15 vs 18

Let me take you on a journey. The journey of sweet espresso, reliable grinders and cost efficiency. With one piece of equipment:
A filter basket.

The post covers 6 months of experimenting with different doses in VST baskets. My hypothesis is that 15g dose tastes better than 18g dose on the same ratio, whatever that ratio would be.

This is not even touching the scientific method. All my thoughts and conclusions are questionable and I'm open for debate.

I did my testing on Victoria Arduino Mythos grinder. To understand the relationship grinder-basket I did my best to explain how this grinder works. I hope some of you will at least take that away regardless of your basket choices.

The article covers:
1. Theory and its practical application for VST baskets

2. Mythos - the way it works, when it overheats, how to prevent it

3. Puck saturation rate and what it means for extraction

4. Thermal equilibrium as key to understanding physics behind saturation temperature-wise

5. Experience of using bigger baskets and bigger doses (a bit of data)

6. Why would I increase the dose?

Theory of VST
VST has put tons of research into their coffee baskets. It feels as if someone for the first time thought about shape, holes' diameter and placement and extensive QC of every single product to follow.

I have been part of a research group working with one of the espresso machine manufacturers. As a part of the research, we've been testing different baskets. VST baskets had the best results. All the testing followed the scientific method. I'm not allowed to share any more of that, but I wanted to show that some people have proven VST baskets superior.

In theory, the variance in hole diameter should compensate for different doses. Total Open Area changes between sizes. 15g dose in 15g VST basket should have pretty much similar grind size to 18g dose in 18g VST basket.

Is that true? I answer that later in the text.

At the moment my favourite espresso grinder. The best espressos I ever had were made using Mythos. Also, Mythos is easier to operate in 12kg a day environment (where I did most of my experimentation).

The grinder was built to create the most consistent grind size and particle size distribution during the day. The idea behind it is to keep the grinding chamber in the 35-55 degrees range. It should never be cooler than 35 degrees. It should never be higher than 55. The grinder has a complex heat management system. It consists of:

Heating element. It's placed in the heat sink. Warms up the grinding chamber. I guess it should turn itself off upon reaching 45 degrees, but I'm not sure about it. Some people like using Mythos with the element unplugged. It does not warm up coffee though, so the beans don't get much hotter before grinding. It just helps the grinder reach and maintains working temperature - you dial in on a warmed up grinder.

Some people "determine" temperature of the grinder by touching the heat sink. And when it seems too hot to touch they say: "it overheats". Not exactly.

The way to measure overheating is by measuring the temperature of the grounds. When those exceed 50 degrees, the grinder has no capacity to cool them down if you keep making coffee. The grinder overheats when grounds are hotter than 55 degrees. Period.

A thermal equilibrium will be too high when grounds reach 55 and above (I explain the term later). The result is as if you used extremely hot water to brew an espresso (like 96 and more). Many bitter compounds in a roasted bean need a high temperature to extract easily. And with temperature equilibrium being too high, you extract too much of them.

The excessive heat is meant to be taken away by fans. There are 2. One, on the back of the grinder, sucks air in - therefore the temperature of the environment is important. The second, on the right side, blows the hot air out. They turn on when grounds temperature goes over 35 degrees. A few doses ground is enough unless the environment is really cold.

Over 40, fans turn on second gear and the air is blown away with significant force. After an hour of continuous service, grounds are over 50 degrees and the blown air over 40.

At that moment the heat sink has huge thermal energy and fans are barely able to keep grounds under 55 degrees. In most cases they are able do it. But at that point, the heat sink has accumulated enough thermal energy to keep the grounds above 50, even if you stop making coffee for a while. It takes around an hour of not grinding for fans to cool grinder to 35.

The fans are not big and powerful enough to drag the thermal energy out of the grinder (that is being improved in the next generation).

There have been very few occasions when I experienced overheating Mythos:

1. Broken fan

2. Unreasonably fine grind size - which causes a decrease in extraction

3. Particularly tricky coffee - light roast Pacamara for example, resulting in grinding time of 20 sec at 15g dose just after 1kg of coffee used

4. Lack of proper cleaning - which I explain in the next paragraph.

Victoria Arduino recommends cleaning behind the bottom plate (the part with moving burr) weekly. The factory-new grinder can get clogged with as much as 15g of coffee behind the bottom plate (also called backplate) after using only 1kg of coffee. I think the grinder should be cleaned every 8kg with a deep clean at least once a week.

