Pressure & Flow Restriction

It was terrific that the last post was so well received. It would seem that from others’ experiences and some more of my own that a lower pressure/flow has the potential to improve extractions irrespective of the grinder. The suggestion being that lighter roast coffees are harder to extract from evenly as a pressurised pour over (aka espresso) and that a lower flow rate helps with this.

The dialogue on the topic of pressure has mostly suffered and been stifled by the conceptual differentiation of flow rate and pressure. I was motivated by this misconception and wanted to explore the language and theory in this post.

We can start by stating that flow rate applies only to water and pressure refers to only the pump. In an espresso machine, the pressure determines the flow rate of water, but this pressure is not the hydrostatic pressure of water.  The pressure in an espresso machine refers specifically to pump pressure: whatever technology the pump is based on. Flow rate and pressure are not the same thing when dealing with gases, but all we care about when talking about coffee extraction is the rate at which water is pushed through the coffee bed.  Before we can explore the physics, we must go on the journey water takes through the machine.

When we make an espresso, water is forced towards the group using some sort of pump.  This force comes from applying pressure to the water.  But this does not mean that the water is pressurised.  Where did the word ‘pressure’ come from in terms of espresso machines?  Pressure is an important measureable for gases and most espresso machines force water through the pipes using some form of pump.  This is very different to applying ‘pressure’ to water.

Now that the water is moving, it progresses through the group, followed by exiting the shower screen.  It then comes into contact with a resistant barrier (the packed coffee dose).  Instead of permeating through the coffee, water is continually pumped into an open head space above the coffee. Once all of this space has been used up and there is nowhere else to go the water is forced through the coffee.

The coffee and the holes in the basket are effectively the exit point for the water. They’re simply resistors, so the flow rate is defined simply by the rate in which the coffee travels through these resistors.  We use liquid water to make coffee, which makes the discussion of pressure somewhat easier. Unlike something like air that will readily compress significantly under pressure, water is almost completely uncompressible. For example, at 4km beneath the ocean’s surface water only compresses by 1.8% by volume. Hydraulic cranes work by filling metal sleeves with water that will not compress even when lifting very heavy objects.  We know that water expands when temperatures deviate from 4 ºC, but, in terms of an espresso machine, the density of water is only a function of temperature, not pressure, and this means that we can measure the flow rate in L/min.  Any object in the way of the flow of this water will slow its rate.

It gets tricky here, because in an open system (where the water can leave by for instance flowing out of the basket) means we must consider displacement. The water entering the basket displaces the water already occupying the basket. Given that the basket (including coffee) is the resistor; the rate of displacement defines the “pressure” in the basket. Note that this effective pressure is simply measured by flow rate, and hence if the pump pressure decreases the flow rate does too.  Not because the water itself is less pressurised, but because the force pushing the water is less.  When we increase pump pressure we increase the flow rate of water exiting the shower screen and vice versa for a lower pump pressure.

There is another way to lower the flow rate coming out of the group head called a flow restrictor.  A flow restrictor is basically a little pipe that the water passes through during its path to meet the coffee. It effectively funnels water into a narrower pipe, and like traffic on a motorway getting pushed into one lane, the same volume of traffic will now take longer to pass the same distance effectively slowing the traffic rate.

If we consider a machine that has a pump achieving 9 bar pressure pumping water through the group. We then install a narrower flow restrictor in the path. This slows the flow of water down at the point of the flow restrictor, which directly decreases the flow immediately after the flow restrictor.  Our  pump pressure remains high,  but the rate at which the coffee brews goes down.  We have effectively lowered the flow rate using a flow restrictor rather than just lowering the pump. There are mechanical issues to consider but in essence the flow restriction and pump pressure impact flow rate the same way through different modes.

It’s much easier to just adjust the pump rather than exchange a flow restrictor.

To help illustrate this problem, below is an image that show that as the flow restrictor becomes narrower, there is a greater difference between incident flow rate (i.e. incoming pump pressure) and exit flow rate (i.e. the flow of water after all resistors have acted on the water).  Furthermore, you can achieve exactly the same effective flow rate with any flow restrictor, Simply by adjusting the pump pressure to compensate for the flow restriction.

This first image displays how different in going flow rates create different out going flow rates with different flow restrictors


This second image displays how flow rate going in can be altered to achieve the same outgoing flow rate when the flow restrictors are different.


