The Resistance of Heat

 “You know there is no such thing as ‘cold’. There is only less heat.” — Alton Brown

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At the Frisky Goat, our day follows a familiar routine. Milk away, mise set, float in the till, music on; all things correct while the Hot 8 Brass Band fills the shop. Then I season the Opera – three groups, three shots; fourth shot on the first group mine to savour. I lean back and sip, nod and smile at our early customers, point politely to our 6.30am opening sign, and at that time open the doors and start the day.

The line starts immediately, either from customers in the door, or text orders through an app. Within 15 minutes our first staff member turns up, and by 7am it’s essential the third is behind the counter. Orders at this point don’t stop until around 11am. There are natural ebbs and flows during the day, mostly dependent on weather (Brisbanites hate the rain) public holidays (our clientele are predominately public servants — and they sure as shit don’t work on a public holiday. And they’ll take the day off before. And the day after) along with the general trends and habits of early morning coffee: breaks, meetings, and those moments required to maintain mental health. For the most part though, from 7.30am to 11am, we’re busy enough to need one person serving, one person pulling shots, one steaming milk, and one pouring that milk and passing out orders. And doing toast. There is always toast to toast and banana bread to butter.

We’re busy. How busy? Our standard dose is 19g in a 20g VST basket, and we serve 38g shots as a “double”, and 21g “ristretto” style shots as a single. There was a long line of decisions made before I took over management of the Frisky Goat, all leading to naked doubles and ristrettos, and no splitting shots. Agree or disagree, that’s our reality — it’s doubles or nothing. We go through 50kg in an average week, 10kg a day over five days Monday to Friday (that’s right: Weekends. Off. I’ve paid my dues; over a decade of babychinos is more than enough). A very busy day would be 12kg, with insanity around 14kg a day. That is not recommended nor something to aspire to though, at least not over four hours. 14kg in four hours is brutal. And we get our fair share of insane days.

A 19g dose works out at around 52 shots per kilo. We serve predominantly 12oz cups (it’s all milk based here in Queensland, Australia) with a smaller amount of 8oz cups, near to no 4oz cups, and an annoyingly large amount of 16oz cups — just under half a litre of milk that requires a 38g shot and a 21g shot just so you can taste the coffee. More than 10kg a day works out at about 526 cups of coffee, and breaking that down to 16oz cups and wastage, we’re easily pulling 500 cups a day. At least 400 of those would be between 7.30am and 11am. That works out to around 114 shots an hour, or just under two shots a minute. And can I stress here, this is on an average day going through around 10kg. Add just two kilos to that, and you’re at almost two and a half shots per minute; add two more kilos on top and you’ll understand why I love the Puqpress so much.

We’re busy. Which is a long way of saying if I were to slap a thermocouple onto my Mythos and start observing temperature trends and correlating them back to shot behaviour, I’ve got a pretty large and consistent sample set to draw from. So that’s what I did, and that’s what this article is about: the resistance of heat, and how it affects your coffee.

The reason I started looking at temperature was a problem I was having with our new Opera: the volumetrics seemed busted. During our peak periods at around 9am, I was having to change the volumetric settings as they were regularly out by 7-10g. This was kinda worrying for me as the Opera was just a few weeks old; and being high volume, I relied on volumetrics to give me a workflow that could handle our volume consistently. It made no sense to me that the volumetrics weren’t working properly — not on a three week old Opera. And over numerous calls and site visits from the skilled San Remo support staff, it became evident the problem lay not with the Opera, but with the Mythos instead.

Just to clarify though, I put a huge amount of work into investigating the mechanics of the Opera, testing and evaluating them, and to the best of my skill set (18 years in this industry … and counting) ensuring the volumetrics were working as intended. I’ve put every inch of those inner pipes, pumps, gears, and flowmeters to the test during the past four months. The Opera is without a doubt, one of the most temperature stable and hydraulically sound espresso machines on the commercial market.

