One of the critical factors in filter coffee brewing is the flow of water through the coffee grounds. This process is governed by a combination of gravity, pressure, and viscosity. As the hot water is poured over the grounds, it begins to flow downward through the filter due to gravity. The rate of flow is influenced by the pressure difference between the top and bottom of the filter, as well as the viscosity of the water.
As water moves downward, it carries dissolved solids with it. If the water moves too quickly (due to channels forming in the bed), you get "under-extracted" coffee. If it moves too slowly, you get "over-extracted" coffee. 4. The Role of the Filter Paper
Different flavor compounds dissolve at different rates and temperatures: acids and sugars are extracted early; bitter alkaloids (e.g., some phenolics) and heavier compounds extract later. Managing temperature, grind, and time controls which compound classes dominate the cup.
The holy grail of coffee physics is predicting Total Dissolved Solids (TDS) as a function of time. This is governed by a simplified version of the for coffee solubles:
