Extraction Science & The Grind

Extraction Yield

Measured as a percentage of dry coffee weight dissolved into the brew. The SCA target range for properly extracted filter coffee is 18 to 22%.

• Under 18%: Underextracted. Sour, sharp, salty, thin. The sugars and body-building compounds have not fully dissolved.
• 18 to 22%: The target window. Balance of acidity, sweetness, body, and bitterness.
• Over 22%: Overextracted. Bitter, astringent, woody, dry, harsh. Undesirable high-molecular-weight compounds have dissolved.

Some modern specialty roasters push extraction to 23 to 25% with very uniform grinders and precise water chemistry. The 18 to 22% window is a guideline, not a law.

Brew Strength (TDS)

TDS of brewed coffee (not to be confused with water TDS) measures the concentration of dissolved coffee solubles in the final beverage. Expressed as a percentage.

Method	TDS Range	Character
Filter coffee	1.15 to 1.45%	Light, clean, drinkable
Strong filter	1.45 to 1.60%	Bold, heavy body
Espresso	7 to 12%	Concentrated, viscous
Ristretto	12 to 15%	Syrupy, intense
Cold brew concentrate	3 to 6%	Dilute before drinking

TDS and extraction yield are related but distinct. You can have high extraction at low TDS (dilute but well-extracted) or low extraction at high TDS (concentrated but underextracted). The brew ratio connects them.

How to Read It

The Brewing Control Chart (developed by the Coffee Brewing Institute in the 1950s, refined by SCA) plots strength (TDS %) on the vertical axis against extraction yield (%) on the horizontal axis. Diagonal lines represent different brew ratios.

The center rectangle is the "Gold Cup" zone: 1.15 to 1.45% TDS, 18 to 22% extraction. Brews that land here are considered balanced by the majority of tasters in SCA research panels.

• Top-left quadrant: Strong and underextracted. Intense but sour.
• Top-right quadrant: Strong and overextracted. Intense and bitter.
• Bottom-left quadrant: Weak and underextracted. Thin and sour.
• Bottom-right quadrant: Weak and overextracted. Watery and bitter.

The Formula

Extraction yield connects TDS to the brew ratio:

Example: 18g dose, 300g brew weight, TDS 1.35%.

This brew is at the upper edge of the SCA target. Slightly overextracted by the standard, but many specialty coffees taste excellent here.

Note: brew weight is not the same as water weight. Some water is absorbed by the grounds (typically 2x the dose). For 18g dose and 300g water in, brew weight is approximately 264g out.

Phase 1: Acids

Extracts first

Organic acids (citric, malic, acetic, chlorogenic) are small molecules that dissolve quickly. They produce brightness, fruitiness, and sharpness. A very short extraction tastes dominantly sour.

Phase 2: Sugars

Extracts second

Sucrose, fructose, and caramelization products (from roasting) dissolve next. They produce sweetness and round out the acidity. The best cups have a balance of acid and sugar that reads as "juicy."

Phase 3: Lipids & Melanoidins

Extracts third

Oils and Maillard reaction products contribute body, mouthfeel, chocolate and caramel tones, and perceived "richness." They also carry many aromatic volatile compounds.

Phase 4: Bitter Compounds

Extracts last

Chlorogenic acid lactones, quinic acid, and high-molecular-weight phenolic compounds. They produce bitterness, astringency, woody dryness, and a harsh finish. Over-extraction pulls too much of this phase.

Grind Size

The most powerful extraction variable. Finer particles have more surface area per gram, meaning more contact between water and coffee. Doubling the surface area roughly doubles the extraction rate. Grind is always the first thing to adjust.

Water Temperature

Higher temperatures increase the kinetic energy of water molecules, accelerating dissolution. Every 1C increase raises extraction by roughly 1% of yield. Range: 85C (low extraction, used for dark roasts) to 100C (maximum extraction, used for light roasts).

Contact Time

The longer water is in contact with coffee, the more solubles dissolve. In pour-over, contact time is controlled by grind size and pour rate. In immersion, it is controlled by steep time. In espresso, it is a function of grind size and pressure.

Agitation

Stirring, swirling, turbulent pouring. Agitation moves fresh solvent to the surface of coffee particles and displaces saturated water. More agitation increases extraction rate. Bloom stirring, mid-brew swirls, and pour height all add agitation.

Pressure

Relevant mainly to espresso and moka pot. Pressure forces water through a compressed puck, increasing contact efficiency and enabling extraction from very fine grounds in a very short time. 9 bars is the espresso standard. Higher pressures do not proportionally increase extraction.

Brew Ratio

The ratio of coffee to water (by weight). More water per gram of coffee increases extraction yield (more solvent to dissolve solubles) but decreases concentration (TDS). A 1:15 ratio is strong, 1:17 is standard, 1:18+ is light. Ratio shifts you diagonally on the Brewing Control Chart.

