This Low Pressure Lark

“If I have seen further, it is by standing on the shoulders of giants”

— Isaac Newton, 1676

I can only imagine how it would feel to find that perfect shirt to wear out. And having found that shirt, ironed it, hailed a cab, headed into town, you turn around and see that shirt. Your shirt, but not yours, on someone else, and not you. Your shirt. But better.

I’m pretty sure it would be similar to how it’d feel to write my post last week, then find this post by Maxwell Colonna-Smalls … which led to this post by Matt North (literally titled “Marginal Gains – Maximising Extraction through Flow Rate and Pressure”) … which in turn cycled back to Maxwell’s post covering 15g basket – which of course included discussion of flow rate and low pump pressure. Let’s not forget to mention posts here and here by Matt Perger also mentioning pump pressure and flow rate. It might be a similar feeling. Or maybe I’m just overstating how shitty it felt to think I’d stumbled upon a Great Truth of Espresso, only to realise I was simply standing tall on the shoulders of others. I spent a sentence making analogies to waterfalls – Maxwell wrote 1000 words, and he had pictures.

Truthfully though in the back of my mind, when following this thread of low pressure I always felt I was playing catch up. Maxwell, both Matt’s, and countless others, had caught up already to the idea of how physics form the basis of the modern espresso shot. I’m still attempting to catch up, and in doing so I think it’s instructive to see how this style of espresso differs when contrasted with the traditional 9 bar shot. So let’s start with that – and if anyone else out there has already covered this, consider this “borrowing” the concept …

Making espresso with 9 bars of pressure means we need to grind fine, with the fineness of the grind itself creating resistance to the pressurised water. This resistance means the water has to work incredibly hard to move through the coffee and get to the other side, out of the filter basket holes, to where it wants to go. On it’s way, this pressurised hot water extracts flavour from the coffee – and therein lies the whole premise of high pressure extractions. The fineness of the grind also enables a larger surface area of the coffee to be exposed, allowing for a fast extraction typically around thirty seconds. But the high pressure creates some problems that need to be worked around through both equipment and technique.

The first problem is channelling. Everyone should have learned back in barista school that water is lazy. Say it after me: “water follows the path of least resistance”. So distribution of the ground coffee is crucial to ensure there are no weak spots or unevenly distributed pockets of coffee or air within the basket. Otherwise high pressure water will simply gush through the puck like a water blaster at a bucket of sand. Along with the actual distribution of the coffee, tamping is equally important. Tamping unevenly will create the same problem as distributing the coffee unevenly – looking sideways at the puck, there will be one side “thinner” than the rest, and conversely an opposing larger side. Water will take longer to travel through the larger side and less to travel through the thinner side, leading to an uneven extraction. The tamp also needs to be fitted to the edge of the basket. A tamp that’s short of reaching the edges will allow another avenue for water to channel through the gap created.

The physical act of tamping presents problems as well. Resistance to pressurised water does not just come from the fineness of the grind, but also with how compact the grinds are themselves. Traditionally we were taught the “thirty pounds of pressure rule”, so we tamp hard and as consistently as possible to around this level to compact the grinds to an even density. But however hard we tamp, it needs to be consistent, as the fineness or coarseness of the grind, along with how hard we tamp, all contribute to flow rate through the puck. A consistent tamp should correlate to a consistent flow rate, or so our 9 bar theory says. But this doesn’t take into account how this density actually plays out, with the physics involved instead distributing the energy towards the middle of the puck under tamp pressure, not evenly throughout the puck. Effectively the harder you tamp, the denser the middle of the puck gets, but not both the top and the bottom.

Also with 9 bars of pressure water moves quickly, which doesn’t allow much time for the water to do it’s job of extracting flavour from the coffee. The temperature also combines to speed up flow. With the boiler set to between 93 and sometimes up to 96 degrees celsius at the group head, this temperature lessens resistance as the heat increases solubility. Our fine grind should balance out both pressure and heat to allow the water to reach some kind of temperature and diffusional  equilibrium with the coffee, but perhaps not enough time. Luckily the pressurised water compresses the puck towards the end of the shot, with this compaction increasing resistance. The pressurised water also blasts the fines – the little bits, or little guys that form part of the particle distribution of the grinds themselves – towards the bottom of the basket, clogging up the filter holes, in a process sometimes known as“fines migration”. Not enough fines migrate so nothing can get through, but enough to aid resistance.

