The Four Rules of Optimal Coffee Percolation

After having brewed almost exclusively pour overs with the V60 and the Fellow Stagg [X] in the past few years, I have come to adopt a style of recipes that best suited these drippers in a way that it took me a while to fully understand. Writing my upcoming book The Physics of Filter Coffee forced me to think more deeply about the limitations of these drippers, and made me aware of their design flaws, if we can still call them flaws in a context where almost every other dripper is more flawed. More recently, I began playing with Decent Espresso Machine’s DE1, with the Tricolate dripper, and with the base of the Aeropress as a gravity dripper. While I will discuss these in other posts, these different devices opened my mind to some aspects of percolation.

After this shake-up of how my thoughts about percolation are organized, I decided to write down some of what I now believe are the most important things to consider when designing a dripper, a brew recipe, or espresso preparation methods. Here, I’ll call them the “rules of optimal percolation”; it does not mean you necessarily have to follow them, but rather I think they are some things we should always be mindful of.

Some of them may be useful in the context of immersion, but I wrote them specifically with percolation brews in mind. It is generally a lot easier to achieve good and even extraction with immersion brews, but immersion is not as potent as percolation to achieve high extractions, and in some cases percolation also makes it possible to achieve a very good beverage filtration. This is why I think it is worth putting up with all of percolation’s difficulties in the first place.

First, let me just state the four rules I have settled upon, and I will then explain them in detail.

1. Avoid Bypass
2. Avoid Clogging
3. Achieve an Even Flow of Water Through the Coffee
4. Adjust your Brew Ratio to your Grind Size

In addition to the four rules above, it is good to remember that using drippers made of insulating materials is desirable when preparing coffee with any percolation method. It is also best to avoid drippers made of materials that can store a large amount of heat, with the exception of espresso machine group heads, because those can be kept at a controlled, high temperature. In general, you just want to be able to control the temperature of your slurry during an extraction. This is famously quite hard to do with most pour over drippers, which is why I tend to prefer the Fellow Stagg [X] or the plastic V60 over most other drippers.

In the same spirit as this caveat about temperature stability, it will always be frustrating and wasteful to brew coffee unless you can repeat your best brews in a repeatable way. It is therefore always preferable to choose repeatable methods, measure your dose of coffee and water, and use a grinder, kettle or dripper that helps you replicate your results precisely. The four rules that I am about to discuss only focus on how to achieve a better evenness of extraction during the percolation phase, and ignore these considerations of repeatability.

1. Avoid Bypass

What I call bypass is any water that manages to make its way around the coffee bed, or most of the coffee bed, and will therefore not participate to extraction. While this effect doesn’t immediately sound alarming—I’ve said in the past it is just like diluting your brew with more water—I now think that it drastically reduces our flexibility when brewing coffee. Worse, bypassing water often touches the outer edges of the coffee bed, and may drag unpleasant flavors by over extracting these parts from the significant flow of clean water.

One of the major problems with bypass is that it will depend on your brew parameters, such as the filter you are using, how tall a column of water you have above your coffee bed, the depth of your coffee bed, and perhaps above all, your grind size. Imagine you are brewing coffee in a V60 dripper: the absolute amount of water that flows around the coffee bed at any moment does not depend on your grind size, if you compare apples to apples (i.e., with the same filter and the same water column heigh). It might be, for example, around 1 gram per second at the moment where you have a 5 cm column of water above the coffee bed (I made up this number). However, how much water passes through your coffee bed depends significantly on your grind size.

If you are grinding quite coarse, maybe you have 5 grams per second that are passing through the coffee bed, and therefore bypass only makes up for 17% of your total drip rate at that moment. However, if you ground fine enough that only 0.5 grams per second are actually passing through the coffee bed, bypass makes up more than 66% of your total drip rate at that moment. This is a recipe to get a weak and astringent brew. I now think this is one of the major hurdles that prevent us from grinding finer and still obtaining a good-tasting beverage with drippers such as the V60.

I have often heard baristas claiming that pouring at the center of the coffee bed avoids bypass—this is simply false. Wherever you pour, if water is able to pool on top of your coffee bed at all, it can find its way to the edges and still bypass. Neither are aggressive center pours a good solution: they will cause a crater at the center of the coffee bed, which may reduce bypass, but it will also produce a very uneven extraction by leaving some of the higher-up coffee particles under extracted.

Another way to mitigate bypass is to divide water pours into many steps, such that the column of water never gets too tall above the coffee bed. While this will definitely reduce bypass, it will significantly reduce the temperature of your slurry, in a way that is hard to control. While lower slurry temperatures may be preferable with darker roasts, I have never enjoyed them with the lighter roasts that I am used to drinking. Using many small pours will also make your brew much longer, because the water traveling through the coffee bed will not be pushed by as much weight on top of it. A longer brew time is not necessarily a bad thing in itself, however, as I will discuss further down.

