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Take a spoonful of freshly ground Arabica beans from the Colombian mountains. Place it in the espresso machine and let the hot water percolate through. The fragrance is already there, filling the room, announcing the most heavenly peaceful moment.

I love these quiet coffee breaks and that is why I acquired an espresso machine some time ago. I am always using the same filter, a cylindrical cup with a fine mesh bottom. 53mm internal diameter and 17mm deep. I always fill it to the top. That is just about 37.5 cm³ bulk volume of freshly ground Arabica beans from the Colombian mountains (well to be completely honest, sometimes I cheat my Colombian friends. This is when I can get, at a hefty price though, marvelous Arabica beans from the highlands of Ethiopia, and that is a real treat indeed).

Before buying the espresso machine, I checked what the requirements for a good appliance were. Pump pressure happened to be the feature to carefully consider. The magic number is 9, in bars - for a drink it sounds sound that it be indeed in bar, as for applying gas pressure to a draught beer tank I wouldn’t use anything else than bar, would you? Well, at this early morning time, let us go back to espresso… - 9 bar pump pressure. So I went straight for a 9 bar machine and of course an Italian brand as a tribute to Angelo Mariondo, the inventor of the brewing process.

Good espresso must be freshly ground, alternative are just none. So I went on to a grinder purchasing exercise. I found a decent grinder with several grinding options, because I wanted some flexibility to check several grinding qualities and find the one that would satisfy my taste. The grinder has four grinding options: very fine, fine, medium, and coarse.

So I decided to try them all. My espresso machine makes 25ml espresso cups. When I tried the different coffee grinds, I noticed that the time to make the espresso - or percolation time - was different:

With the same coffee blend, the taste is somewhat different as well. I eventually found out that the fine grind was my favorite and that I was not busy enough so that time was not a parameter that I would consider relevant in the espresso brewing engineering.

Anyway, I found that I could calculate the flow rate for different cases, should I get involved in the espresso business at an industrial scale (actually I am not that interested).

 (Side exercise: consider the price of $ 1.50 for a 25ml espresso, calculate the revenue one can make a day with the machine, varying the grinding method)

Of course to grind the coffee finer, I needed to put more energy into it. As a penitentiary inmate told me once, “the more you hit it, the smaller it gets”, and this is something that you might consider in case you want to save energy on the grinding operation.

Also, I soon noticed that it is actually taking more time to grind the coffee, throw the used packed grind to the bin and fill up the filter again with fresh coffee and wait for the machine who inadvertently had timed out to warm again. No doubt that in an espresso shot, we pay for service, not for coffee. So the economics figure above makes no sense whatsoever.

Anyway, if the water has more difficulty to flow through a finer grind, it means that there is a characteristic associated with the coffee pack and its grain size. A characteristic that governs its ability to have the hot water flowing through it. Well actually it is not the grain size, but the void left between the grains that allows more of less water to flow freely between the grains. However, you probably noticed that the size of the void left between the balls in a tight pack of soccer balls is much larger than the size of the void left between the particles of ground coffee, even though for the same bulk volume, the total void volume may actually be the same. Small pores create more friction, hence more time is necessary for the same liquid volume to go through. This ability of a porous medium to let the fluid flow through is what we call the permeability, and surprisingly there is an optimum permeability of Colombian Arabica grind to my coffee taste.

This feature (permeability, not coffee taste) was actually not discovered by an espresso drinker because the first patent of the machine was filed in 1884 in Turin while Henri Darcy had been buried 323km away across the Alps, in the city of Dijon 26 years earlier.