Fluctuations in pump pressure from the rotary pump to the pre-heating boiler are mitigated by a “Content Delivery System” from the pre-heating boiler to the individual group head boilers. This CDS has gear pumps at the centre of its impressive operation, the rotary pumps “pushing” water to them, the gear pumps “sucking” water out and into the flow meters. This is achieved through the rotation of the gears at set RPMs. The RPMs are calibrated and confirmed with a Scace device, the speed of the revolutions relating back to a set pump pressure. This allows for two things: almost every millilitre of water through the flow meters is accounted for, if you want 70ml of water to hit your puck, you’ll get 70ml; and if you want that 70ml to be at 9 bars of pressure or 6 bars of pressure, you’ll get it at 9 bars or 6 bars. Every time. Regardless of whether the pre-heating boilers, steam boiler, or group head boilers are being refilled during your shot, or if the second or third group head is engaged during your shot. With a probe less than one centimetre away from your maximum water level in the individual 500ml group head boilers, and that boiler less than 3cm away from your puck, you have a steady and consistent supply of heated water to that puck. If the chrono display says 90.2°C, then you’re receiving water at 90.2°C onto your puck.

The temperature and hydraulic stability of the Opera is sound, and the volumetrics operate as they should. They deliver a set amount of water to your puck. The individual group boilers deliver that water at the temperature you’ve set it, with minimal (yet always visible to you) fluctuations. The gear pumps deliver that water at the set pump pressure, at a set velocity — with no variation. Which is to say the Opera is almost the perfect machine to highlight any flaws in your system that arise from other causes. While the Mythos wasn’t the flaw — that was the inconsistent beverage weights I was getting — I was sure the behaviour of the Mythos was the cause.

But first I had to slap some thermocouples on the Mythos and hook them up to a temperature display. I used K-type thermocouples attaching one probe directly beneath the grinds exit chute. In this way all grinds coming out of the Mythos would first pass pretty much straight past the probe. The second probe I placed directly over the right side exit fan. I not only wanted to know what temperature the grinds were at, I wanted to know when the fans kicked in (when the display temp started moving up) how high that temperature got, and when it would start to come down again in. Within a day the outline of a trend became clear; by the end of the week I had a good idea of what was going on, and after four months of empirical evidence, observation, and logging I can tell you very confidently the temperature of your grinds is probably far more important than you realise.

It’s easier to first conceptualise how this affects your shots by first assuming the fans have no effect, and you have a constant supply of coffee through the grinder. Both of these assumptions are false, and probably why it’s initially quite difficult to start correlating the behaviour of the Mythos temperature management to your shot quality. But for now let’s put that to the side and start looking at what those probes were showing me.

The Mythos itself has a tiny 9 watt heating element, its only purpose being to ensure your grinds temperature never drops below 35°C if you have a break in between shots. While it’s a little unfair to say it wouldn’t dry a wet sock, it definitely does not heat up your grinds — the friction created by the burrs takes care of that. Leaving the Mythos off at night, then turning it on the next morning but disconnecting the element, it takes around 15 to 20 minutes of continuous shots (just over one dose ground per minute) to hit 35°C. Leaving the Mythos on for half an hour in the morning, with the elements connected, takes that time down to around 10 minutes. Leaving the Mythos on overnight, and with the element connected, within minutes I’m shooting up to 30°C and before five is up, I’m hitting 35°C. During our morning peak the temperature crests 50°C, but hardly ever over 55°C (on one particular busy day it did max out at 60°C — but we were 15 days post-roast, very much degassed, and on a particularly fine grind).

On average though, I think you could draw a line on a graph in your head, starting from 30°C up to 55°C. That’s our starting point for this conceptualisation: a line from a low grinds temperature up to a high grinds temperature, the x axis representing a continuous supply of coffee over time through your grinder up to your peak period, the y axis a generic low to high as it extends upwards.

On this graph you’d need to add a curve, starting from up high, going low, then back up high again. That’s your resistance to pressurised water from your puck, and yes — this resistance is a curve. At the start of your morning, with a 30°C grinds temperature and starting from an arbitrary zero point on your dial, you’ll start grinding finer with a continuous volume of coffee through your grinder, as the temperature of your grinds continues to travel up the line on your graph. You’re grinding finer to increase resistance, as the temperature of the grinds is decreasing resistance. As Matt talks about here though, you inevitably end up at a point where you hit too much resistance and you bottom out on your curve.

