Bed depth: why it matters
As third-wave coffee has evolved over the past 25 years, we’re probably at or near the extremes of a few long-term trends: roasts have gotten lighter, roasters use fresher green, on average, than ever before, menu offerings have decreased in number, often down to just three or four offerings at a time, and filter brews have gotten smaller, especially in the US. All of these trends have caused welcome improvements in cup quality, but trying to brew smaller and smaller amounts of coffee has sometimes caused quality problems.
I’m all for brewing very small amounts of coffee. I’d rather taste several small portions of different coffees than one large mug of one coffee. But I often see cafes trying to batch brew less than two liters in their Fetco and Curtis brewers, or brew very small pourovers, such as using 15g doses in a Kalita 185 or a NextLevel Pulsar, or Filter3. The problem with these efforts is their bed depths are too shallow. (Side note: the recent increase in the number of drippers designed for smaller doses is encouraging.)
Why does bed depth matter?
If a bed is too deep, one has to use a very coarse grind. The result in the cup may or may not be good enough, but the extraction level will be low, which is inefficient, in the sense of wasting coffee material. More often, when I see people brew with beds that are too deep, they don’t grind coarse enough, and the brews end up being astringent due to filter clogging and the resulting channels the liquid creates to get around the clogged areas. This is especially common with lower-quality home grinders that produce too many fines, and with very large-batch batch brews. (It’s surprisingly challenging to convince some businesses that they may need to use the coarsest setting on the grinder for some brews.)
Shallow beds are more prone to astringency. Let’s assume that in almost all percolation brews, some version of a channel forms numerous times per brew. What matters is less the formation of channels than whether those channels reach the bottom of the coffee bed. Hypothetically, many channels will dead-end at denser areas of the coffee bed below the channels. Those channels end up being relatively harmless. Channels that manage to reach the bottom of a coffee bed carry larger, astringency-causing compounds out of the bed and into the cup. As you can imagine, if deeper beds offer more opportunities for channels to dead-end, shallower beds allow more channels to reach the bottom.
The old rule of thumb I learned in the 90s was to have a bed depth of 3cm—5cm in a batch brew. It’s hard to say what is the optimal bed depth for a one-cup pourover, especially given the variety of pourover shapes and sizes these days, as well as the variety of filter-paper types in use; some filters clog more easily than others, and some may trap a greater number of astringency-causing particles than others.
Personal experience would imply these doses offer something of a sweet spot for each brewer:
NextLevel Pulsar: 30 — 35 grams
Kalita 185: 25g seems ok, 30g seems better
V60: 20g — 25g
Fellow Stagg X: 25g
Kalita 155: 15g—18g
Pulsar Mini: 15g (soon to hit the market!)
Aeropress: 18g — 20g
Filter3: 20g—22g
Orea: I know it’s popular, but I honestly can’t keep track of all of the variations on offer, so I’ll pass on this.
*These dose/brewer combinations apply to dense, lightly roasted coffee. I cannot say what is optimal for darker roasts.
Of course you can use any dose you want, in any brewer. I’m not here to tell anyone what to do.
The point here is that if your bed depth is substantially less than would be created by the dose/brewer combinations above, the risk of astringency increases. These doses are “safe” in the sense of providing a combination of adequate bed depth to reduce risk of astringency, and a fine enough grind to provide reasonably high extractions.