| Compound | Solubility in water | Extraction timing | |----------------|---------------------|-------------------| | Citric/Malic acids | Very high | First 30 seconds | | Sugars (sucrose) | High | 30–120 seconds | | Caffeine | Medium | Throughout | | Chlorogenic acids | Medium-high | After 90 seconds | | Tannins/Bitters | Low | After 3 minutes |
Key insight: A longer brew does not make coffee “stronger” in a good way—it adds bitter compounds.
Fine particles are mobile. They move downward with water and can lodge between larger particles, slowing flow dramatically (fines clogging). Mitigation: Use a flat-bottom brewer (e.g., Kalita Wave) or avoid aggressive swirling. The Physics Of Filter Coffee Pdf
Coffee grinds are not uniform; they follow a Particle Size Distribution (PSD).
The Physics: As water flows downward, it exerts a drag force on particles. Because fines have a lower mass-to-surface-area ratio, the drag force can overcome gravity, suspending them or pushing them deeper into the bed. This often leads to "fines migration," where small particles clog the bottom of the filter, reducing permeability ($k$) and causing the brew to stall (Channeling). | Compound | Solubility in water | Extraction
| Variable | Physical role | Practical effect | |---|---:|---| | Temperature | Affects solubility & extraction rate | Higher temp → faster extraction; risk of bitterness if too hot | | Grind size | Controls surface area & permeability | Finer → stronger, faster extraction; coarser → weaker, slower | | Flow rate / Brew time | Sets contact time & removal of solubles | Faster flow → shorter time → under-extraction | | Filter type | Controls particle/oil retention | Paper → clean cup; metal → fuller body | | Pour technique | Influences turbulence & saturation | Even pour → consistent extraction; poor pour → channeling |
As the stream hits the crust of grounds, the Weber number (We) predicts whether the water will penetrate or splash. Fine particles are mobile
For filter coffee, you want We < 10 to avoid droplet formation. A high We (caused by pouring from too high a height) creates micro-droplets that cool below optimal extraction temperature (90–96°C) before even reaching the coffee.
PDF Takeaway: A proper physics PDF will include a Pour Height Nomogram—a chart linking kettle spout diameter, flow rate, and optimal height to maintain laminar, non-splashing flow (typically 3–7 cm above the slurry).
