. One study measured under 7 mg of lipids in a 150 ml cup of paper-filtered coffee, compared with 50 mg through a metal screen and 60–160 mg in boiled coffee and espresso. Less sediment and oil means a lighter body and a clearer flavour.

Should I rinse a Chemex filter?

Yes. Rinsing with hot water washes out loose paper fibres and paper taste and warms the glass so the brew does not lose heat. Pour the rinse water away before adding coffee.

Why is my Chemex brew draining slowly?

The usual causes are a grind that is too fine, a fine-heavy grinder, or the filter sealing against the glass so air cannot escape from the carafe. Grind coarser, make sure the thick, multi-layer side of the filter faces the spout, and keep the air channel clear.

Is Chemex coffee healthier than French press?

For people watching cholesterol, paper-filtered coffee is the safer choice. French press coffee contained about 3.5 mg of cafestol per cup in one study, while drip-filtered coffee contained negligible amounts. Researchers estimate that 10 mg of cafestol a day raises serum cholesterol by about 5 mg/dL.

Keep Learning

• Coffee extraction science: TDS, extraction yield and the brewing control chart
• Every home brewing method compared
• Water chemistry for better extraction
• Brew ratio calculator
• Moka pot extraction physics

Sources

• Urgert, R. et al. (1995). Levels of the cholesterol-elevating diterpenes cafestol and kahweol in various coffee brews. Journal of Agricultural and Food Chemistry 43(8), 2167–2172.
• Ratnayake, W. M. N., Hollywood, R., O’Grady, E. & Stavric, B. (1993). Lipid content and composition of coffee brews prepared by different methods. Food and Chemical Toxicology.
• Rendón, M. Y., dos Santos Scholz, M. B. & Bragagnolo, N. (2017). Is cafestol retained on the paper filter in the preparation of filter coffee? Food Research International.
• Rendón, M. Y., dos Santos Scholz, M. B. & Bragagnolo, N. (2018). Physical characteristics of the paper filter and low cafestol content filter coffee brews. Food Research International.
• CHEMEX Corporation. Official site: history, materials and Bonded filters.