Basic 9 bar extractions are simple enough to understand in theory, but putting it into practice there are so many points of possible failure there – all going back to the fact that it’s highly pressurised water blasting though a small puck of coffee in a tightly confined space. I mean that’s how we get espresso, but repeatability is such an elusive outcome here. So taking a different tact, let’s actually list what needs to happen to get an even extraction, after we grind:

  1. Evenly distribute the coffee in the basket.
  2. Tamp the grinds down in a level manner.
  3. Tamp the grinds down in a level manner, to an even density.
  4. Tamp the grinds down in a level manner, to an even density, with a fitted tamp.
  5. Tamp the grinds down in a level manner, to an even density, with a fitted tamp, one coffee after another.

Now look at what needs to happen while we actually pull the shot:

  1. Our grinds have been evenly distributed, tamped down in a level manner, to an even density, with a fitted tamp, consistently one shot after another.
  2. Enough fines then need to migrate to the bottom of the basket, in a consistent manner shot after shot, to slow down the flow rate to enable temperature and diffusional equilibrium.
  3. The puck also needs to compact at the right time, to the right amount, in a consistent manner shot after shot, to also help in slowing down flow rate allowing temperature and diffusional equilibrium.

For shits and giggles, what about looking at what needs to happen before we grind or pull the shot? In the hopper here’s what we need:

  1. Your beans are roasted to an adequate development and roast level.
  2. Your beans were stored in a bag with a one-way valve.
  3. Your beans were sufficiently degassed, or ‘aged”.
  4. Your beans are from the same roast date, and better yet, the same batch.
  5. You have enough beans in the hopper to defeat the “popcorn effect” (I’ve yet to find an auger that manages this consistently).

What about the grinder?

  1. Your burrs are not blunt – better yet, cleaned regularly.
  2. Shit, your entire burr carrier chamber is cleaned regularly.
  3. The burrs themselves are aligned correctly (An Ahab action if ever there were one).
  4. Heat has been managed as effectively as possible (I could go on about heat, and how it affects variability, but for now I’ll keep that conversation for another time).

All your shots are extracted from your machine, so what about that?

  1. Your machine is clean. And I mean clean – factory fresh clean. Any part of the machine that has been in contact with the coffee has been cleaned with coffee detergent, then cleaned with fresh, hot water.
  2. Your seals are functional.
  3. Your screens are functional.
  4. Your brew water temperature is consistent.
  5. You’re using VST baskets. (And stop it. Just fucking stop it. VST baskets or piss off. Not IMS baskets. Not La Marzocco Precision baskets. Not the stock standard shit-ass sucky things that get packaged up with your home espresso machine or whatever they give to you with your $25k Slayer. You’re using VST baskets. No one in the industry has put more research and development into baskets than Vince Fidele. They have a proven track record, tried and tested, repeatedly – you’re using VST baskets).

What you have listed here are twenty-two potential points of failure for an even extraction – give me an hour, an espresso, a glass of water, and a captive audience, and I could double or triple that amount. And even with what I have listed, I know damn well half of you reading this are thinking “what about this or what about that?”

You see, if you get none of this right, if none or only some of these variables are lined up, you have no hope of getting a consistent shot. How that shot performs informs how you approach grind changes. This should be self-evident to any barista who’s spent time on the line. If none of these variables are accounted for, or interpreted in the wrong way, you will most certainly be lost in the noise. Tamp not level, shot gushes out, change the grind to fine – next shot chokes. Tamp down too hard, shot slows down, change the grind to coarse – next shot gushes. Your tamp is not fitted, temperature inconsistent, burrs are blunt, not using VST baskets, beans too young, not enough in the hopper – you’ve got no fucking idea – so enter stage left the Great Coffee Superstitions: “It’s a Tuesday, shots always suck on a Tuesday”, “Yeah it’s quite humid today, maybe close a window”, “Do these beans look darker? I think they roasted this batch a little darker”, “This machine is totally broken, did someone fuck with the volumetrics? And all the way to my all time favourite: *shrugs shoulders* “Just one of those days, you know? Coffee huh? it’s a motherfucker”.

All variables funnel out from that shot, built upon one another, after one another. Look at it as an even extraction being on one side of a window, you on the other, and all those variables muddying your view. It’s only when we clear the mud that we can see an even extraction.

You can clear some mud easily enough. You can thoroughly clean your machine – and that should be part of your daily routine. You can clean your burrs regularly, keep a record of how many kilos have been through them, vacuum it out regularly, align them with shims. You can get consistently roasted beans. Keep your hopper filled. You can buy VST baskets, a fitted tamp, have your machine serviced regularly. None of this is too hard, or too much to get done. These are variables you can control, and should be controlling.

Controlling your technique though, making it repeatable? And this after attempting to deal with a particle size distribution of your grinder that you cannot change? 9 bars of pressure, blasting though your puck, with the only thing standing between a 20% extraction and a ruined shot being you, this Rube Goldberg technique and workflow, and the physical limitations of your grinder? That’s hard. But that’s how we do it, because 9 bars of pressure is how we get shit done round here.

But Maxwell discovered 6 bars of pressure enabled sweet extractions, with little to no variability. Matt North moved down to 6 bars and discovered his average extraction went up to 21.28% – “In fact, extractions below 21% are now in the minority rather than the normal occurrence”. Matt Perger, when referencing the difference in pressure from the top of the basket and the bottom of the basket – “I love even extractions. So why not lower pressures?”

Exactly. Why not lower pressures? And if you do lower pressure, what difference is there compared to 9 bars of pressure? I tried answering this in my previous post, but I think in trying to broadly outline my argument, I messed up my delivery. I need to simplify it. So here’s another attempt:

First and foremost, a low pressure means a low flow of water. This enables a number of things, either happening concurrently or consecutively:

  1. Low flow of water allows for a slow infusion of water into the puck. This slow infusion mitigates uneven particle size distribution, as rather than blasting the little bits to the bottom of the puck and blocking up the filter holes, the natural expansion of the beans as they saturate means they expand “in place”.
  2. Any uneven distribution of force when tamping is mitigated, as rather than have these high or low density areas either allowing water through or blocking water, they instead follow the even same expansion upon contact with the water.
  3. Steps 1 and 2 mean that once the puck has fully saturated and the pressure built to whatever you’ve set, this pressure spreads in an omnidirectional manner evenly throughout the entire puck.
  4. The slow flow of water also enables the water to reach temperature equilibrium with the coffee within a timeframe that allows for even extraction for the particle size distribution of your grinder.
  5. Not only does it allow for temperature equilibrium, but also diffusional equilibrium between the water and the coffee within a set timeframe, again allowing for even extraction.

The variability of the particle size distribution of your grinder, or the set mode of that distribution, has been mitigated by low pump pressure rather than exposed by high pressure. The variability of your tamping technique has been mitigated by low pump pressure rather than exposed by high pressure. The variability of your tamp pressure has been mitigated by low pump pressure rather than exposed by high pressure. Is this starting to make sense? What this logically leads to is variability between your shots plummeting, while extraction yields go up.

This has an immediate and exponential effect. With a significant amount of mud cleared from your window, machine and grinder cleanliness, bean quality and management, technique, grind changes – all this sharply comes into focus. You tighten up, with a clearer vision of what needs to be tightened up, all without any background noise. It’s a virtuous cycle, all caused by one thing – less variability in your shots facilitated by low pressure.

Less variability will not come however without some adjustments and experimentation to your technique, workflow, and recipes:

  1. Any calcium build-up in your machine that 9 bar pressurised water will have blasted through, now creates a barrier low pressure water will have a harder path to pass through. Keep an eye on flow rate – you may get an earlier-than-before signal that it’s time to service your machine. Have a look at your filtration system also while you’ve got the technician there.
  2. Distribution is still incredibly important. You need to be palm-tapping at least, and collapsing if your dose is still too high in the basket. The finer you grind the easier this distribution becomes.
  3. If your tamp is not fitted to the basket, the slower flow rate will allow the water more time to channel through the gap caused by the under-sized tamp. This runs kind of counter-intuitively against the idea the grinds saturate then expand “in place”, I can only theorise that perhaps the water channels too quickly through the gap before the puck is saturated enough. Either way, swapping out a 58.4mm and above tamp for a 58mm tamp had dire effects on extraction yields. At this point you really should have at least a 58.4mm tamp.
  4. Your brew water temperature will need to come down to compensate for the extended dwell time caused by the slow flow rate.
  5. This will need to be calibrated to how long you pull the shot for, which in turn will affect your yield. If your grind is kept at the same grind setting as your 9 bar shots, you’ll initially want to go coarser to allow water to actually flow through the puck – the slower flow rate caused by low pressure means the smaller grind size increases resistance.
  6. And this will cycle back to grind size and tamp pressure …

Which leads to two remaining questions. It may be a lark but you can go low, with 6 bars at least allowing entry to the club. But once inside you quickly realise it’s now about how fine you grind and how hard – or how light – you tamp. One allows less dispersion in grind particle size, the other allows that smaller dispersion in the first place. Both lead to even less variability with high extraction yields that are easier to obtain. All other variables can be worked out in a systematic manner, but these final two are rather tricky when thinking of repeatability and consistency. “How fine you grind and how hard or light you tamp” may not make too great of a title, but once you go low you quickly realise that’s a question in need of an answer.

Michael Cameron

(You can join the conversation over on the Barista Hustle Slack, or on the Barista Hustle Facebook page. Alternatively feel free to lodge your dissent or agreement with me in 140 characters or less over on twitter; you can find me here. As always an abandoned puppy finds a new home every time you double – tap over on Instagram).