It would appear that discussing pressure and flow restrictors over complicates the discussion. In terms of sharing brewing parameters it makes more sense to describe the amount of water that exits the group in a given time with no resistance. This way we implicitly understand the relationship between pump and flow restriction on a given machine.  It also removes the obvious variable of grind size, which directly determines how strong a resistor the coffee bed is.

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15g baskets

We have moved over to using 15 gram baskets to brew all of our espresso in store.

I have explored in other posts how our recipes and practices have changed over time. How the value of ristretto style shots became questioned , especially if the goal is the fullest character in the cup. Then how the very definition of espresso has become more malleable and ranging. Longer shots, lungos and the like have gone from being seen as travesties to tentatively and logically considered, to then being fully recognised as positive ways to brew coffee.

Of course, brewing is never in isolation. It is linked to the coffee, grinders, water and roasting. Light roasting can work very well but it is also in danger of not fully developing the coffees character, which is reason in the first place to move away from darker roasting. But baked coffees start to pop up and under developed coffees become very common, both in the flavour form of light grassy sourness or as a brothy vegetal toastiness(where the coffees isn’t developed  but has been roaster darker to compensate).

It can sound obvious, but many roasters I speak too are looking to now roast the coffee as fully as possible before they get the taints associated with pushing it too far – ashy, toasty, heavy. This is a seemingly small but significant conceptual move away from roasting it as light as possible before it is too light. After all, its the roasting process that harnesses the coffees potential and produces flavourful by products. Roasting is a tricky subject and I am excited to explore it in depth ourselves shortly. It’s also great to have more openness about roasting theories within the community, much like the world of brewing has seen, especially as it is all intrinsically linked.

A full even extraction is part of this process of understanding. It’s a complex feedback loop. Roast the coffee to be more soluble and you can achieve shorter shots with higher extractions and vice versa, in that less soluble coffee will need more water to achieve a higher extraction. Seriously underdeveloped roasts will struggle to reach high extractions at all, regardless of the amount of water. Solubility is an interesting indicator of roast and it’s a piece of terminology that is becoming more and more used. Roasting coffees to the same solubility is a concept that I have seen more of. It is also curious to consider how the same solubility could be achieved with different flavour development. But like Extraction numbers in brewing, it has the potential to improve the data and the conversation.

There’s a lot I want to taste in a coffee, I am looking for espresso to have intense complexity accompanied with sweetness whilst also being clean. I also want it to have the body and tactile qualities that set espresso apart (I’m a big fan of lungos, but see them a s different drinks).

In store we have moved to brewing all shots on the EK43. Our preferred recipes often ended between 35 and 45 percent brew ratios. The only downside is that the shots were getting long. Simply put, I didn’t like the way they sat in the espresso cup, they were also often too much. We have In our store always presented espresso as a one size beverage  as opposed to doubles & singles.

Over the years we had dropped down from 20g baskets to 18g baskets. We then thought, hell, why not go even lower?

So we got hold of some 15g vst baskets. The plan was to simply scale down. Keep the same types of ratios and extractions but just have less of the same drink in the cup, effectively reaching a desired portion size.

My initial concern was that it wouldn’t be this simple. The diameter of the dose remains the same, but the depth decreases. This presents questions for both flow and temperature.

Is a shallower bed less or more likely to extract evenly? Also, with a higher percentage of the dose now being immediately accessible to the water, as well as less overall temperature decreasing energy from the dry coffee, will the overall temperature of extraction remain higher?

Our biggest challenges with the drop in basket size have all been to do with restriction in the basket and the ability to get consistently high and even extractions. VST acknowledge the fact that as the basket depth decreases and we use less coffee, the amount of resistance the water faces, changes. The pressure in the basket effectively decreases as the amount of coffee decreases. VST aim to counter this by changing the frequency and size of the exit holes. The goal being to achieve the same resistance basket to basket. Ideally, the same grind should result in the same ratio to coffee to water in the same shot time.

