“Most of the world’s observables are easily described by a simple set of physical laws. But, as we begin to focus on more detail we quickly realise that our simple laws are not refined enough to describe the more complex interactions.
It is these complex ideas that frequently show up in a cup of coffee.”
— Christopher Hendon, 2016
Over on the Barista Hustle Facebook page, there’s been some discussion surrounding flow rate and water debit. I was going to reply there, but a 1900 word comment to a post may have been a little over the top … Instead I’m going to lay some of my own thoughts out here. So settle back in your chair then and get comfortable – I’ve had a lot to think about …
I liked the explanation given on the difference between “water debit” and “flow rate” over on the post in question (have you signed up yet? Go do it!) – water debit being “the measured flow rate of water out of an empty portafilter with the pump on”, flow rate “the measured flow rate of espresso out of the portafilter during a shot”. Pump pressure was left out of the initial explanation but I feel it’s an important contributing factor – you can’t talk about water debit without talking about pump pressure. However they’re easily confused, or misinterpreted.
When it comes to pump pressure I like going with the analogy of a waterfall, with “flow rate”, or “water debit” (which I’ll be calling it) being the amount of water that rushes over the edge of the cliff, within a set time. How hard the water hits the lake or river below is what I’d call pump pressure. That second definition there is important when thinking of how we measure “flow rate” during a running shot. It’s not through measuring how many mills in how many seconds as we do with water debit. Rather we measure that output in grams as the shot yield – a ratio to the dry dose also measured in grams, correlated to the time it took for the shot to reach the yield we wanted.
I think it’s important to see that water debit is only important up to when it hits the puck. At that point, with a fine enough grind, it would cease to flow based on the resistance it’s met. It’s the pump pressure that picks up the baton and starts moving the water through the puck. Restrictors on your machine (literally restricting the flow of water out of the group) are one of the few options available to slow down water debit. Until you do install restrictors it’s best to see water debit as a set variable you need to work with.
It’s worth pointing out that water debit and pump pressure both lie, and operate on, different spectrums: slow to fast water debit, and low to high pump pressure. So ignore pressure profiling and pre-infusion for a moment, and instead focus on restrictors slowing down water debit, and your pump pressure being lowered. So now we have slow water debit and low pump pressure. This is where things get a little hairy because we need to look over to the side at our grinder.
The grinder’s particle distribution is set – a gross simplification but this being the mix of small bits of coffee and big bits of coffee that result when the beans hit the burrs. I know – very scientific. Heat will mess with it, but until grinder technology definitively deals with that issue, we need to look at grinder particle distribution as another set variable we cannot change. Pump pressure, a variable we can change and fix in place, needs to now be looked as accommodating the set variables of particle distribution and water debit. As far as grinders go embrace it’s particle size distribution, and hope that at the very least whatever that distribution is, it produces it consistently. If you know your particle distribution creates a large number of fines (looking at you EK43) or a large number of boulders/intruders (hey Mythos One) then we can start slowly pulling all this together and connect the dots.
So lots of small bits and not so many big bits mean you dial in your shot based on the extraction of the small bits. You don’t worry about the big bits because at best within the time frame it takes to extract the small bits, there hasn’t been enough time to extract anything from the big bits. At worst the even extraction of the small bits drowns out the under extraction of the big bits. With lots of big bits, you dial in your extraction based on the big bits, not worrying about the small bits as the over-extraction of the small bits is drowned out by the even extraction of the big bits. It can be a bit of a head scratcher but once you grasp the concept you realise what we’re trying to do is optimise whatever majority of particle size we have – either small bits or big bits.
Still with me? The particle size also has important implications for distribution in the basket. Now we need to go up a level from particle size distribution, and focus instead on what we all know – grind aperture, or how fine/coarse your grind is. A finer grind is less size disperse – so more small bits, less big bits. Less difference in size between those small bits and big bits has an important implication for water debit and pump pressure, by placing less importance on initial distribution in the basket, and tamp pressure compacting the puck.
