The Physics of Filter Coffee — Jonathan Gagné (2020)
Sources: The Physics of Filter Coffee by Jonathan Gagné (2020)
Jonathan Gagné is an astrophysicist who turned his data-gathering and modelling skills toward coffee brewing. The book was published by Scott Rao in 2020. It is the most physics-rigorous reference on filter coffee extraction available in the specialty coffee community. All conclusions are grounded in empirical measurement or established physics equations, and Gagné is careful to flag where evidence is preliminary.
Chapter 1 — Extraction
The central metric is average extraction yield (AEY): the fraction of dry coffee mass dissolved into the beverage. The traditional 18–22% window (Lockhart, SCA) has shifted slightly higher in specialty practice; Gagné empirically finds good cups at up to 23.5% with high-quality grinders, and notes that AEY around 15% can be preferred, followed by a dip in preference from 15–18%, then another positive zone above 18%.
Two physical mechanisms drive extraction:
- Advection: solubles carried away by flowing water (fast, at particle surface)
- Diffusion: concentration-gradient-driven migration from particle interior to surface (slow, rate-limiting)
Different chemical compounds have different diffusion coefficients (Einstein-Smoluchowski relation). Temperature raises all diffusion coefficients — but in different proportions — changing the profile of extracted compounds, not just the speed. This is why temperature and grind size affect flavor in distinct, non-interchangeable ways.
Fines (broken-cell particles) dump all solubles instantly; they disproportionately dominate flavor even when few by weight.
Percolation vs immersion:
- Percolation: continuous fresh water → advection dominates → efficient, clean
- Immersion: no flow → diffusion-limited → gentler, more body, less efficient
The Liquid Retained Ratio (LRR) — water retained by spent grounds divided by dry coffee weight — is typically ~2.2 for V60. It must be accounted for in accurate AEY calculations; using the wrong formula can misstate AEY by 10% or more. See Extraction for formulas.
Chapter 2 — Water
Water chemistry affects extraction in ways invisible to taste tests of the water alone. The two critical parameters:
- Total Alkalinity (bicarbonate HCO₃⁻): resists pH change. High alkalinity chemically modifies desirable coffee acids as they extract — the most damaging water parameter for cup quality. Target: 20–70 ppm as CaCO₃, with the specialty community using 40–50 ppm.
- Total Hardness (Ca²⁺, Mg²⁺): positively charged ions believed to increase diffusion of specific compounds. Ca favors body and sweetness; Mg enhances flavor complexity. Hard water above ~200 ppm as CaCO₃ is problematic.
Softer water mimics lighter roasts — brighter acidity, less body.
Most city tap water worldwide is too hard for specialty brewing. See Brew Water Crafting for mineral recipes.
Chapter 3 — Grinding
Particle size distribution (PSD) is the key grinder quality metric. The 10th percentile particle diameter (D₁₀) governs hydraulic resistance of the coffee bed more than average particle size — small particles slow flow far more than large ones speed it up. Lower-quality grinders produce wider PSDs; higher-quality grinders produce narrower or bimodal distributions.
Larger burr sets produce narrower PSDs. Room temperature shifts grind size (warmer → more ductile beans → coarser effective grind). Grinder seasoning (first ~dozen pounds) is required before a stable dial-in is possible. See Coffee Bed Hydraulics and Grind.
Chapter 4 — Percolation
Channeling — regions of lower hydraulic resistance — causes local overextraction and leaves other regions underextracted. A flat, even coffee bed and consistent pour technique minimize channeling. The bloom (pre-infusion) degasses the bed, wets particles evenly, and prevents dry pockets during main brewing. See Channeling.
Fines migrate downward during brewing and progressively clog the filter, increasing resistance and slowing drawdown over time.
Chapter 5 — Filters
Thicker paper filters = cleaner cup (less body), slower flow. The coffee bed itself acts as a self-filter for undissolved particles in percolation. Tabbed vs. tabless paper filters produce measurably different results. Filter brand consistency matters for repeatability.