Why does the curve on my graph then start going up? Because when met with resistance we’re all taught to grind coarse, so that’s what you do. To keep shot time static, 28 seconds, 30 seconds, 40 seconds, whatever your target — if the shot chokes you grind coarse to keep on target. So your curve starts heading back up again as you approach your peak grinds temperature, as you’re now decreasing resistance to combat the increase initially caused by the temperature of your grinds. Though now at this peak you’re at a coarser grind setting than at the start of the morning. You could probably add another curve to your graph, inverse to the resistance curve, and label it “grind setting”, going up in fineness then down in coarseness.

This grinds temperature line on your graph won’t just affect your resistance. This increase in heat will also affect your grind profile. Figure 4, panel B, of Christopher Hendon et al’s paper on grinding physics shows this effect quite clearly. As beans get hotter, the way they shatter produces less fines, but the fines that are produced are bigger. Decreased fines, increased size. So now we can add two more lines to our graph. One line representing fines volume, an almost mirror image of the grinds temperature line going from a high volume down to a low volume. The other line represents the size of the fines, parallel to the grinds temperature line, starting from a small size up to a large.

Again, ignoring the fans for now and assuming a constant supply of coffee through the machine, this decrease in fines and increase in size causes two things. First, this is the reason you grind fine as the grinds temperature increases. You’re losing fines, and those fines are aiding in resistance. But because they’re also increasing in size you’re left with the peculiar effect of the actual grind change itself having no added effect on resistance; you’re just lifting the mean fines volume and proportional size up — essentially “jogging on the spot”. You could be aggressive and change the grind finer, but you then jump to the middle of your resistance curve and over-shoot the mark, choking the shot. However, even if you weren’t to change the grind setting and just leave the heat of the burrs to change the grind profile, after a certain volume of coffee you’ll hit a grinds temperature of around 40°C, and your resistance will increase. Whether this behaviour in the puck is because of increased diffusion and/or dissolution, fines migration, percolation pathways, or other reasons, for now the literature is unclear (that’s being worked on though!). Either way, it’s inevitable your resistance curve will bottom out and you’ll start grinding coarser.

Not only will this change in grind profile affect your shots, it’ll also affect your grinder dose time. So if at the start of your day you set a 20g dose at 7.50 seconds on the Mythos display, it will only give you 20g as long as the grinds temperature stays where it was when you set that timer to 7.50 seconds. I think “dose debit” sounds just as cool as water debit, and that’s what you’re dealing with here. As the grinds get hotter and your fines decrease in volume yet increase in size, the time it takes to exit the chute also increases. Your dose debit goes up. Whether it’s because the fines are larger so they travel slower, or because there’s just less of them therefore less exiting within the time set — either way you will not get a consistent dose as the grinds temperature increases. So add another line to the graph, parallel again with grinds temperature and fines size and heading in the same direction, calling this one “grinder dose time”. It goes up as the grinds temperature goes up. Having two fuck-off big arrow buttons to adjust your grinder dose time, right at eye level on the front of the user interface of the Mythos, was perhaps a design decision made in light of this fact. Serendipitous if not.

At this point I think it would be beneficial to look at our graph and add some constants in your head. I know it’s already pretty crowded, but with the addition of a few variables it may become clear we need to stop ignoring temperature when it comes to our current view of making espresso.

In specialty coffee in 2016 there’s probably one of two ways you’re making coffee — either using volumetrics or weighing your shots. One is predicated on the idea that the volume of water used for every shot going into the puck is the same, the other instead has the volume of espresso exiting the puck constant. Both “triangulate” the volume of water in or espresso out by keeping dose consistent while manipulating the grinder setting to keep shot time static.