So grind fine and distribute with palm tapping and collapsing. This is more to prevent initial weaknesses in the puck the water debit may expose (especially if it’s fast) which the pump pressure will exploit later when it kicks in (especially if it’s high). Next, when it comes to tamping you need to look away from the idea of “maximum density” as something we wish to achieve, and instead focus on “maximum amount of force required to distribute energy evenly throughout the puck”. This is an important distinction. You have big bits and small bits in the puck – tamping hard will not move the big bits past the small bits, no matter what the force is. While with some grinders there are by far more small bits than big bits (thereby mitigating this factor, and hey again EK43) a heavy tamp also runs the risk of squishing the density together in the middle of the puck, and not at the top or the bottom. Variable pressure will simply move these “high density” or “centralised energy” points around to different parts of the puck. Once you have reached full pressure – be that low or high – the great equalising force of pressurised water can only be exerted throughout the puck based on either the initial evenness of tamp pressure energy distribution, or worse the variations in that distribution. We need a different solution.
If you’re still with me, thanks for the company and your perseverance, as this is where I finally get to the point. Low water debit along with low pump pressure is important because it eliminates both particle distribution and tamp pressure as variables, or at least lessens any negative impact they may have. Low water debit cancels out any imperfections in the puck as it’s initially presented to the water, meaning when the low pump pressure kicks in, it will evenly begin saturating the puck. If you can control the tamp pressure and evenly distribute energy throughout the entire puck, then the low pump pressure will also evenly start passing through the puck, enabling an immersion style of brewing. The slow and even saturation of the puck will promote even expansion of the coffee, either mitigating variable tamp pressure, and/or furthering even extraction. The slow contact time promoted by low pressure water enables the entire puck to reach temperature and diffusional equilibrium between the brew water temperature and grinds. Given the nature of modern espresso machines, that temperature remains constant throughout the entire extraction as the column of water passes through the puck. Once the puck is entirely saturated flow will begin speeding up due to conductivity.
So with a fully saturated puck – whereby tamp pressure energy variability has been mitigated or eliminated, and with the puck also reaching temperature and diffusional equilibrium – you can grind finer to compensate for what would have been a faster flow at a coarser grind. A finer grind, and with that less dispersion in particle size, means you can now dial in an optimum time for how much contact the small bits/big bits ratio of coffee should have with the temperature controlled water. You dial in how much time this needs by measuring output of the shot by weight. Shot weight is varied by grind size. Triangulate your fixed dose to this shot time and beverage weight, and dial in your brew temperature to compensate for the increased contact time the coffee has with the water. You’re now making the low pump pressure work with the limitations you have, rather than high pump pressure work against those limitations.
High pump pressure was preferable once as it was reasoned that by grinding fine you create resistance to the initial water debit and the high pump pressure. It was this resistance that would work to extract flavour from the coffee. Along with this pressurised “hard work” it would also blast the small bits to the bottom of the basket clogging the holes up, thereby artificially slowing down water debit, and the ensuing flow rate of espresso. It was this clogging process along with the fine grind resistance of the entire puck that enabled the temperature of the water to have enough contact time with the coffee. But when viewed through the lens of particle size distribution, actual coffee distribution in the basket, and even distribution of force throughout the puck, you can start to see how this theoretical basis works against the physical realities of your equipment and the over-reliance this has on technique being consistent. Again – we need a different solution.
So this is worth repeating – with a slow water debit and low pump pressure you can target the small bits for optimum or maximum extraction, while ignoring the big bits. Alternatively you can target the big bits, and hope the small bits will be drowned out by the big bits. In this way you can ignore the variables you can’t change, tweak some of the variables you can change, and fix in place the remaining variables that you can change, but probably shouldn’t. You achieve all of this by slowing down water debit, lowering pump pressure, grinding fine, and focusing on a tamping technique whose energy produces an even distribution of force throughout the puck.
None of this is really new, it’s part of basic espresso theory we’ve all been taught since the days of David Schomer,”Espresso Coffee: Professional Techniques”, and Illy “Espresso Coffee, The Science of Coffee”. What is new is our understanding of the physics involved, and for that we need a new paradigm of thinking. 9 bars worked when we thought we had to make the grinder fit the espresso machine. I think we need to now look at the espresso machine fitting the grinder, and an integral part of that is water debit – but more importantly, pump pressure. And this begs a parting question: in the immortal words of Chuck D, “how low can you go?”.
(I hate comments under articles – it’s a personal thing – so they’re closed. Instead, make sure you join the conversation on the Barista Hustle Slack, or the Barista Hustle Facebook group. If you feel like perusing my past conversations with the Optus twitter support team before engaging in 140 characters, you can find me under @strivefortone. Alternatively, feed my inner narcissist and double-tap that shit all over the show while admiring my #epichashtaggame on Instagram – an angel gets it’s wings every time I get a like … )