Basket Shapes & Water Quality: Filter Coffee in The Science Lab

What do you need to do to brew better filter coffee? That’s one of the many questions UC Davis Coffee Center is setting out to answer, through a variety of research projects. After all, there’s nothing like hard science for an answer you can trust.

Professor William Ristenpart, the centre’s Director, agreed to talk me through his current work and what we can expect to see in the future. From the impact of coffee filter baskets to water quality and temperature, read on to discover what I learned.

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The UC Davis Coffee Center. Credit: UC Davis

What Is The UC Davis Coffee Center?

The UC Davis Coffee Center is a multidisciplinary university-based research centre dedicated to the specialty coffee industry. As Prof. Ristenpart says, “The Coffee Center is poised to do for coffee what UC Davis did for wine and beer: to become the leading source of scientific expertise in the study of coffee.”

Everything started with the Design of Coffee, the first course he gave about coffee. This introductory class was created six years ago and aimed at freshmen (first-year) students of Chemical Engineering.

While the module began as what Prof. Ristenpart calls “a teaching exercise”, the increasing complexity of the course and the enthusiastic response from the coffee industry led UC Davis to “realise that there was such a tremendous need for both research on coffee science and more advanced education in coffee”. The result was the UC Davis Coffee Center.

The centre has 35 faculty members, 14 listed areas of expertise, and a range of undergraduate and graduate courses. But what I really wanted to learn about was Prof. Ristenpart’s research projects.

The classroom used in The Design of Coffee module at UC Davis. Credit: UC Davis

Rethinking The Brewing Control Chart

The Brewing Control Chart is designed to help brewers understand the relationship between extraction, strength, brew ratio, and coffee flavour. However, it was also designed in the 1950s. Fortunately, Prof. Ristenpart has been working on a two-year research project with the Specialty Coffee Association (SCA) to update it.

When Professor Earl Lockhart, a food technology PhD from the Massachusetts Institute of Technology (MIT) developed it, it was some of the most advanced coffee research of the time. However, Prof. Ristenpart tells me that his project will be the chart’s first revision in the seventy years since its creation.

Moreover, he explains that there were several problems with the initial study: the sensory evaluations were based on panels primarily made up of housewives brewing with percolators, which, as he tells me, tend to over-extract coffee. The temperature the data was taken at isn’t specified.

“What many people recognise is that it has… technical problems in terms of how the numbers are represented,” he adds. He pictures the improved brewing control chart with a z-axis for temperature.

Filter Basket Shapes: An Impact You Can Taste

The professor has also recently finished a five-month preliminary investigation with the SCA into filter basket shapes. He tells me that there have been passionate arguments about what the best basket shape is – and now, science will be able to weigh in.

“There are two analogists, for the first time, generating hard data corroborating beliefs and right now we are doing a series of more digital physical measurements trying to answer the question: why do people taste a difference? What is it about the basket geometry that affects the final brew?”

The team was able to show that regular people, who aren’t trained testers, were able to tell the difference between coffee made with different basket shapes when everything else remained constant: the coffee beans, grind size, water temperature, basket manufacturer… and more.

“There was an appreciable difference in taste,” Ristenpart tells me. “They were able to taste the difference in the shape and they were not able to taste the 25% difference in the grind size… There was much less sensitivity to that than to the basket geometry.”

Behmor Brazen Plus coffee brewers being used for testing filter basket shapes at the UC Davis Coffee Center. Credit: UC Davis

Prof. Ristenpart tells me that a lot of this comes down to flow rate. They kept the flow rate of the water being delivered to the top of the grounds the same. However, the rate at which the brew came out in the carafe depended on the basket shape. Since the feed flow rate was the same, the delivery flow rate was treated as a function of the basket shape.

“What I hypothesise,” the professor says, “is that, in the future, more advanced coffee brewers will, just as a matter of course, allow you to dial in very specific flow rates. Some more advanced brewers already do that; most do not… but I think the flow rate will be a control parameter that more and more people will be aware of as affecting the final flavour.”

