Extraction II

Care must be taken with this variable since, sometimes, we focus too much on the temperature of the water and not on the temperature of the solution, which is the most important. As logic invites us to think, we extract faster the higher the temperature.

Various factors can influence the temperature of the unit: the temperature of the water, the temperature of the ground coffee, the temperature of the container that contains both, and the temperature of the outside if the device is open to the environment. In the case of some filter systems, it is necessary to take into account the passage of time and how this can affect the drop in temperature of the assembly. Scott Rao recommends extracting between 91 and 94ºC, although if we adapt all the other variables (grinding, time, etc.) to another temperature range there would not be a problem either, as has been seen in various Aero-press championships, where many participants choose to lower than 90ºC in your recipes.

On the other hand, Matt Perger defends the use of boiling water* for filter systems, arguing that in this way extractions will always be carried out at the same temperature (goodbye Bonavita) and that it is foolish to think that with such hot water we burn the coffee, as long as all other variables are appropriate to it.

* Eye! Water boils at different temperatures depending on pressure/altitude. In Madrid, the boiling temperature does not reach 98ºC. If we go to 2000 meters, we will have boiling water at 94ºC.

In the case of espressos, it must be taken into account that the temperature at which we set the group will not be the same as that at which the extraction takes place. This is because many machines do not have saturated groups and the temperature that we set as a reference has a fairly wide margin of error before and during extraction. The fact that the machine's PID temperature probe is usually installed at the boiler outlet and not in the group can also lead us into error. Depending on the manufacturer, there may be a heat loss of 0.5 to 3.5ºC. (The PID or proportional-integral-derivative is a mechanism responsible for maintaining a desired value within an error range, in this case the temperature of the water leaving the boiler).

No less important is the effect of the temperature of the freshly ground coffee that is going to come into contact with the water, since it will make the temperature of the liquid drop a few degrees until both reach a thermal equilibrium as the temperature passes. time. Depending on the quality of our grinder, the wear of the grindstones, the amount of coffee we have ground in the last pull and the outside temperature we can have a grind at 1oºC or 50ºC. Matt Perger designed a calculator to find the equilibrium temperature at which the extraction is carried out, although personally I see it only as a curiosity since that information is not going to help us greatly to make better coffee. In any case, it is curious to say the least to see how some manufacturers and baristas obsess over the temperatures of the machines and ignore the fact that in the same morning, the temperature of the set can vary by up to 4ºC.

In my opinion, when we exceed 95 degrees in the water of espresso machines, I find the coffees somewhat astringent. Below 90ºC I find them flat, with annoying acidity and a very short aftertaste. I don't usually play too much with the temperature of the machine and generally leave it at 92ºC, although I recognize that it can be useful when you have to work with coffee that is very lightly roasted (the temperature would increase) or over-roasted (the temperature would be lowered or the temperature would be thrown away). the coffee in the trash).

The grinding .

We could make a blog just about this section and without a doubt we will talk about this repeatedly, but for what we are dealing with on this topic of extraction, we must know that a finer grind extracts faster than a coarse one, since the Specific surface area of ​​the coffee is greater and there is more contact with the water. The particle size distribution greatly affects the quality of the extraction, but to avoid complicating the matter I will refer only to the average of that distribution.

It is important to take this variable into account when making espressos, as it will not only influence whether we extract more or less, but also the flow of the liquid to the cup, so the grinding will affect us at the same time. total extraction. For the same dosage parameters and weight of the espresso, by refining the grinding point and delaying the total fall time we will achieve a more concentrated drink (with more %TDS) and with a higher extraction percentage. This information will be very useful to us when playing with our recipes in Extraction III.


Simply put, the longer the water is in contact with the ground coffee, the more it will be extracted, although it must be taken into account that the extraction/time curve decreases exponentially. For the same dosage and grinding point, the body will also reduce as the extraction time passes.

Water: quality, concentration gradient and agitation.

We have a pending post about water quality. The recent publication of the book Water for Coffee by Maxwell Colonna Dashwood and Christopher Hendon has revolutionized the global coffee scene, although many (including myself) are still trying to decipher the complexity of what is stated in that work. For now we will work with the SCAA recommendations for infusion water and in the future we promise to get involved with a post about the conclusions of Maxwell's book.

For extraction, the concentration gradient of the water that surrounds our coffee is key. If the solvent surrounding the surface of the ground grain is already saturated, it is more difficult to extract anything than if the water is in motion and “emptying” the solute. For this reason, it is so important that we have distributed and compacted the coffee as evenly as possible in our espresso pot, because if not, the water in a continuous flow will sweep some areas more than others. For filtered coffees this issue is vital, since the concentration gradient increases very quickly with stirring and, likewise, extraction. That is why you have to be very careful when stirring a water-coffee mixture or when turning an Aeropress... to give two everyday examples.

All of the above leads us to think about ratios. The higher the coffee:water ratio, the more we will extract for the simple fact of having more solvent (water) for our solute (coffee) and increasing the concentration gradient.

Pablo Caballero

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