I deep clean Mythos grinders for over a year - without any negative consequences. Only in that way you can ensure the grinding chamber is 100% clean. The part is designed to be taken off regularly, both the screw and thread bear unscrewing better than 4 front screws for heat sink (the part with static burr).

The tricky part about the deep clean is a reverse thread on auger (which is a nut for the bottom plate), but the shape of the bottom plate helps with determining that. Also, you have to be careful with putting the plate back on the shaft. Its shape is almost fool-proof, but I've seen it assembled wrong - the result was a deformed bottom-plate made from soft aluminium. The fix required 2 people, screwdriver, hammer and a new bottom-plate (£25). Along with working grinder, you get complimentary dents on the grinding chamber.

Some heavily used and badly maintained Mythos grinders have trouble spinning the spindle. I suspect coffee dust being pushed into the motor and its bearings. Additional resistance on the motor makes the grinder slower and potentially heating up even more. The motor is one integral part and it's quite expensive. Also, it requires a qualified technician to install (not sure if you'd even get it if you're not one). That could've been avoided with proper cleaning.

Now that we know how Mythos works, let's dive into puck saturation rate. My favourite topic in the last couple of months.

Puck saturation rate - how quickly the entire puck gets saturated with brewing water.

The faster puck saturation rate the more even and higher extraction (given reasonable dose/grind size/temperature/pressure etc). If grounds across the entire bed have the same contact time with water, in theory, they should extract more similarly. As opposed to the top of the puck being in contact with water much longer than the bottom.

But what's crucial when we talk about saturation rate is temperature. The temperature of slurry (brewing water and coffee) is not uniform across the puck. The temperature is highest at the top of the basket and lowest at the bottom of the basket. We can talk about temperature equilibrium in espresso - but that's an average, not an absolute value.

Now, coming back to the grinder for a second. Grinding increases the temperature of the grounds. Mythos keeps those grounds in 35-55 range - which in itself creates more consistency. Warming up the grounds makes the saturation a bit faster - good! So why not bring up the grounds as hot as possible?

Gwilym Davies said that when developing the Mythos they experimented with different temperatures. And grounds above 60 degrees never made good espresso. Also warming up the coffee changes particle size distribution, not in a way, we would like to.

The factor that affects saturation rate most is the depth of the coffee bed.

Lower coffee bed means better saturation.

When water at 93 degrees gets in contact with grounds at 35 it cools - thermodynamics. The further down the puck, the more it cools, even down to 40 degrees. Colder water has less energy. Less energy = less extraction. Of course, that water is displaced with fresh hot water. But even that new water won't reach past equilibrium point - brewing lasts only so long.

Temperature equilibrium
I've touched on this subject earlier, discussing the saturation and temperature of the grounds. But reaching full saturation is far from reaching thermal equilibrium (TE). Understanding TE is key to understanding saturation. Although it's only about temperature.

Thermal equilibrium is when 2 things at different temperatures mix with each other. In our case, ground coffee and brewing water. To calculate it you need weight, temperature and heat capacity of both materials.

Heat capacity sometimes referred to as specific heat, is how much energy is needed to change the temperature of an object. Specific heat of water is 4.18 joules per gram per Kelvin. Specific heat of coffee is assumed to be around 1.4 j/g/K (like dry wood, following Matt Perger in that matter).

The specific heat of water is higher than that of coffee. What does that mean?

It's much easier to heat coffee than water. It makes sense when you think about how much heat grounds absorb from grinding. During extraction, brewing water is the same temperature. But grounds temperature will vary during the day (within 20 degrees if you're using mythos).

In theory, regardless of your dose (keeping the ratio the same), thermal equilibrium is the same. That thermal equilibrium is only 3-4 degrees less than the temperature of brewing water. However, this temperature never is reached. Let alone the loss of heat, time of extraction is too short. So what we can think of is how quickly the puck gets acceptably close to the desired temperature.

The lower the mass in the basket, the faster puck reaches the equilibrium. And you can derive that using logic or transforming the formula.

What happens during extraction: top of the dose is brewed with very hot water. The bottom is extracted with much lower temperature water. The difference between those 2 measures evenness of extraction - bigger difference means less even extraction. With higher dose, not only it takes longer to reach thermal equilibrium, but the longer time for those values to get closer to each other.

The bigger the difference between the temperature at the bottom and the top of the basket at the end of extraction, the less even extraction. The less even extraction, the lower extraction.