For us it didn’t work. This may be to do with the grind distribution or the roasting styles. With the coffee burrs we just couldn’t get the shot slow enough, and the extractions high enough. We then moved to the Turkish burrs. This did allow us to achieve slow enough flow rates. The extractions went up but they weren’t consistent. The crema was erratic. Finer again didn’t help matters. We were hitting a celling on extraction and it would appear that by going so fine we were actually lowering the extraction, with the bed creating pockets of clumped fines that hindered the flow of the water, in essence having a coffee puck full of channels. This was something I had heard of before but had not encountered so obviously.

Next we took the dose up to 16-17g to increase resistance/pressure so that we could grind a bit coarser and hopefully achieve more even extractions. We collapsed heavily to keep head space, evenly distribute the bed and worked a nutating tamp in to the mix. Results improved. We also dropped the temperature from 95 to 93 to compensate for the potential increase of overall brewing temp and this also slowed the flow down again. This was all with the Turkish burrs. I still preferred the results I could achieve with the coffee burrs in bigger baskets.

The real success came by taking the pressure down to 6 bar. I was thrilled to have someone suggest this to me. On the San Remo Opera this gives us a flow rate of 190g of water in 30 seconds (straight out of the group with no handle inserted). If the pressure and flow are relative to restriction then it makes sense to lower pressure as restriction is lowered.  This has allowed for use of the coffee burrs again. The results in the cup are top notch.

The exploration of this basket size, was as you can see not without its problems, but the successes are multiple – espresso beverages of our desired serving volumes, with full, even and flavoursome extractions, that saves us coffee and money. Happy days.

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Remarkability and Game Mechanics

A book about word of mouth, trending, and all round contagiousness was fittingly passed on to me by a customer of ours recently.

I really enjoyed reading this book, and felt myself saying “Yes, that’s exactly what happens” on almost every page I read. Needless to say I highly recommend the book, and this blog is about a few but not anywhere near all of the concepts and ideas explored in the text, which is Authored by Jonah Berger and titled Contagious.

The book looks a lot at social currency and influence. Using psychology and statistical research to build a comprehensive guide as to why some things catch on and why others don’t.

Two concepts that struck a particular cord with me are those referenced in the title of this blog.

Remarkability, commonly defined as “notably or conspicuously unusual; extraordinary” & “worthy of notice or attention”, is in many ways flowing through the veins of speciality coffee.

The pursuit of character and flavour in coffee is all the more remarkable when juxtaposed with its more commoditised counterpart.

This presents an alternate positive perspective in regards to serving coffee, swapping the notion of restriction and lack of meeting expectation for that of increased Remarkability. For example, a speciality coffee that works solely as a black filter and struggles to be personalised and manipulated in the common way could be seen as a failure to meet customer demands, but on the flip side also has the potential to be remarked upon. All of this of course depends on framing and presentation. But questions of ‘why’ begin. “Why doesn’t it do what most other coffee does? Why does it taste floral, you have to taste this coffee it’s so interesting, I never knew coffee was so complex” and so on. The Remarkability can cascade from here. After all, there is so much that is noteworthy.

Of course, there is potential for much Remarkability in more commercial settings, it just changes nature. The sharing or the noteworthy element may be more about the seasonal concoction or the new loyalty scheme.

Speciality coffee on the other hand has this kind of natural Remarkability, and it’s this very aspect that made me want to work with it and start a company based around it. In fact that shareableness of coffee’s Remarkability is at the core of our concept. We often get asked whether we were concerned about doing things differently. Taking a risk etcetera,  we honestly didn’t think about it. We believed in the Remarkability of speciality coffee and in the concept of making that Remarkability more visible. We struggled to see how that couldn’t catch on, how that couldn’t connect with an audience.

It is intriguing that the talking points of coffee that allow it to stand out are often covered up in an attempt to make it more approachable. The danger with this, clearly, is that the excitement, the word of mouth power can get lost.

Of course, unusualness and extraordinariness are relative. A coffee with bright acidity and sweetness is in the larger scheme of things pretty rare, but when immersing oneself in speciality coffee for a while, these can become somewhat normal. The goal posts change. This isn’t a concern for speciality coffee, as the rabbit hole is endless. The drinker just becomes a player in a complex and engaging game, one that benefits from repeat rounds.

This leads nicely into the concept of game mechanics. Game mechanics are everywhere in our lives, sometimes in surprising places. They can be part of the conversations we have and the topics we interact with.