Another trick can be used to reduce the impact of bypass: agitation. By causing enough agitation of the coffee bed, a barista can force even the deeper parts of the coffee bed to encounter fresh water, and increase the efficiency of extraction before much of the water can actually get around the coffee bed. While this solution certainly works, it is not without drawbacks. Too much agitation can allow coffee fines to get trapped in a paper filter, and cause clogging. We will come back to clogging in the next rule—but basically, this is the main reason why we never agitate 100% of the coffee bed for a full brew, because this would be a sure way to clog it.

Even Fellow’s Stagg [X] dripper, which I have come to prefer over the V60 in part because it suffers from less bypass with the appropriate modifications, is not completely free from bypass. I have only realized this after brewing coffee with drippers that actually do not bypass.

2. Avoid Clogging

Whenever too many coffee fines get trapped in the pores of a paper filter, the drip rate of a coffee brew can go down drastically. This is not only a problem because the brew becomes much longer: the bigger issue is that whatever water is still able to pass through will do so along smaller, and unchanging paths through the filter. This means that large regions of the coffee bed will potentially stop receiving fresh water, and will remain under extracted, while other regions will receive the bulk of the flow and contribute astringency or other unpleasant flavors caused by over extraction.

It is often not easy at all to avoid clogging; using a high-quality grinder that generates less coffee fines is a viable solution, but even with those you will encounter some coffee beans (e.g., decaffeinated or Ethiopian coffees) that still generate enough fines to potentially clog most paper filters unless you are careful about it.

Having your water pass through a large surface area of paper filter is one great way of reducing clogging, as well as using thicker paper filters. This is true because both of these tricks will increase the total volume of paper filter where fines can get trapped before clogging occurs. This is often referred to as the loading capacity of a filter. Using creped filters is also a good trick, because the rippled surface of the filter will increase the surface area of contact between the coffee particles and the coffee filter at the very small scale.

One big design flaw that I often encounter in drippers is that the concentrated water can only pass through a small region of an otherwise large paper filter. For example, the thin Kalita filters and the shallow ridges at the bottom of the Kalita dripper often cause the filter to sag down, and sit on top of the three small holes of the dripper. Before this happens, water is free to flow through the full bottom of the filter, providing a large-enough surface of filter to avoid clogging, but as soon as the filter sits on the holes, water begins flowing only through the paper directly above the three holes. This drastic reduction in the filter surface is responsible for the Kalita’s infamous clogging issues. The Chemex, Stagg [X] and Stagg [XF], among others, suffer from this problem. The design of the V60 makes it extremely robust against clogging because it has a gigantic, cone-shaped surface area of filter that is well lifted from the dripper walls by ridges. However, doing so makes the V60 extremely vulnerable to another major problem: bypass.

3. Achieve an Even Flow of Water Through the Coffee

Rules 1 and 2 above are only useful because they are in a sense required to achieve an even flow of water through the coffee bed. However, there are other ways in which water can flow unevenly through a coffee bed: having a non-level bed of coffee, a non-level dripper, a very uneven particle size distribution, bad puck preparation in the context of espresso, can all be further causes for an uneven flow of water through the coffee bed. It is therefore important to always be mindful of these potential issues.

Problems of uneven flow are often grouped within the term channeling. While technically, channeling may only refer to a hollow in the coffee bed (either microscopic or large) that allows for a large local flow of water, this problem really is of the same nature than bypass, or any uneven flow. In my experience, it is relatively easy to avoid channeling in the true technical sense with gravity-driven brews, other problems like bypass and clogging are not easy at all to deal with. In the context of espresso, channels in the true technical sense are not as easy to avoid, because mistakes in puck preparation combined with the higher pressure will favor channels.

There are some general ways of improving the uniformity of flow inside a coffee bed that are unrelated to clogging and bypass. For example, blooming the coffee bed properly to start percolation after it is entirely wet, and using a grinder that produces a more even particle size distribution are two ways to do this. A more uniform particle size distribution will not only it will make it directly easier to extract evenly in the first place, but it will also lead to a more even flow of water. Another important consideration is the drip rate of your brewer; for a fixed dripper cross-sectional area, the much slower drip rates will tend to give rise to a less even flow of water inside the coffee bed. I discussed this in a bit more details in this past blog post, and we’ll come back to this idea below.