Let’s look at volumetrics first. The goal is to maintain a constant beverage weight, and we use volumetrics as a proxy for that weight. We don’t know our beverage weight for every shot, we’ve outsourced that job to volumetrics. But the way volumetrics work is by sending a set amount of water to your puck — and that’s it. Your pump pressure manages the velocity of the water, and your tamp force and distribution technique, along with your grind setting and grind profile, will dictate the flow rate through the puck. Whatever comes out the other side will largely depend on these factors. But half that equation — your grind profile, your grind setting, and your flow rate through the puck — is effected by your grinds temperature. To the hard left of your graph was when you probably set the volumetrics, at a zero point for your resistance, a zero point for your grinder setting, and at a low grinds temperature. There is absolutely no way you’ll maintain a consistent beverage weight, with a constant supply of coffee through your grinder, given these conditions.

With a consistent volume of coffee through your grinder, your grinds temperature increases, leading initially to less resistance in your puck; so you grind finer to increase resistance to keep your shot time static. And that right there is your fatal flaw. As you increase resistance with a finer grind you decrease the amount of water that can get through the puck. So if you have a theoretical 80ml of water, volumetrically set for each shot with a 20g dose, under perfect conditions you’ll get around half of that in your cup as espresso — let’s say 40g. With increased resistance you’ll maybe only get 35g in the cup, at worst perhaps 30g,10g under your target. If you’re checking beverage weight while using volumetrics (and let’s be honest here — you only do that with those 30g shots because 10g under you can notice visually; 5g not so much) you’ll blame the short shot on the flow meters, the weather, the grinder, or you tamped too hard. Or you just ignore it, whisper yolo, move on thinking it was just a one-off.

But keep looking at that resistance curve. At around a 40-45°C grinds temperature it starts to bottom out; you’ve gone too far with your fine grind changes as you “jog on the spot”. Now your shot times are starting to blow out as you choke up the machine. So you grind coarse to get that resistance curve smiling again. Only now, with less resistance, you’re letting more water from your volumetrically set 80ml through your puck. Not only that but because of the increase in grinds temperature, regardless of the macro change in the grind setting to coarse, your grind profile – with less fines that are bigger in size – has become a micro coarse setting. If you keep your shot time static, at your peak period of the day, you’ve opened the flood gates to the 80ml of water to gush right through your puck. You won’t get the whole 80ml, but your target of 40g now becomes 45g or 50g. And just like you’ll notice 30g, you’ll also notice 50g. But instead of looking at how heat from the grinds might be affecting your resistance you’ll go back to blaming the machine, the flow meters, your technique, static build up; anything else but the real reason. Your volumetrics are working just as they should – your resistance has changed due to the change in your grinds temperature.

Weighing your shots — using gravimetrics or simple scales — has the opposite effect. You keep shot time static, and if you’re lucky your machine shuts the shot off for you when it hits your target beverage weight, or you do it manually yourself. No problem here, you always hit your target beverage weight, and you adjust the grind setting to keep within your target shot time. Only now you have a large difference in the volume of water that passes through your puck, based on where you are along the x axis of your graph.

Imagine for a moment if you were to take three 20g samples of coffee, all ground at the same temperature and brew each with 50g of water, 100g of water, and 150g of water. In this example you could easily manipulate the grind setting to achieve the same beverage mass for each sample. You would go from coarse with the 50g sample to fine for the 150g sample, and this is exactly what you do as you move along the x axis of your graph. If you’re triangulating your beverage weight with shot time, as your resistance decreases you’ll grind finer, necessitating more water through the puck to hit your target, and less water as resistance decreases. If you were to test those three samples the extraction yield would be different for all three, and depending on what side of Matt’s extraction hump you’re on, you’ll be going from worse to better, better to worse, or skip the middle altogether and just go from bad to terrible.

It’s possible that the finer grind at the start of your graph will mean more surface area and tastier shots, but this will diminish as the grind profile changes with increases in your grinds temperature. It’s also possible that the lower surface area from this profile at high heat could be mitigated from the heat itself — better extraction due to higher diffusion and temperature equilibrium. But the dilemma remains — good to bad, or bad to good, or possibly even good to good — with a shitty hump of badness in the middle. Whichever way you look at it you may be getting a consistent beverage weight but you won’t be getting a consistent flavour.

