The pore sizes in standard paper filters range from 10 to 20 micrometers. This successfully traps insolubles (coffee sediments) and high-molecular-weight lipids (diterpenes like kahweol and cafestol).
If the coffee bed is unevenly packed, water takes the path of least resistance. It carves out high-velocity "channels," over-extracting the coffee along the channel walls while leaving the rest of the bed dry and under-extracted. 4. Thermodynamics: Temperature and Solubility the physics of filter coffee pdf full
is the pressure drop across the bed (driven by gravity and fluid height). is the dynamic viscosity of the water. is the thickness (depth) of the coffee bed. Permeability ( ) and Fines Migration The permeability ( The pore sizes in standard paper filters range
) within the cellular matrix of the bean. When water infiltrates these pores, it displaces the gas. The Displacement Problem If you do not allow the CO2cap C cap O sub 2 is the dynamic viscosity of the water
| | 📚 The Physics at Play | ☕ Actionable Tip | | :--- | :--- | :--- | | Grinding | Percolation, Particle Size Distribution | Focus on grind consistency to create uniform pore spaces for even extraction. | | Water Quality | Solvent Chemistry, Dissolution | Use water with the correct mineral content (e.g., low alkalinity, moderate hardness) to act as an effective solvent without imparting off-flavors. | | Pouring (Pulse vs. Constant) | Turbulence, Mixing Dynamics | Pouring in pulses or smaller circles increases agitation and mixing, promoting a higher and more even extraction yield. A steady, slow pour with minimal turbulence reduces extraction. | | Bloom Phase | Surface Tension, Gas Expulsion | A proper bloom (using ~2x the coffee's weight in water) allows water to fully wet the grounds, expel CO₂, and prepare the bed for even extraction. | | Brew Time | Advection, Diffusion, Mass Transfer | Adjust your brew time (e.g., by changing pour speed) to control how long water interacts with the grounds, impacting the final strength and extraction percentage. |
A dry filter wets slowly, causing initial bypass of water around the sides. This is why you must pre-wet your paper filter to eliminate paper taste and create a uniform flow field.