4. Adjust your Brew Ratio to your Grind Size

When grinding coffee finer, you are exposing more cells of the coffee beans to the surface of the coffee particles, and they can therefore extract more easily. This means that the finer you grind, the less solvent and the less time you will actually need to extract everything before you start to draw out the unpleasant components that extract slower and taste worse. In other words, I believe that optimal brew ratios will be smaller (less water) when grinding finer. This also means that beverages prepared with finer grinds will necessarily be much more concentrated, and generally have somewhat higher average extraction yields.

If I had read this particular rule a year ago, I would have though “what the hell is this guy thinking?”. I would have thought that because, in practice, preparing a V60 with very finely ground coffee would taste quite bad, even if one uses much less water. But remember: grinding finer with a V60 actually causes a different problem: it increases bypass, and significant bypass will result in a weak and astringent brew. I now believe that this is why it took me so long to uncover this idea of how adjusting brew ratios to the grind size is important. Every dripper I had always used had design flaws that simply made it impossible to achieve good beverages with some combinations of ratio and grind size, because of either bypass or clogging!

It is only at the beginning of 2020 that this idea of adjusting brew ratio to grind size hit me, which prompted me to write about it in a past blog post, but lacking the proper drippers to explore it further, it remained just that, an abstract idea. However, this came back to me when I started alternating between espresso and Rao allongés on the DE1, and wondering how such different grind sizes could taste good when using different ratios, but similar brew times and pressures. When I thought more about this, I suggested that there may exist a family of good-tasting recipes, where the flow rate of the DE1 would be adjusted as a function of the grind size.

However, when I wrote about this and designed DE1 “adaptive” shot profiles to explore this family of brew recipes, I did not immediately grasp the importance of the changing brew ratios. It is only after having brewed a lot with these adaptive profiles that I came to realize the brew ratio was a lot more important than keeping a fixed brew time along all possible brew recipes. Coarser-ground coffee tasted better with 1:4 to 1:6 ratios, whereas finer-ground coffee tasted better with 1:2 to 1:3 ratios, and there didn’t seem to be any grind size that doesn’t taste good, as long as the proper ratio is used, and the puck preparation was good enough to achieve an even flow of water through the puck.

Brewing with the Tricolate made me even more confident about this. To be sure I was not experiencing any bypass, I placed some food-grade silicon around the bottom part of the dripper, and I started experimenting with brewing finer-ground coffee with a shorter brew ratio. And indeed, the results were very encouraging: I was obtaining very good-tasting coffee, free of astringency, at both higher concentrations and extraction yields that I was ever able to achieve with my more classical pour over brews at a 1:17 ratio.

It is important to stress again that you must free yourself from the other problems like bypass and clogging, before you can actually fully explore the family of good-tasting coffee beverages with different grind sizes. So far, the only drippers I encountered that allowed me to do this are: the DE1 espresso machine, the Büchner funnel, the Tricolate, and the bottom of an Aeropress used as a gravity dripper (although it is hard to avoid clogging with it). In other words: don’t expect this rule to be very useful if you are using drippers like the V60 or Stagg [X] that do not completely avoid bypass. The first two rules therefore act as some kind of a barrier that you need to cross before you can really use the concept of adapting ratio to grind finer than typical pour over grind size.

You will also have to open your mind about brew time; we are used to think that very long pour over brews (above 4 or 5 minutes) are always bad, but this is only true because they normally indicate you ground fine enough to cause significant bypass, or that your filter has clogged. In other words, they are once again different underlying problems that are not directly related to brew time. If you brew with a dripper that is completely free of bypass with a filter that does not clog, you can obtain very good tasting brews that are much longer. I personally had a few really good brews that took as long as 10 minutes recently, with the Tricolate and the bottom part of an Aeropress used as a gravity dripper.

If you try to brew something as fine as an espresso with only the force of gravity, you may obtain brew times so long that they will become truly problematic; maybe the flow of water will get so slow that flow will be uneven inside the coffee bed, or the slurry will become too cool to achieve good extraction. Scott Rao has experimented a bit more than me with the extreme end of finer grind sizes with the Tricolate, and he seems to be finding a limit, much finer than what we are used to for pour overs, but still a limit, where the brews start tasting flatter. The use of pressure with a Büchner funnel or espresso machine is probably needed at some point, and with the increased uses of pressure, there is also probably a point where tamping becomes important to avoid uneven flow. However, using pressure will almost always lead to a loss of beverage clarity, because the added pressure, and the resulting fast flow of water in the pores of the coffee bed at the microscopic level, will allow coffee fines to migrate even through relatively thick paper filters. This is not necessarily a bad thing; I was really surprised recently at how much I like Rao allongés that are not clear brews at all.