On the face of it, volumetrics seem like the worse option to use — as with weighing your shots you’re at least always going to hit your target beverage weight, only with the hump to worry about. You’re unlikely to worry much about that shitty middle hump though, as by mid-morning you’ll be knee-deep in dealing with the fans. So let’s take away our earlier assumption that the fans won’t have an effect, and deal with the reality that unfortunately, those fans are going to mess up our entire graph.

The fans on the Mythos do one of five things:

  1. With a grinds temperature below 35°C, they do nothing.
  2. At 35°C the Mythos temperature management kicks in and they turn on. While they’re definitely on, most the time at this point you won’t even notice them — they’re just gently keeping the grinds temperature at 35°C.
  3. By 40°C the fans are at full force, attempting to actively constrain heat by pushing the airflow out of the grinding chamber, resulting in the grinds cooling. If you’re lucky your volume of coffee through the grinder is high enough that the temperature will continue to climb, but the fans won’t cool, and you’ll hit a cozy equilibrium with a constant grinds temperature. This almost never happens. You’re just jogging on the spot riding up to peak temperature madness.
  4. If you’re not lucky and it’s actually a normal service, then you have breaks in between shots due to the natural ebb and flow in your shop. The fans are now actively cooling, attempting to bring the temperature of the grinds back down to 35°C. That is until your volume of coffee picks up and you head back to step 3, and ever back and forth again. You’re stuck in a one step forward, two steps back scenario, changing the grind coarse and fine, cursing the fans as your fines production continually changes in volume and size. Now you’re on board the Mythos Train heading up to Peak Temp Territory. Choo choo.
  5. Final step, you’re over 50°C and the fans are fucked. Even with a small break in service, there’s enough residual heat in the grinds chamber that it takes only one or two shots to bring it back over 50°C again. The fans ain’t doing jack shit about temperature, as you’ve hit the maximum amount of shots you can perform per minute combined with the maximum heat output from the burrs and electronics. You’ve reached your final destination: Peak Heat.


Your graph now becomes vastly different. While the trend lines will continue on their upward or downwards trajectory, it’ll be jagged the entire way. Between 35°C and 50°C, using traditional espresso parameters with volumetrics or weighing shots, there will be no consistency beyond consistent frustration.

This is being unfair to the Mythos though. That consistency is the frustration — you see the same pattern repeat itself day after day. And that’s because the Mythos does what it says on the box: constrains heat between 35°C and 55°C. At high volume that’s testament to the design, construction, and implementation details of the Mythos that this swing in grinds temperature is only 20°C. It can be, and is, much worse on other grinders. This consistency though, matched with an understanding of how heat affects resistance, allows you to try something a little different and this behaviour into a workflow that, well, works.

If you look at your graph again and take a thin slice from the far left, you have a snapshot in time of when you dialled in your coffee to taste the best. This snapshot included a volume of water to your puck, a set temperature of the grinds in your puck, and therefore a set grind profile at that grind setting. This created a set resistance that dictated the flow rate of the water passing through the puck, resulting in a set beverage weight in your cup, all in a set amount of time. That snapshot is the key to using the Mythos, or any grinder for that matter (they all get hot) during high volume. But to do this you need to let shot time take care of itself, and embrace both volumetrics and scales.

Rather than run more or less water through your puck to hit your desired beverage weight, rely on your volumetrics to work as designed — they will deliver a set volume of water to your puck. (Granted there’s around a 3% failure rate of flow meters due to electronic or mechanical failure, and of the 97% of water delivered as designed, for the Opera there is another 3% that will give an incorrect volume based on water hanging from the shower screen or trapped in the wastage line. Of the 97% of water that makes it through your flowmeters, on an Opera at least, 97% of that water makes it to your puck and is counted. So trust your volumetrics).

If you keep your volume of water delivered to your puck constant, you still have that variance in resistance to deal with. But the only way you can see that variance is by the checking your beverage weight. In the morning if this was set at a 1:2 ratio of 20g/40g when the grinds temperature was at 30°C, but your beverage weight now is 30g with a grinds temperature of 40°C — you’ve got variance caused by that 10°C shift up in grinds temperature. Your beverage weight is showing your variance.

