The extraction of a coffee describes process of dissolving soluble compounds from ground coffee in water. To brew a good coffee, the degree of fineness and the quantity must be correct on the one hand, and the quantity and temperature of the water on the other.
To measure the degree of extraction, the TDS value (total dissolved solids) can be used to describe what percentage of the ground coffee is in the final coffee. A so-called refractometer can be used to measure in PPM (parts per million) how many particles are dissolved in the liquid.
However, when it comes to extraction, the wrong preparations can quickly lead to over-extraction or under-extraction.
When we talk about over-extraction, we mean that too many substances (and especially too many bitter substances) are dissolved from the coffee grounds. This may be due to water that is too hot, such as boiling water, or to the grind being too fine.
In the case of under-extraction, on the other hand, too few substances are dissolved from the coffee grounds. Since the acids contained in the coffee bean are the first to be dissolved in the extraction process, the result is a sour taste. This may be due to water that is too cold, coffee grounds that are too coarsely ground, or a brewing time that is too short.
