Espresso is something you never really understand. Like almost anything in coffee, all the knowledge available merely scratches the surface. But sometimes you stumble upon an idea you think you can prove. 

This article is a two-part series aimed at getting a better understanding of bed resistance. This one focuses on theory and the following on the testing.

How is it possible that coffee lipids affect resistance? That's something that I never thought about. But I have some other questions that I keep open for years. One of them is this:

Why after the whole busy morning of making the grind finer suddenly the resistance goes up and I have to make it courser?

What's even stranger is that at some point, on the busiest days, I was going finer again.

But I could never find out why that was happening. We had general ideas on how warmed-up grinder works differently. Probably due to the metal expansion under the heat.

Bean temperature
The partial understanding of the issue came from the grinding paper - The effect of bean origin and temperature on grinding roasted coffee

As the temperature changes so does your particle size distribution (PSD). Cold beans when ground result in smaller fines, but there's more of them. The opposite is true with warm beans. The reality is a bit more nuanced but in this case, we only consider a narrow range of bean temperatures.

The hopper placed above the grinding chamber and motor is likely to get hotter when it's busy. The warmer beans will grind differently. It's more about the temperature of the coffee beans than the grinder itself. 

How does that apply to espresso?
Let's suppose you've dialled-in a coffee to have 35s of contact time at a given ratio and grind setting. The beans were at the room temperature. Keeping the ratio and grind setting constant you pulled more shots of the same beans. But the beans had a different temperature.

Some of them were kept in the fridge, some were microwaved before grinding.

Beans from the freezer produced 10s longer contact time. Beans from the microwave run 10s faster!

The change in temperature determines the change in PSD. There's a linear correlation of the temperature of the beans to the contact time. Since all other variables were kept constant, the contact time corresponds with the bed resistance.

Colder beans - more resistance. Hotter beans - less resistance.

That explains why you have to keep fining up the grind size to reach the desired contact time.

Until something unexplained happens and you have to go courser against all logic.

The resistance of heat
Michael Cameron published a fantastic article - The Resistance of Heat. That article shifted the perspective to heat as resistance.

Here's his explanation of the curve of resistance:

"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.”

I have analysed this a lot. I was glad to see more people encounter the counterintuitive resistance increase around 40 degrees. Michael's post explains that from a perspective of volumetrics. The change in brew water retention and thus espresso yield is significant. But what about weighing every single shot? How would a gravimetric machine behave?

You always have the same yield, but a different amount of water passing through the coffee bed. The lower surface area of grounds is probably compensated with higher temperature of grounds.

But I think we should separate the temperature of beans (and their PSD) from ground coffee temperature (which we believe affects the resistance on its own).

The change in PSD makes a lot of sense. Grounds that are hotter are changing the dynamics of extraction too. The pressurised system of solid mass, gas and liquid is very complex even without considering the heat. There are certainly no linear correlations. It also seems like there are 2 different processes happening simultaneously.

Beans vs. ground coffee temperature
A while back, when I was measuring the effect of bean temperature on its resistance I noticed something very interesting. All 3 samples (beans ground at ~0, 25 and 60°C) had a similar temperature after grinding. All within 10 degrees. Which is very peculiar given that the beans varied by nearly 60 degrees! Look at this table:

The values in blue, grey, green and yellow are respectively: cooled in the freezer, room temperature, heated and cooled down, microwaved.

It was at the end of a busy day, so the grinder was quite hot - both fans engaged at full speed. The grounds were measured with an infrared thermometer.

It seems like the thermal energy within the grinding chamber has a greater impact on the temperature of the grounds than the beans. There's a lot of frictional heat generated during grinding too.

We've established both the temperature of beans, grinder and heat generated during grinding play a role in resistance. Having considered that, what exactly does the grounds temperature affect?

To paint the full picture and make the issue even more complex, there's one more thing correlated to temperature.

The post-grinding degassing plays a role. Not only Carbon Dioxide but volatiles too. It's in my experience that ground coffee left to degas offers less resistance. This is sped up by a higher temperature. To see a perspective of someone way smarter than me on this, check out Christopher Hendon's comment on that:

“letting the coffee equilibrate to the same T sounds, in principle, like a good idea and minimizing variables but actually at the T range, you were looking at *this is your biggest variable*. Chahan and his talented postdoc Samo showed conclusively that the rate of certain volatile losses occurred on the seconds (rather than 10s of seconds) time scale and the process is kinetically limited. This means at high T, cooling, vs RT sitting there, the chemical composition of the volatiles has vastly changed. Volatiles are one of the main reasons shots run at different rates - less water, solid interface, lower extractions, etc. When you grind hot you drive off more of these, and while your EY doesn't change (let's not talk about that measurement though... that's clearly problematic given the experiment I just described) your flavour profile does”

[posted: August 2019, edited for clarity]

This comment has been posted under this video, where James experiments with the temperature of beans for grinding. It confirms that the temperature of the grinder is important - in his case the cold grinder sucked in a lot of heat.

The last piece of the puzzle
About a month ago I stumbled upon a piece of research in Illy's book (Illy, 2005). I think that answers what's the unknown in Cameron's article.

The major part of coffee lipids fraction is composed of palmitic and linoleic acids and stearic and oleic in lower percentages. This blend is very viscous at room temperature, but becomes liquid above 40 degrees. At higher temperatures this liquid can easily flow through micro cracks and cover the outer surface of the particles (which can become semi-solid when cooled down). This phenomenon affects the cohesive behaviour of ground coffee when compacting, hence influencing the hydraulic resistance of the cake. Overheating on grinding may thus be responsible for erratic percolation.

The lipids affect the bed resistance. I think we're getting somewhere.

The 'erratic flow' could be also caused by clumping. As the grounds get more sticky clumps are formed more easily. The grind size and the resistance on the outlet from grinding chamber (sometimes controlled by a clump crusher) plays a major role in that as well. It makes sense, but clumping doesn't affect bed resistance unless it's extreme (like in the photo above).

I've put that to test using WDT (ref. 5) to break clumps and it follows the same resistance pattern as when palm-tapping.

What's my hypothesis?

As the temperature of the grinds increases and the lipids transform into a liquid state the resistance increases due to that transition.

Espresso flow changes somewhere around 40-50 degrees mark (it will depend on how you measure it). The resistance increases and the turbulence of flow increases (due to clumping). These two seem correlated. In this instance, they occur simultaneously. 

But they are not correlated below 40 degrees. Unless your grind size is very fine (at the point of diminishing returns, usually 45s+ shots), lower resistance induces more turbulence. Think about those 18s shots at open ratios.

If you're using Mythos grinder, both fans are at full speed when you reach 45 degrees. It's designed to keep the grounds temperature below that point. Which it really struggles with in high volumes - I've seen more than 65 degrees when putting through 14kg/day. Any grinder would overheat with that. The point is, there's a good reason the Mythos tries to cap the temperature below 45 degrees. I believe plenty of testing has determined this point.

I think baristas should really focus on that point of transition into increasing resistance. Make sure the contact time stays within a reasonable range, be careful with puck prep and discard any shots that look subpar. It might be worth to adjust the brewing water temperature too.

Click here for the next part, where I explain my testing to derive the pattern of heat resistance.

1. Illy A., Viani R, (2005) Espresso Coffee: The Science Of Quality, Second Edition; Grinding p.228
3. Uman, E., Colonna-Dashwood, M., Colonna-Dashwood, L. et al. The effect of bean origin and temperature on grinding roasted coffee.Sci Rep 6, 24483 (2016).

Title Photo by Thom Holmes on Unsplash


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