You got there by either grinding too fine, or from the grind profile changing, the fans kicking, or a combination of these factors. Either way, your resistance at this point has increased, making it harder for your set volume of water to get through the puck. To solve this, you need to lessen the resistance by changing your grind coarse, and allowing a little more water through the puck. Conversely, if you find you’re now over your target beverage weight, then you need to increase the resistance and grind finer.

To simplify, in the morning set your volumetrics with a scale after dialing in and finding a tasty recipe (individually, left to right — it’s unlikely that the flow is consistent through each). From then on, change your grind setting based on checking the beverage weight every third or so shot (I do it every chance I have). If it’s over your target weight — grind finer to increase resistance. If it’s under your target weight — grind coarser to decrease your resistance. Whatever your starting grinder dose time was, increase it as your grinds temperature goes up, to keep your dose into your basket or cup relatively consistent. As you start winding down from your peak heat, you’ll find yourself adjusting your grind setting again as the resistance curve reverses, and you’ll lower your grinder dose time with it.

This all of course flies in the face of conventional wisdom, as I’m ignoring shot time here. A curious thing happens though when you follow the curves and lines of the graph, and arrive at the far right. At this point, you’re at your highest grinds temperature, therefore at your lowest volume yet largest fines size, and you’re at your highest grinder dose time. If you were to keep your grinder setting and volume of water constant the whole ride up from 35°C to 55°C, then at your highest grinds temperature your starting shot time of say 28 seconds will now be hovering around the 40 second mark.

Put another way, ignoring the Mythos Train on the ride up, with volume of water and beverage weight held constant, you’ll be at the same grind setting at 35°C as you are at 55°C, but with a 12 second difference in shot time. In this way it would seem you can separate shot time from your grind setting. There is a relationship between the two, but the mechanism for change is directly tied to grinds temperature, and the effect that has on resistance.

The reality though is on that ride you would be manipulating your grind setting — to either keep shot time and volume of water static while ignoring beverage weight, or by keeping shot time and beverage weight static while ignoring your volume of water. One is volumetrics, one is weighing your shots. One will blow your beverage weight out by around 10g, the other will blow your volume of water out by around 10g — both will be at a far coarser setting than you were when starting the morning. That’s if you don’t ignore shot time and instead keep it static. I say ignore it.

And that’s ok! Matt always placed shot time at the bottom of his “Hierarchy of Espresso Needs” (sorry Matt, I study psychology so of course I think that’s a cool name …) and so do I. You’ve dialled in your coffee to taste good at the start of your morning, and probably hit around 28 – 30 seconds if your grinds temperature is around 35°C. By the time you’ve hit your peak grinds temperature you’ve lost those smaller fines at the start of the morning that provided all that tasty flavour. To allow the water access inside the bigger fines so you can keep hitting tasty, you’re going to need more time. So if you ignore time and concentrate on triangulating beverage weight with volume of water and dose, you’re gonna get that time regardless. To keep your shot time static you’re compounding an already micro coarse grind profile by going with a macro coarse grind setting. Again, ignore shot time — it’ll take care of itself.

The start of the morning is problematic though if you don’t leave your grinder on overnight, or at the very least allow it enough time for the element to heat up. If you don’t, your difference in temperature from early morning to peak heat will be larger than the already pain in the ass 20°C shift. Your goal should be to get that difference lower, not higher. So leave your grinder on overnight.

Spent coffee grinds in the crevices of the burr chamber are an effective insulator as well — they suck up a lot of heat created by the burr friction. Christian Klatt talks a lot about this here. So keep your grinder clean and aligned. At The Frisky Goat, going through at least 50kg a week, I clean the burrs out once a week — soap, a toothbrush, isopropyl, then aligned properly — but it’s becoming increasingly noticeable by the end of the week that it needs a clean. It’ll be twice a week soon. Someone wrote a small novel here about that. There’s some nice pictures here. Clean and aligned you’ll get through the ride up to peak heat faster.

A cleaner Mythos and therefore cleaner burrs also means you’ll have less clumps. Clumps in your grinds block the water path through your puck, leading to inconsistent beverage weights … That you blame on your grinder setting, the weather, the beans themselves — when it’s just your dirty janky burrs instead. Clean them. Often. And when you keep getting clumps after you’ve cleaned and aligned your Mythos, you need to change your burrs or replace your clump crusher, or both. The clump crusher should really be self-explanatory — and its role is absolutely crucial. Don’t neglect changing out that little piece of plastic, nor underestimate its importance. It crushes clumps — look after it.