The higher your dose, the higher depth of the coffee bed, the less even saturation, the less even extraction, the lower extraction.

Therefore: higher dose implicates lower extraction.

53mm baskets
When I competed in UKBC I used a La Spaziale machine with 53mm baskets. Thus the same dose as in 58mm basket means a higher depth of the puck. Espressos from La Spaziale are more on the bright, fruity side, less complex, less sweet, less coating, easier to get lost in milk.

Regarding that last thing, my milk beverage was 5oz drink with 20g of espresso. It was cappuccino so the ratio of coffee to milk was even stronger as it was more aerated. Practising on 58mm baskets I achieved synergy between espresso and milk. But, using 53mm I got feedback of espresso getting lost in the milk.

Would it be reasonable to create even wider baskets? Nope. As we know it's not easy to create even dispersion of water from the grouphead in the pressurised system. Also, the headspace volume would be altered.

It doesn't matter how much you extract but WHAT you extract.

Let's consider 2 shots on the same ratio but with a different dose. Using a refractometer and 2 calibrated people to evaluate.

17g dose/ 40g yield

TDS: 8,64 (pretty much standard, good for the ratio)

Extraction %: 21,18 (quite high)

Scoring according to WBC:

Acidity: medium to high, unpleasant

Sweetness: low to medium, caramel-like

Bitterness: medium to high, without any flavour attached

Balance score: 3

Aftertaste: 3,5

The body high, but tactile not pleasant, harsh, lacking smoothness

Flavour clarity: flat flavours, lime, chocolate, caramel, no depth or complexity

15g dose/ 35,5g yield

TDS: 8,07 (lower, but still very good)

Extraction %: 19,89 (low, yet better than most London cafes though)

Acidity: medium (pleasant, lime-like)

Sweetness: medium to high

Bitterness: low to medium

Balance score: 5 (that looks optimistic but definitely better than the previous one)

Aftertaste: 4,5 (long-lasting, dragging on lime zest and bakers chocolate)

Body lower on weight, but very smooth, silky, rounded.

Flavour clarity: high, +5 flavours discernible, deep, complex

That example shows that WHAT you extract is much more important than HOW MUCH. Sweetness, flavours and complexity are parts of great espresso. Not a 21+ extraction percentage.

Let me share with you a little table. Different espressos, a similar number of extraction, very different sensory results. Conclusions to be drawn on your own.

That doesn't prove anything and is nowhere close to the scientific method, but gives a bit of context for why higher doses are less sweet.

Sweetness is perceived as weight and smooth texture before our taste buds can determine what taste is it. Therefore it positively affects tactile of the espresso. You can test it yourself tasting mild solutions of sugar.

Would you swap?
What if you consider swapping to bigger baskets for a more drastic change in dose?

I actually did such an experiment.

In the busiest coffee shop I've ever worked in, doing on average over 1200 drinks a day. The workload is split between 2 machines and 5 baristas. One of the machines had 15g VST baskets, the other 18g VST baskets.

We did it for a week. Only. Let me tell you why.

Baristas ended up dialling in on more closed ratios. It seems as if they are used to certain yields and won't let go. I had a similar observation when changing to smaller baskets in the past. Because the ratios were more closed, the intensity of the espressos sky-rocketed. So baristas made grind size unreasonably course to make up for that. And those were some of the most underextracted shots I've tasted in that shop.

That gives a hint that you may want to increase your dose when using darker roast - but I believe it's not your way of making coffee, is it?

15g dose/35g yield is a 2,3 ratio - that's where the blend we used for testing tastes best. On bigger baskets baristas tended to go for 18/36 which is 2 instead of 18/42 (2,3). Any time you increase the dose, you may want to think about the ratio of espresso to milk in milky coffees.

Grinding higher doses gives more strain on the grinder. Total Open Area in VST basket compensates for different depth of the coffee bed. Thus grind size remains pretty much the same on dialling in between baskets. But given the same grind size, grinding higher dose takes longer.

Experimenting on how Mythos heats up, I derived that grinding time has a much bigger impact than grind size. That's why the prototype with high-speed motor doesn't heat as quickly as low-speed. Reaching 8s of grinding time upon peak period makes the heat sink very hot (65-70 degrees) and grounds are at 55.

15g dose on average gives 5,5-6s grinding time on Mythos used for testing (low-speed version).
18g dose started at 7.9s with too course setting (18/36 ratio) and 8.5s at the correct setting (18/42).
That means grounds at 40 degrees after dialling in and a huge potential for reaching 50 degrees in an hour (it takes 2,5h on 15g dose).