In service businesses we can find these game mechanics all over the place. In fact if you as a member of staff can recognise the rules of the game, the job can move from mundane to rewarding, because you can become a player in a number of games. This is dependent on specific environments and structure. For example there’s nothing more frustrating than learning a set of rules and those patterns changing for no apparent reason within the construct of the game. This could be witnessed in a management decision within the business, but also by a customer whose participation doesn’t stick to the rules of the game they thought they were taking part in. This could be because rules weren’t clearly laid out or understood or because the business doesn’t consistently deliver.

Barrier to entry presents an interesting question regarding both game mechanics and exclusiveness. Flavour notes provide all sorts of game mechanics. But can seem overwhelming and exclusive. This barrier to entry is often discussed in speciality coffee. We want everyone to take part, but can forget that we are committed players.

Take two games as an example. Articulate and magic the gathering. Articulate has a low barrier to entry. The concept is pretty intuitive with lots of potential complexity. Magic the gathering however requires a player to learn quite a lot. The rules and dynamics are more complex and less intuitively understood.

We have all sat down to play a game and thought ” I’ve got no idea whats gong on”. The truth is we are all capable of understanding the rules and taking part. But do we have the time and inclination to initiate ourselves with the game, such as would be the case with Magic the Gathering?  The same question could be asked of speciality coffee.

Speciality coffee is intriguing in that it presents layered levels of entry and education. It’s easy to taste coffees and see what you think, but of course many people may have no interest in entering a flavour dynamic with a barista as host. If the choice is made, the depth of the game can expand quickly beneath your feet. The drinker can begin to learn more tasting language, to draw links between processing and flavour. On the other side of the counter , brewing itself presents a barista game theory. Getting better at brewing, better at tasting, better at the coffee game is something we all strive for, it’s part of the craft. The game here can be more practical, in a commercial environment, speed, cleanliness and service present a constantly malleable and challenging game dynamic. Busy service never gets boring.

Remarkability and game mechanics raise interesting questions about accessibility. Perceived exclusiveness can be a good thing as the topic or game may be more notable and shareable. Ironically, types of exclusiveness could increase accessibility and popularity.

We hit a big audience in store. But its fair to say most visitors still feel like they are privy to something special, a game for connoisseurs. Luckily for us, being a spectator to that world, dipping your feet in, appeals to a lot of people. Many take up the game, others enter the profession, and many watch from the stalls. Importantly though, people want to talk about it.

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The Heat is On – More Grinding Puzzles

The reaction to my last post indicates the sheer amount of interest in grinding concepts. It has also presented me with some completely new and surprising information. Out of all of the concepts and speculation put forward within the article, it was actually a seemingly well-established and widely accepted idea that sparked debate.

Let me summarise:
As we get busy the grinder gets hot and we find that we have to adjust the grinder to a finer setting to achieve a similar extraction to what we initially dialled in, earlier that day. Without going finer, the flow rate of the shot speeds up, with the recipe of dose to shot weight kept the same. Every barista around the world will have worked with this day to day, frequently being irritated by the circles they find themselves running round in.

There are two main theories that are circulated to explain this phenomenon. That is i) the heat of the grinder expanded the metal composition of the grinder and that the burrs move apart or ii) possible change in efficiency of the grinders motor as it beds in for the day.

The world of twitter comes into its own here, allowing for quick, albeit short and sometimes confusing conversational exchanges. Some people questioned, quite aggressively, this notion regarding heat and grind changing. In fact, it was implied to not be true at all: This is very intriguing, because if this is the case, then we want to know why!

It appeared that this knowledge is coming from research carried out by grinding manufacturers; proprietary studies where the results are often not public. But, if the studies are true; that is, the grinder is not changing, nor is the particle sizes coming out of the grinder, the ramifications are massive.

If the grind doesn’t change then some other change is having a huge impact.

The popular counter argument is that hotter grind results in hotter puck temperature. That the grind stays the same and heat energy changes flow dramatically. For instance, yesterday in store we had a medium busy day and we had to alter the grind dial on the EK43 approximately one whole number by midday to achieve the same flow rate that was achieved in the morning. If the grind is not changing, we have then made our grind a hell of a lot finer. This means that the mode of all the particles in the puck is smaller: we will have dramatically changed the surface area within the dose, the contact time will be similar as we have kept the flow the same by going finer. This means our extraction should be considerably higher in the afternoon compared to the morning.

