Coffee Knowledge Base

20 February 2024

Perfecting coffee extraction: balancing over- and under-extraction factors

Perfecting coffee extraction: balancing over- and under-extraction factors

In this blog we will talk about perfecting coffee extraction, more specifically over- and under-extraction. We'll explain the different parameters and their influence on flavour.

The flow-through time is very important for a filter coffee’s flavour. The flow-through time is controlled by parameters including the temperature, the number of grams used, the degree of grind, the amount of water, the pressure and the filter structure. If the coffee runs through too fast, we call it under-extraction. In other words, you are not using all the coffee’s characteristics optimally and flavour is lost. If the coffee is running through too slowly, we call it over-extraction.


Extraction is the degree to which the soluble substances are extracted from the coffee beans and dissolve in the solution, i.e. water, in this case. This immediately brings us to the difficulty involved in making coffee in the proper way. The correct extraction percentage for a good coffee is between 18 and 22%.


So the wrong level of extraction can lead to a poor coffee. Technically, it is possible to extract up to 30% of the coffee and dissolve it in the water. But that coffee will be over-extracted and therefore undrinkable. Its taste will be extremely bitter because all the unwanted flavours will also be extracted and dissolved into the coffee you will drink.

Causes of over-extraction:

  • Excessive contact time between the water and the coffee.
  • Coffee that is ground too finely, which will prolong the flow-through time too much.
  • Insufficient coffee in relation to the volume of water, making the coffee ‘overused’ or ‘over-extracted’.
  • Excessively hot water.


On the other hand it is just as important that you ensure a minimum degree of extraction or it will be under-extracted. Under-extraction means simply that you extract too few of the soluble substances and the coffee is partially unused.

Causes of under-extraction:

  • Insufficient contact time between the coffee and water.
  • Excessively coarse-ground coffee, which makes the flow-through time too short.
  • Too much coffee in proportion to the water, which in itself blocks full use of the coffee and results in under-extraction.
  • Water temperature too low.

The parameters 

Grams used

The number of grams used is, of course, very significant to the intensity of your coffee. The more weight you use, the stronger the result. You can usually reckon on 60 grams per litre. But we will elaborate on this for each brewing method a little later.

Water quality

About 98% of coffee is water. So water is a factor to which we often do not attach enough importance. The composition of the water that you use has a great deal of influence on the taste of filter coffee. You should always use filtered water anyway. And if you would like to use bottled water, we recommend Mont Roucous. Of course, the amount of water also has a major effect on the intensity of the coffee.

The degree of grind

The finer you grind the coffee, the more difficult it is for the water to flow through the ground coffee, and the more you extract from it.


The water temperature has a great deal of influence on the level of extraction. As general rule, we can say that you should never use boiling water. It will release or accentuate the bitter substances. And that is even the case with lightly roasted coffee, which in fact contains fewer bitter substances. On the other hand, if the temperature is too low, an unpleasant, sour taste can arise. 


The type of filter you use and its specific thickness and permeability are also determining factors for the coffee’s flavour. Filter papers with denser or thinner woven holes let the water flow through slower or faster. The original Chemex® paper has a very thick structure and filters the maximum amount of insoluble substances and oils out of the coffee. The paper used in an AeroPress® is much thinner and therefore lets more solids through, and that creates coffee with completely different ‘body’ or mouthfeel.

The filter apparatus

The shape of the device and the opening, as well as the number of holes, determine the flow-through time.


With the traditional filter method, gravity alone serves as the pressure driving the water through. With an AeroPress® the pressure is independent of gravity. And that, of course, has a different influence on the result.


By turbulence we mean the manner, pace and power with which the water is poured onto and mixed with the ground coffee. You can pour it on very calmly or with a powerful flow. And you can do so in a circular motion or not.