For today’s post I will be focusing on an element of brew which I actually did not have as one of the fundamentals of brewing in my book. This is all about brew device shape, and essentially why I did not provide comparative analysis of the the various shapes/styles of dripper out in the world.
To start off I want to say that I do believe different shapes can impact the final brew’s flavour, but because of numerous other variables it is extremely difficult to give a truly comparative look at what is happening. If you have been around SCA events or news in the past year you may have been introduced to the study which was performed at UC Davis in California comparing flat-bottomed vs conical shaped drippers. While the data is not incredibly well explained, there are indications that they found flat-bottomed drippers to produce more favourable flavours. Additionally they seem to have determined that the “non expert” tasters could not differentiate between a medium and medium-fine grind.
I applaud the incredible amount of resources that went into this study, as making and tracking hundreds of brews, as well as having tasting panels give structured feedback, is no simple task. Unfortunately I do have some serious questions about the effectiveness of this study, and while I don’t want to dwell on what I find to be a less useful study outcome, but I’ll run through a few of my questions:
If the grind size difference was difficult, or impossible, to differentiate, was it all over or under extracted tasting overall? (Essentially, if they all had a similar flavour character they would be quite difficult to differentiate.)
On that same note, it indicates that the “non expert” tasters could not differentiate, which seems to imply that the “expert” tasters could. What exactly was asked of the non experts? Just to identify the odd cup? Even experienced coffee tasters can have trouble with this. Also, how long were their tasting sessions? Did they experience palate fatigue?
In order to keep the comparison fair, what was the filter paper used in flat/cone drippers? Did they cut their own from the same source, or use off the shelf options? This is quite a big impact on your brew in my book, and if even slightly different paper was used it could easily impact the study.
Some other data on this study, which I believe I saw at the SCA show in Boston, indicated that the TDS and extraction yields between the two shapes were quite different. This brings further concerns as to the effectiveness of the study in the first place, but my conclusion is simply this:
It would seem the coffee used, temperature, water chemistry, device material, filter, turbulence, and grind profile, ratio, etc was in favour of the flat bottomed dripper. It is unlikely this was by design, but because of this study there will be a large influence on dripper manufacturers in the near future to trend toward flat bottomed dripper shapes.
Now that we have raised questions about what is almost certainly the most in depth scientific look at the topic we have at this moment, let me tell you why I didn’t go about this type of comparative data in the first place.
I go into this quite a bit in my book, and in it we find there are significant differences in major paper filter brands around the market. Because I already knew I needed to compare specific types of filter paper, I also knew that I needed to find the same exact paper to use between multiple different shapes. In my pre-research phase, I realised that this would not happen without some major manufacturing help.
Looking at filters from a Kalita Wave, Hario v60, Chemex, and Melitta truncated cone, it is apparent that these are widely different in both porosity and thickness, so I couldn’t use stock filters to perform a test. You can however buy roll-stock filter paper and cut/fold/crimp it to fulfil your needs (and perhaps I will do this in the future). This will be a huge amount of work, and I find the data comparing the actual filters to be more use. If you want to do this yourself, I would be happy to discuss your findings.
While we want to look at the dripper shape, what we are ultimately looking at (most of the time) is the paper’s particular shape. The dripper is more like a shell that keeps the filter in its position, as well as adding thermal stability (or loss) from the material.
Look at a flat bottomed filter and what do you see? A flat base with flutes (or ripples if you like) around the exterior. If you have ever brewed with one of these you probably have noticed that a significant amount of coffee particles get trapped in the flutes, preventing them from interacting with turbulent water and the rest of the slurry. If you wash these grinds out, you are likely pouring water past the filter in the negative space of the flutes, which brings further inconsistencies. Long story short, it is very difficult to maintain a consistent slurry that is extracting from all of the grinds with the flat bottom filter.
Now look at a conical or truncated cone filter. It has smooth edges which are uninterrupted and easy to pour in a way that pulls most or all of the grinds into the slurry. The only bypassing of water could potentially come from washing the filter itself and creating a spot where the water can flow through the filter more quickly, which may have large or negligible impacts. Regardless, this is significantly less troublesome in terms of getting all the grinds into the slurry and interacting with the turbulent water. The trouble the conical filter has is that it stacks the grinds in a way which makes it unclear if the entire dose is being extracted evenly/fully.