Just like placing your finger over a hose will result in a higher pressurised flow firing out from the smaller exit, clumps under high pressure lead to channeling — and more inconsistent beverage weights. Lower your pump pressure. Someone wrote another small novel here and here and here about that, with some more words on the subject here, along with a brief how-to here. There is a whole world of deliciousness beyond less channeling when you go low.

With no clumps and low pressure, even extractions are still predicated on the idea that your coffee distribution in the basket is even too. Palm tap. Please. And while I don’t agree with collapsing the bed before tamping (I think it leads to inconsistent density — let your tamping take care of that) you could always consider lowering your dose and adjusting your beverage weight if you’re struggling to get your entire dose into the basket. VST baskets have a +/- 1g rating for each basket — we dose 19g in a 20g VST basket. What about IMS baskets, LM stock baskets, or the like? Don’t know, don’t fucking care. Use VST baskets.

You’ll still get channeling, inconsistent density in the basket, and a non-level distribution if you have the wrong size tamp and use inconsistent force when tamping. Which will lead to inconsistent beverage weights. I use an Eazytamp at home with a 10kg tamp force spring, while at work I use a Puqpress, also set to 10kg. At high pressure the wrong size tamp will blow out water down the sides of your basket. Use a Pergtamp, a 58.4mm tamp at least; anything but an undersized tamp base. There is no point in changing your grind setting to achieve a consistent beverage weight if you can’t produce a consistent tamp force or provide an effective tamping surface.

With all this in mind if you were to consult the graph, write out all these steps, and evaluate it all, you’d realise there’s actually a metric shit-tonne of variables to account for. It’s daunting, and therefore quite easy to stick to old dogmatic views of how to pull espresso shots. 9 bars or bust, 30ml in 30 seconds, tamp as hard as you can, adjust fine or coarse to speed up or slow down your shot time. There is a seductive simplicity in that view.

But it wholeheartedly ignores empirical evidence. It wasn’t an expensive temperature logger and thermocouples that I bought, but it showed me a zero point, and the movement up and down from that point. It was nice to see when it showed 35°C on the display that I could also see the fans turning on, because that’s what Colin said here. I could see the zero point on my grinder dial, and tell that at the busiest time of my day I was at a coarser setting than from the start of the morning. And I could weigh that shot and see my beverage weight had blown out. I have a display on the Opera that shows me the pulses of the flowmeters represented as millilitres, and I could see those millilitres go up and down with the change in grinds temperature if I kept my beverage weight constant. On any given day I can see the resistance curve on the graph going down then up, I can see the temperature changing and my shot times extending, while my beverage weight and volume of water stay static. If I don’t change the grind setting I can see my shot times still blowing out, giving me a view of the grind profile itself, while my grinder dose time grows larger as the grinds temperature rises.

I can see all of this, and the thing is — so can you. Any frustration aimed at the Mythos is because of its consistency, which really isn’t fair. The Mythos does what every other grinder does — it heats up the grinds from the friction caused by the burrs. But when used right, it consistently constrains heat between 35°C and 55°C. If any other grinder could do that, you would’ve heard about it by now. But that ride up to 55°C is painful, and what you’ve read is the best way I’ve found to deal with it. This isn’t theoretical for us — this is our day-to-day practice. But don’t take my word for it. Just observe. With one day of well-designed solid research, you’ll see more than you probably have throughout your entire career. And if you keep your eyes open you’ll observe the same behaviour I have, and that’s the resistance of heat, and how it affects your coffee.

Michael Cameron

(Find me on twitter here for all your sub-tweets and critiques, or join the lively conversation on boundary pushing here on the Barista Hustle Facebook page. Alternatively you could just watch all the daily shenanigans at The Frisky Goat here, along with pics of my freezer full of coffee goodies. Occasionally there’ll be a picture of my wife, or our cat, Daisy. But it’s mostly coffee.)