Sieving the day: filtering caffeine from your coffee

The humble coffee filter has the relatively simple job of letting the coffee through and leaving the grounds behind. But researchers from the ARC Centre of Excellence for Carbon Science and Innovation (COE-CSI) are exploring whether a filter operating at the molecular scale can do something much more difficult: separate the caffeine from the coffee itself.

It’s no ordinary challenge. A cup of coffee isn’t simply water and caffeine; it’s a mixture of various chemical compounds, many of which contribute to its flavour, aroma and other characteristics. Removing caffeine while leaving the things that make coffee taste like coffee means distinguishing between molecules at an extraordinarily small scale.

For UNSW Team Graphene master’s research student Yihan Tian, the challenge had plenty of appeal. Not only is she a huge fan of coffee, but the science itself also excites her.

“What attracted me most is that this research connects fundamental membrane science with a very practical, everyday product,” she said.

Working alongside Dr Tongxi Lin and COE-CSI research fellow Dr Xiaojun Ren, Tian developed a membrane using graphene oxide—an atomically thin, sheet-like carbon material that can be assembled in layers. Tiny channels between those layers allow water and other molecules to pass through the membrane and, by incorporating sodium alginate and crosslinking it with calcium ions, the researchers were able to change those channels and influence which molecules could make their way through.

In effect, they created an extraordinarily fine sieve.

Taking caffeine from coffee itself

Where an ordinary coffee filter separates relatively large particles of ground coffee from liquid, the graphene-oxide membrane attempts to separate molecules already dissolved in the coffee.

Of course, removing caffeine from coffee is nothing new; conventional decaffeination generally takes place before the coffee beans have even been roasted. But separating caffeine from a coffee bean—without also adding or extracting other compounds—has always been part of the challenge, because some of them contribute directly or indirectly to the flavour of the end product.

Rather than removing caffeine from the beans, Tian and Ren approached the problem from the other end. They wanted to know whether it could be removed from coffee that had already been made.

Finding the right balance

The researchers produced several versions of the membrane, but quickly encountered another problem: making a membrane better at one thing could make it worse at another. Removing as much caffeine as possible, for example, is of little use if the membrane also removes too many of the compounds you want to keep.

Rather than simply asking which membrane removed the most caffeine, the researchers assessed them against several measures—including caffeine removal, retention of selected compounds and water permeance—to identify the best overall performer.

The best reported membrane was able to remove around half of the caffeine from the coffee.

Other compounds, including trigonelline, N-methylpyridinium, choline and hydroxymethylfurfural, remained detectable in the coffee after passing through the membrane.

Promising progress

Removing around half of the caffeine is an encouraging result, while further work is still needed to move towards a practical decaffeination process.

The result does not necessarily represent the limit of what the membrane can do. In this study, filtration time was kept consistent so the different membranes could be compared fairly. Further optimisation of the membrane and filtration conditions could improve caffeine removal while maintaining a practical processing time.

Removing around half of the caffeine while retaining several other coffee compounds gives the researchers a promising starting point for further development.

The researchers also plan to explore how filtration affects coffee’s taste and nutritional properties. Sensory testing could guide the next stage of development, with a familiar goal in mind: removing more caffeine while keeping the flavour people expect from their cup.

From the laboratory to something larger

The researchers have also taken an early step towards exploring how the approach could work at a larger scale.

They incorporated their membrane material into hollow fibres—thin tubes that allow a large membrane surface area to be packed into a relatively small space. Hollow fibres are already used in commercial filtration systems, giving the researchers an established membrane format to build on as they develop the technology towards practical use.

In a continuous filtration experiment, the modified hollow-fibre membrane consistently removed more caffeine than an unmodified graphene-oxide membrane—an encouraging result for taking the approach further.

The experiment also helped identify priorities for the next stage of development. Coffee compounds accumulated on the membrane over time, slowing the flow of water—a process known as fouling. Future work on cleaning, reuse and membrane lifetime could help maintain performance for longer, while larger-scale trials could establish the costs of running the process. Further investment and industry collaboration could support these next steps towards a practical coffee-filtration system.

For Ren, the results offer good reasons to keep going.

“We know that it needs more experimental research for our membrane to work in the real process of decaffeination, but we are optimistic,” he said.

More than coffee

Coffee offers a familiar starting point for a much bigger idea. The research group has spent years investigating graphene-oxide membranes for applications including water treatment and organic solvent systems. Working with coffee brings that nanoscale science to a complicated material almost everyone immediately understands.

And the promise extends beyond caffeine. A membrane that can selectively separate one molecule from a complex liquid while allowing others to remain could open up new approaches to food and beverage processing—giving producers more control over what they remove and what they keep.

There are further opportunities to explore within coffee processing itself. Removing water while selectively separating caffeine could potentially concentrate coffee at the same time, while the separated caffeine could be recovered as a useful by-product. That could mean producing more than a lower-caffeine drink: it could also mean making better use of the ingredients coffee contains.

These possibilities give the researchers plenty to build on. The technology has significant potential for industry applications in the coffee and broader food and beverage sector. Further research and collaboration with industry could help bring the approach to a larger scale and explore where else it might make a difference.

For Ren, applying nanomaterial research to something as familiar as coffee is part of what makes the work exciting.

“It is so related to our daily life,” he said. “Everyone, when they hear it, already knows what’s happening.”

The humble coffee filter has long decided what gets into our cup and what stays behind. The next generation could give us more choice over both—with possibilities reaching far beyond our morning coffee.

Find out more about this research study via the recent publication.