This would then mean that every shop carefully adjusting their grind to achieve a consistent recipe all day long have got it all wrong, myself included. Being proven wrong is a good thing, if it helps us have a better understanding of what we are doing.

It would actually mean that we need to let the flow rate speed up during the day in acknowledgment of the grind size/surface area being the same. It raises some other questions though. Is the increased heat and its potential impact on extraction counter acting the decreased contact time from a speedier flow rate? Or would we need to slow it up, but just not as much as we do.

In all honesty I haven’t consistently tested the extractions at the lowest and peak busy times in store. We have tested it at various times and not noticed a dramatic correlation, but we haven’t steadily tested it with a good data set. I have a coffee post lunch every day though, a coffee I drank in the morning and at the same recipe, it is often a little different but it tastes good. I may not be picking up on something though, which is always a sensory concern. Would the extraction really stay the same if I kept the weights of the dose and shot the same but let the flow rate go as it pleases? One comment on twitter suggested that this new understanding led the individual to actually go coarser in the busy period to counteract the posited increased extraction from a hotter puck….

Wait a minute… what are we talking about? I am now confused. The grinder is not physically changing aperture with temperature, yet I am grinding a lot finer to achieve the same extraction when the grinder is hot. And you are telling me that I the particles coming out of the grinder are not changing in size or distribution as the temperature increases, yet the flow rate suggests otherwise?

Something is fishy here.

The source

The problem with this discourse however was the lack of source material/data. We needed to see the data that the grinding companies have obtained. Grind distribution graphs can look hugely different depending on what axis have been used (i.e. log scales, volume %, total count, etc) and because a lot is going on at once the interpretations can be troublesome. As mentioned in the last post, changing the axis from volume % (where fines are minimal contributors because they are very small) to surface area % (where fines have a huge surface are to volume ratio and thus contribute massively) completely changes what we see as significant. The fines bump which looks so small and uninteresting in the volume % presentation, becomes the most important part of the graph. They contribute the most to surface area and extraction. Not by a little bit. By A LOT. They are 70% of the grind, minimum.

Myself and Chris were kindly sent the graphs which were at the source of the theory that grind distribution does not change with temperature. They are fascinating, and although I want to present that data here, I will not yet, because we want to write this story concisely and scientifically.

However, I will explain our interpretation of this graph whilst referencing the ideas outlined above.

The first amazing thing that they indicate is that yes: The burrs don’t move apart during the day/heating. BUT they also do not show that the grind stays the same.

Part of the grind stays identical, but part of it changes.

The larger particles and therefore the mode stay the very similar. This is expected, because if the aperture of the burrs remains constant, the large particles coming through should also. So what’s changing? Again, it appears to be all about the fines!

The general shape of the particle distribution is pseudo-Gaussian. It looks like an ant hill. However, the hot grind and the cold grind have slightly different shapes. The tails (the fines) of the cold grind are about twice the volume % of the fines tails of the hot grind.

It means the cool dose produces way more fines and therefore more surface area (slower shot) and the hotter dose appears to produce far less fines and therefore less surface area (faster shot) even though the mode is similar (statistically the same). So in essence the original idea about the grind changing is right, just not for the reason we thought. The burrs don’t move, but the coffee breaks up differently.

Why is this happening?

Chris describes it like spaghetti, a hard cold piece of spaghetti shatters into more uneven pieces through being brittle. When heated, the hotter piece of spaghetti (without added water) is more malleable and less likely to splinter into tiny bits. This actually holds true for just about every organic material. This is well founded and published in Physical Review Letters (the premier physics journal);

It looks as though hot ground coffee is less likely to produce fines. Amazing!

This means when we change our grind to finer we are making more fines to make up for the fact the hot burr ground coffee produces less fines. This will of course change the whole grind profile as we have to bring down our mode size to achieve this. That would infer that a grinder running hotter all the time would be more consistent.

It doesn’t mean we can conclusively say heat of the puck isn’t also affecting flow rate and extraction, but we can say that the grind profile does change at different heats. Chris would speculate this affect is not a major contributor to the extraction.

There we have it, when I wrote that blog and posted it yesterday I didn’t conceive nearly all of which is written above. Thrilling really. May the search for answers continue.

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