Chapter 6 — Kettles and Agitation
Gooseneck kettles allow precise pour rate and placement control, directly affecting slurry agitation and extraction evenness. Turbulent pours increase evenness; overly agitated slurries can cause fines migration and clogging. Slurry temperature is approximately 5°C below kettle temperature in most drippers.
Chapter 7 — Drippers
Dripper material affects slurry temperature retention: plastic > ceramic > glass. Each dripper geometry has an optimal dose range tied to bed depth. Gagné finds plastic V60s and the Stagg X dripper produce higher slurry temperatures than ceramic/glass drippers at the same kettle temperature.
Chapter 8 — Freshness
Bean age affects degassing rate and bloom behavior. CO₂ released during bloom can impede initial extraction if bloom duration is insufficient. Lighter roasts may benefit from longer blooms. See Coffee Freshness.
Chapter 9 — Roast, Terroir, Varieties, Processing
Key empirical findings from Gagné’s logged brews:
- Darker roasts → lower average extraction yields across all origins and brew times. Gagné hypothesizes that lighter roasts may have higher soluble content or structural differences favoring diffusion in filter brewing.
- Natural process coffees extract slightly lower than washed on average.
- Decaffeinated coffees extract significantly lower — already chemically depleted.
- Dense/hard beans (many Ethiopians) generate more fines → longer brew times and clogging risk.
Variety flavor profiles (from 1,500 bag database via Firstbloom app):
- Kenyan SL28: redcurrant, berry
- Ethiopian Gesha: peach, floral
- Caturra: fruity, broadly pleasant
Processing flavor patterns: natural process coffees associate more strongly with blueberry and cherry descriptors; washed coffees skew toward citrus and floral.
Relevance to Kaiserblick Specialty Coffee: light roast profiles favor higher AEY in filter brewing; natural process micro-lots will extract slightly lower than washed; dense high-altitude Salvadoran varieties may behave more like Ethiopian hard beans.
Chapter 10 — Technique and Practical Applications
Consistency habits: weigh everything (not volumetric), control water composition, control grind temperature, preheat dripper consistently.
Gagné’s V60 reference method:
- Dose: 22 g
- Brew ratio: 1:17 (water:coffee by weight)
- Kettle temperature: 99°C for light roasts; 88–96°C for darker roasts
- Bloom: 45 seconds, nest-shaped bed
- Pours: bloom + 2 pours for plastic V60
- Swirl after each pour to reset channels and flatten bed
Brew ratio guidance: 1:16–1:17 typical; Kenyan coffees (high solubles) can go to 1:17.5–1:18; decaffeinated coffees better at 1:15–1:16. See Pourover Technique.
Chapter 11 — Instruments and Data
Refractometer (TDS measurement) is the primary instrument. Gagné details zeroing protocol, sample cooling (15 min in glass dropper), and alcohol prism cleaning. Bluetooth scales for pour rate tracking. Custom grind analysis app for PSD from scanned images. See Coffee Refractometer.
Key Contradictions / Tensions with Existing Wiki
- Extraction states ideal range as 18–22%. Gagné’s data and practice show good results up to ~23.5% with quality grinders — the upper bound has shifted. Updated in Extraction.
- Brew Water Quality framed alkalinity mainly as an equipment/flow issue. Gagné makes clear it is the most important parameter for flavor. Updated in Brew Water Quality.
- Grind lacked D₁₀ and hydraulic resistance concepts. Added in Coffee Bed Hydraulics and Grind.
New Pages Added by This Ingest
- Advection and Diffusion — physical mechanisms of extraction
- Channeling — causes, physics, and mitigation
- Coffee Bed Hydraulics — hydraulic resistance, D₁₀, fines migration, LRR
- Brew Water Crafting — mineral recipes and measurement methods
- Pourover Technique — consistency protocols and dial-in method