All in all, the nature of how the filter paper is shaped has implications beyond the shape of the dripper itself. There is not truly perfect shape for a “paper filter in the shell” type pour-over brew at this point.
Ridges, bumps, and channels
Since we’re pointing out that the particulars of the paper shape don’t tell the story of the dripper, we should not ignore that many drippers have varying internal structures, such as ridges, bumps, and channels. These are commonly described by manufactures as features meant to prevent clogging, speed water flow, or even avoid vacuuming elements occurring as you brew. By allowing some air to flow around the exterior of the filter it is assumed that the brew will behave better.
Despite my personal skepticism that these features have less impact than most other variables, they are still there and could potentially create inconsistencies. Since these features tend to be manufacturer specific, it is very very difficult to compare the exact features next to each other, or in the case of my desired testing, be able to realistically compare only the dripper shape without delving into other unaccounted variance.
In my testing for this book I tried my best to truly compare only a single variable at a time, and as you may guess this can be much more difficult than it looks.
With all of those details out of the way, I do want to go through the nature of a different dripper shape and what you might expect with their use.
Flat bottom brewers create an even slurry bed to filter through, despite the issues with the flutes as discussed above. Many people find a slower flow of water exiting the device to be desirable, which I believe is why the December Dripper was originally created. By slowing (adjusting really) the flow of water at the exit, you can increase the time of steeping in the slurry, which is likely to increase the body of the brew a bit. There may be simultaneously be a decrease in distinct flavours, similar to what is experienced in a full immersion brew. This of course would be dependent on numerous other factors though. Another advantage of flat bottom drippers is that the bloom is more likely to be fully saturated with little or no manual mixing.
Conical brewers make a stacking of the grinds, which results in a strong amount of grind filtration at the base and less at the top portion of the filter (where the water may pass the paper more easily). Essentially there is a tendency for varied amounts of extraction simply because of the nature of the shape. Most conical drippers have a large hole at the base for the paper filter to fit through. The aforementioned ridges/features on the interior of the dripper will have an impact on how effective that large hole is at letting brew liquid pass through. Due to the fact that the grinds are stacked at the base of the filter, some sort of mixing is common during the bloom in order to ensure all grinds are wet prior to continued pouring. I have always found these drippers to be dependent on at least some level of skill in their use, as small changes to the way you pour tend to show up in the finished cup quite easily.
The truncated cone shape was once the standard in the industry. It behaves similarly to the conical shape in the fact that it is easy to keep the grinds interacting in the slurry (no fluting). The grinds bed is essentially half stacked like a conical, and half flat like a flat bottom. This makes for slightly more efficient wetting during the bloom, though they commonly still need some mixing. Truncated cones are fully enclosed by the device material, much like most flat bottom devices. This will add an element of insulation once the dripper is fully heated, but comes with a drawback. The paper for these devices is crimped on the sides (like most conical filters), but also on the bottom. The bottom crimp is usually folded over so that the filter will sit flat during brewing, and since we are creating a very thick layer of paper we should also look at where it tends to sit. Indeed, the very thick folded, crimped paper tends to sit directly over the holes where the coffee will drip through. This is creating a higher level of resistance to the liquid exiting the dripper, once again increasing the time for steeping. This could perhaps explain why the Melitta filters tested in my book were made with larger holes punched in them, as it would theoretically speed up the flow (but I have my doubts on how it actually functions).
Now I have tasted delicious coffee from all of these device shapes, and I would associate this with user skill rather than the advantage or fault of the device itself. Of course some shapes are simply easier to get a great flavour with than others, so that may guide your decisions as well.
The last thing I want to cover on this post is about the nature of comparing extractions. When reading my book some have asked “Why not adjust the extraction TDS and yield for each device and then compare the flavour?”
The answer for this is quite simple. We would then not be comparing the nature of the device, but rather numerous variables being manipulated to get a desired number. While I am certain some would taste better than others, the point is to give an accounting of a single variable and how it impacts the extraction. By changing the grind, brew temp, pouring turbulence, or brew time, you would be dialling in a brew rather than comparing the variables. Being able to dial in a brew was my goal for the book itself, so if you are doing that, mission accomplished! But this may also speak to the difficulties of the UC Davis study I referenced earlier. Keeping so many variables in order is difficult, and it did in fact seem that they did a pretty good job of keeping many variables consistent. In the future hopefully we will get more and more well-tuned studies.
Shameless self promotion…
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