Let's look at the data below. The first picture shows recipes before the swap. The second recipes on 18g VST baskets. I suspect someone was moving a grinder setting dial, thus numbers on the first picture vary a lot. On the second it is trustworthy.

When the grinding chamber temperature increases, the particle size distribution changes. You have to decrease the distance between the burrs to keep contact time the same. Which increases grinding time even further. And because that heats up grinder, even more, you end up having a FINER grind size on 18g during peak time than with 15g.

Real life story:
My favourite customer walked in. I nodded and started making his coffee - I asked my colleague for a double shot in a piccolo glass. She tried. 3 shots in a row failed to meet a bottom limit on contact time (29s in that case). Barista on shots sworn to God that she had just made it finer. The fourth try was servable - I was glad I had known the order before ordering.

Then the service picked up, grinding time went well past 9 seconds, which slowed down the workflow. The downward spiral of killing the grinder began. The grounds were at 55 degrees at least. The grinder was about to overheat. We tried dropping water temperature, but it deprived espresso of any sweetness. It seemed the equipment couldn't make coffee with more sweetness than bitterness. Then the grinder started making weird noises - the setting was extremely fine - the strain on the motor was horrendous.

That day the 18g baskets equipped machine got cleaned first. During clean-down I opened the grinder, holding the heat sink through wet cloth to avoid burns. That grinder didn't look like cleaned the day before (it was).

Let's taste it!
On the sensory side, baristas were not happy with the taste balance of the shots. Lacking sweetness and pleasurable tactile. In milk drinks, the synergy deteriorated. Not a single flat white scored above 3,0.

Having decaf on Mazzer Robur on the 18g basket side I got back to how old baristas were teaching me dialling in. "Make it somewhere between sour and bitter". Being unable to make espresso scoring more than 2,5 is a shitty feeling when the beans in the hopper are decent.

On the financial side, 3g higher dose meant 7kg more coffee used to produce the same amount of drinks in the week.

Why would you increase the dose?
I heard that in Prufrock everyone on the shop floor has to agree if somebody wants to change the dose more than 0.5g (is that true by the way?). Initially, I thought it's ridiculous.

After further contemplation I understood. Changing the dose changes the mechanics of extraction. Different headspace, different resistance, different saturation rate. Too much to deal with.

I almost never change dose more than 0.5g on one coffee and more than 1g between different coffees. If I reach extremes that are either down to roast, equipment or water quality.

As Ben Kaminsky found, roastiness in espresso is hardly perceivable above 10% TDS. Very soluble roasts will extract easily and be more bitter - you can counter that with underextraction. I assume darker coffee of lower density will have the lower thermal capacity - reaching thermal equilibrium faster. Makes sense when you think about a contact time for very dense coffees like high-grown Kenya).

Dull burrs will increase bitterness and underextraction could be the key to balance. But that should not happen in the first place.

Water with high ppm will produce less sweet coffee. Sometimes mineral, very often flat. Having more closed ratio will increase TDS of espresso, decreasing perceived minerality (like roastiness). Flavour clarity reaches its best at underextraction with hard water.
Water's impact on coffee goes beyond that. Your water might be ok, but much different from water the roaster used to test that coffee. I like to think that water and roast work in conjunction with each other.

Today I agree with Prufrock's approach. Unless something extreme is going on, big changes in dose are unreasonable. Assuming baristas know what they're doing and are calibrated with each other.

Different sizes VST baskets will use similar grind size. But the amount of ground coffee will affect the time of grinding. Time of grinding is a key factor in heating up in Mythos.

Low depth dose will be saturated quicker. The difference between the top and bottom of the basket will be smaller. And the entire puck will approach thermal equilibrium point faster.

Faster saturation will allow for more even extraction. Even extractions are higher than uneven extractions. But WHAT is extracted is more important than HOW MUCH. We aim for sweetness, flavour clarity and nice mouthfeel - not necessarily weight.

Change of dose is a risk. Make sure, you have very good reasons to do so. Check your equipment, water system, train your baristas on puck preparation and sensory of espresso. Keep in mind that change of dose has to change your ratios and will change unit costs.

All of the above expresses my opinion at the very moment of writing it. I hope to deepen my knowledge and tune my ideas about the topic. I consider it all debatable and I'm genuinely looking forward to the conversation.

There are many things I didn't cover around the topic. Most of that can be found elsewhere:


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