Strength is not the same as flavour. In coffee, the strength is measured in Total Dissolved Solids (TDS) and usually it is marked in percentages (%). When we measure TDS, we are evaluating soluble concentration, so how much from coffee’s solubles have concentrated into the cup. Put simply, how concentrated (strong) or watery (mild) the coffee is.
It is quite impossible to say what is drink's TDS just by looking at it so to measure it we need a refractometer. A refractometer measures the angle of lights’ refraction through a liquid and by that it can tell how thick or thin the liquid is. Refractometers are usually used in laboratories to test, e.g. plasma protein in blood or to identify the materials in a gemstone. It works really well for coffee as well, but they’re not exactly lying around most peoples houses!
How to manipulate TDS?
TDS works pretty much the same way as extraction. You are able to increase TDS with these ways:
Brew ratio -> more coffee in relation to water -> higher TDS
This is pretty basic and quite logical for everyone: If you use more coffee in relation to water, you will end up with stronger coffee (higher TDS).
Finer grind -> more surface area to extract from -> higher TDS
Grinding finer means that you have more surface area where the water can extract things. If you cut a coffee bean in half, you will double its surface area and it will be twice as easy to extract things from it. As it is easier to extract, the strength will also increase.
As you grind finer it will easier to extract but also you will increase the brew time for espresso and filter brew methods which will also increase extraction. The more water and coffee spend time together, the more coffee will give for itself to the water. If you are using a plunger then you can increase brew time by, well, letting it brew longer before pouring out of the plunger. Note that you will get more bitter flavours, generally described as over-extracted.
Higher temperature in brewing water
You have to have energy to extract the coffee. The most common and easiest energy source for extracting is water’s thermal energy. More temperature, more extraction, more strength.
Turbulence (stirring) when making filter coffee
Turbulence means some kind of movement in the coffee bed when making filter coffee. Moving the grounds and water will expose more surface area to the water so that it is possible to extract more. More turbulence, more extraction, more strength. But, more turbulence may mean that the water goes through the filter faster, reducing brew time. So it’s counterproductive.
Appropriate brewing practices -> avoiding channelling
Channelling means that water is running through the coffee bed unevenly. There are numerous reasons for channelling such as poor dripping/water distribution in drip filter coffee or cracked coffee puck in espresso making.