Collecting brewed coffee samples with the Behmor Brazen Plus at the UC Davis Coffee Center. Credit: UC Davis

Flow Rate, Mass Transfer, & Basket Shapes

So, why does flow rate have an even bigger impact on flavour than grind size? That’s another one of the questions the team at UC Davis is attempting to answer.

But their work isn’t easy, because flow rate isn’t just about time: it’s also about the mass transferred and the quantities of different chemicals and molecules within that.

Prof. Ristenpart tells me, “One of the things that I study is understanding how the fluid flows through the coffee grounds, how the convection affects the diffusive mass transfer out of the coffee grounds into the coffee that we’re drinking.

“It’s actually a really complicated problem,” he continues, “because there are so many different molecules that are undergoing mass transfer. It’s not just one or two… there are about 1,000 different flavour molecules that are identified in roasted coffee so, from a chemical standpoint, it is very complicated.

“From a physical standpoint, it is also very complicated because of the basket geometry. The flow is not the same everywhere so scientifically it’s a very challenging problem – but a fun one.”

Students study coffee brewing as part of the Design of Coffee module at UC Davis Coffee Centre, a course that draws heavily on chemical engineering. Credit: UC Davis

The Relationship Between Water Temperature & Basket Shape

Water temperature is just one more variable affecting the professor’s research. While most brewers could tell you that higher water temperatures increase the rate of extraction, there’s a lot more to the topic.

For example, both viscosity and mass transfer proficiency are affected by water temperature. Prof. Ristenpart tells me that they have “exponential temperature dependencies; in other words, small temperature changes have a very big impact on some of the physical and chemical properties in the system”.

And coming back to filters, the basket geometry will affect the heat transfer within it. Even though the water temperature delivered may be consistent across different filters, that doesn’t mean the temperature of the water the grounds are reacting with is the same.

Prof. Ristenpart tells me that he and his team found “so far about a three or four-degree temperature decrease when using the flat bottom vs the conical [baskets]. So, three degrees by themselves is a lot, but it is actually quite a bit in terms of that exponential temperature dependence.”

For now, they’re using a constant temperature for their experiments. In the future, they also intend to study the effects of different water temperatures.

Measuring the temperature of coffee samples brewed in a Behmor Brazen Plus. Credit: UC Davis

Water Quality: One More Factor to Consider

The coffee industry is waking up to the importance of water quality, with awareness growing among coffee-lovers over the past few years. Books like Christopher Hendon’s Water for Coffee have helped this trend.

Prof. Ristenpart tells me that it’s been well-established that water hardness can affect coffee brewing. Hardness refers to the number of ions – especially calcium and magnesium – that is in a water sample.

Yet more recently, he says, researchers have been noticing that the composition of the ions is also important. Two samples may have the same overall hardness, but one may be richer in magnesium while the other has more calcium. “Because the ions are different, they affect the details of the mass transfer.”

It’s early days for this research; as the professor tells me, they have qualitative data and theoretical work but “there is not a lot of hard data yet so that is definitely one of the goals that we have.”

And in the future, we can expect to hear more about water quality from the UC Davis Coffee Center, which is in the process of partnering with companies to create a water lab. “We recognise that the water quality is crucial for making good coffee,” Prof. Ristenpart says.

With a dedicated research and education centre for coffee, there’s great potential for the future of brewing. As we begin to glean insights into the science behind our favourite drink, we can improve both the quality and consistency of it. Prof. Ristenpart has set out to treat coffee the same way as we do wine and beer – and already we are seeing great results.

Filter basket shapes, water temperature and quality, and new brewing charts: these are excellent first steps in a journey towards better coffee. So, here’s to a future with even more research.

Enjoyed this? Check out Coffee Science: Breaking Down Where Flavour Comes From

Written by Angie Molina.

Please note: This article has been sponsored by Behmor.

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