What changes the tactile of espresso?




There's little research on tactile in coffee. It's been a bit neglected compared to attributes like a flavour or taste. It's easy to predict how the balance will shift when changing your recipe. However, it's not that simple with tactile.

Throughout this article, I will be using names of attributes from WBC sensory scoresheet. I separate tactile into weight,  texture and aftertaste. You may notice I have a preference for a higher flavour clarity and better taste balance over big weight and texture. I don't mean one is better than the other, I just want to make my bias explicit before you read further.
Also, I decided to simplify some things in an attempt to increase clarity.

From the most basic perspective, we tend to associate the tactile sensations with strength. But there are more variables at play with texture and aftertaste. Can we even isolate variables in the perception of weight? It's interconnected, but let's try to break it down a bit.

Weight and strength

A perceived weight of espresso is directly correlated with its strength - TDS. The higher the TDS, the bigger body. Most of the time. The weight can be adjusted with the ratio of coffee to brewing water. Lower TDS, found in more open ratios, makes it easier to recognise the flavour.

More open ratios extract more, have more flavour clarity and have a lower weight. The main problem with a high TDS espresso is its ability to hide imperfections - roast defects, lower-quality green, poor dial-in etc. While sometimes you may want to do that, it's not fair if you're quality-focused and your choices regarding green coffee, roast and extraction level are meant to represent that.

Why do we stick to 1:2 ratio?

We are as an industry able to pull high extractions (above 22%) with a 1:2 ratio. The concept of roasting coffee to high solubility is not just about darkness anymore. Grinders and espresso machines (Decent in particular) are better, just like our ability to calibrate and set them. And we have revisited puck prep and started developing tools to improve it. The boundaries are pushed and that's fantastic. But that's still difficult to do. Probably you're not able to do it in a busy cafe, over the course of the day, across the whole team.

So more open ratios are better, right?

They are easier to work with. They give you a chance of extracting more. And it's not a marginal gain.

Flavour clarity will increase as well. You citrus is more like pink grapefruit, your floral is actually lavender - pretty cool! With that, any imperfections will become present. And that's the 'negative' of high flavour clarity.

For example, maybe suddenly you taste roast. You think you've extracted 'too much', but in reality, someone was not careful in quality control.

What do I mean by that?

Unfortunately, some roasters approach espresso roast as 'it should be balanced at 1:2 ratio' - which makes their job very difficult because reducing acidity to a balanced level at this ratio usually drives profiles into overroasted range. Fundamentally that ratio sets you up for a very low extraction.

Open ratios, channelling and aftertaste

Another challenge of open ratios is astringency. Drying aftertaste is not desirable, even if you like it (I'd recommend red wine and black tea instead). Astringency comes from tannins - which are difficult to extract. However, if there's a substantial amount of turbulent flow due to what we call channelling, tannins extract.

What is channelling then?

In short, it's the formation of locally increased flow due to poor puck prep and pressure/ flow related issues.

More open ratios will highlight astringency. Primarily because of the higher clarity - there's nothing to hide it.

But also because channels and their effect on aftertaste are not linear. They don't cause dryness immediately. You could pull a 14s shot that looks awful and perceive no astringency, even though you could swear there's a ton of channelling happening. If you could stop the shot when channel appears, you'd have little astringency. The concept is not very different from cutting your hand brew short when it chokes.

There's a lot of benefit from more open ratios. Including more flavour clarity and generally higher extractions. If you're like me you will enjoy lower weight too.

But that's enough for ratio. What else changes mouthfeel?

The dose and contact time

Contact time as a variable in the tactile seems very easy to explain. As you grind finer, you increase surface area and resistance - water has more particles immediately available for erosion and more time to extract. The more you extract, the bigger the body.

Except I'm not sure if having more time increases extraction in espresso. Espresso extraction is almost 100% surface erosion. It happens almost immediately when water comes in contact with particle. Theoretically, it's a waste of time to slow it down with increased resistance. It would have to be an uneven flow that would need more time to extract fully. 
Is it the grind size being too fine causing that unevenness? Probably.

Recently we have people all over the world pulling 15s espressos and getting pretty high extractions. Try it yourself! The balance and flavour are there, but weight is very low.

It seems like there's something else at play that has little to do with a level of extraction. It's not about what gets dissolved - it's about what gets into the cup undissolved. Let's have a look at it from the other perspective - dose.

How does the dose affect the body?
(my untested theories)

When you have a high dose you have theoretically more surface area available for erosion. Now, there are plenty of reasons why high dose (say 20g) is not efficient for extraction (I wrote a little novel about it here). But let's assume it's not a factor for a moment. Even if erosion is not efficient (uneven extraction) it would extract something.

Does something from the very beginning of extraction cause high weight? I'm guessing it has something to do with a molecule size and molecular weight - like in homogenised vs. unhomogenised milk (we perceive bigger globules as heavier).

How about lipids?

Fat is not very well soluble in water. But with high pressure, temperature and turbulent environment it would appear some would end up in the espresso. Some people even separate it and taste it - apparently, it's not very good. 

It's safe to assume that you would get more lipids from a higher dose. In cooking, fat products increase weight. So it would make perfect sense that more lipids means more weight in espresso too (maybe that's why lower dose espresso tastes the way it does?).

And let's not forget about the baskets! They certainly let a lot of colloids flow through their holes. Colloids are well known to increase perceived weight. You get more of them from a bigger dose, especially if your basket manufacturer changes total open area (amount and diameter of the holes) to compensate for resistance change. One might think that also lets more lipids in. I have not considered the effects of fines migration on that.

Going back to contact time. I could theorise that with lower contact time you get less lipids into the cup. Probably less colloids too - after all, they have to squeeze through tiny holes in a limited amount of time.

If that was the case, the length of extraction would have less effect on weight than grind size and its implications. It's proven that you don't need the 30s to extract 20% and above. But maybe you need that contact time to achieve the weight of espresso as we know it?

What do you think? Do you agree? Let me know!


Resources and inspirations (links):
  1. Cameron, Morisco, Hofstetter, Uman, Wilkinson, Kennedy, Fontenot, Lee, Hendon, Foster (2020), Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment
  2. Stanley, Araujo, Costa, Andrade (2003), Fluid Flow Through Disordered Porous Media
  3. van Vliet, van Aken, de Jongh, Hamer (2009), Colloidal Aspects Of Texture Perception
  4. What is Astringency?
  5. If Not Channelling, Then What?

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