Particle Size Distribution

No grinder produces perfectly uniform particles. Every grind contains a distribution of sizes. The ideal is a tight, unimodal distribution: most particles near the target size, with few outliers.

Bimodal distribution: Most burr grinders produce two peaks: a main peak near the target size and a secondary peak of "fines" (very small particles, often under 100 microns). This bimodal pattern means some particles over-extract (fines) while others under-extract (boulders) simultaneously.

Fines: Particles under ~100 microns. They extract almost instantly and contribute bitterness, astringency, and sludge. In espresso, fines migrate to the bottom of the puck and restrict flow. In pour-over, they clog the filter and slow drawdown.

Boulders: Particles much larger than target. They under-extract, contributing sour, underdeveloped flavors. They also increase channeling risk in espresso because water flows around them easily.

Measuring Particle Size

Laser diffraction particle analysis: The scientific gold standard. A laser passes through a suspended sample of ground coffee. The diffraction pattern reveals the full particle size distribution. This is what grinder engineers and researchers use. Not practical for home use.

Kruve Sieve System: A set of precision sieves with mesh sizes from 200 to 1600 microns. You grind a sample, sieve it, and weigh the fractions. This tells you what percentage of your grind falls in each size bracket. Practical for enthusiasts who want to quantify grinder performance or remove fines/boulders.

Practical calibration: For most people, the test is the cup. If drawdown is too fast and the coffee tastes sour, grind finer. If drawdown is too slow and the coffee tastes bitter, grind coarser. A consistently even bed after brewing (no craters, no high-and-dry zones) indicates good distribution.

Flat Burrs

Two parallel discs with cutting teeth. Coffee enters at the center and is flung outward by centrifugal force, passing through progressively finer cutting zones. The particles exit at the perimeter.

• Flavor character: Clarity, separation, brightness. Each flavor note reads distinctly. Less blending of flavors.
• Distribution: Tighter unimodal distribution (fewer fines and boulders) at larger burr sizes (64mm+).
• Heat: Higher RPM generates more heat. Large flat burrs (83mm+) mitigate this with mass.
• Retention: Higher retention (more grounds left in the grinding chamber between doses). Purging is necessary.

Common sizes: 54mm (entry), 64mm (mid-range), 75mm (prosumer), 83mm and 98mm (commercial).

Conical Burrs

A cone-shaped inner burr rotates inside a ring-shaped outer burr. Coffee is pulled downward by gravity through a narrowing gap. Lower RPM, lower heat.

• Flavor character: Body, sweetness, roundness. Flavors blend and meld. Traditional espresso character.
• Distribution: Broader bimodal distribution (more fines). The fines contribute body and crema in espresso.
• Heat: Lower RPM means less heat generation. Better for heat-sensitive light roasts.
• Retention: Lower retention due to gravity-assisted exit path.

Conical burrs are found in most hand grinders and many high-end espresso grinders. The Mazzer Robur and Mahlkonig EK43s use large conical burrs.

Ghost Burrs & SSP Geometries

Ghost burrs: Aftermarket and custom burr designs that modify the cutting geometry for specific goals. Some optimize for unimodal distribution (filter brewing), others for bimodal (espresso body).

SSP (Sim Sung Precision): A Korean burr manufacturer that produces aftermarket flat burrs in several geometries:

• High Uniformity (HU): Tightest distribution. Maximum clarity. Designed for filter coffee and competition.
• Cast (v1/v2): Moderate fines production. Balanced for both espresso and filter.
• Multi-Purpose (MP): Intentional bimodal distribution for espresso body with decent filter performance.
• Lab Sweet (LS): Emphasizes sweetness. A middle ground between HU and MP.

Alignment & Retention

Alignment: The two burrs must be perfectly parallel. Misalignment means one side of the burr set has a wider gap than the other. This produces more boulders and fines, broadening the distribution. Factory alignment varies. Enthusiasts shim or lap burrs for perfect alignment.

Retention: Grounds that remain in the grinding chamber between uses. Retained grounds go stale and contaminate the next dose. Low-retention grinder designs use steep chutes, bellows, or single-dose workflow (weigh beans in, grind, verify output weight).

Exchange: Related to retention. When you grind a new dose, some of the output is actually old retained grounds from the previous dose, and some of the current dose is retained for next time. For consistency, purge 2 to 3g before your first shot of the day, or use a single-dose workflow where you weigh both input and output.

How to Measure

1. Stir the brew thoroughly. TDS stratifies. The top of the cup is different from the bottom. Mix completely before sampling.
2. Let the sample cool to below 40C (most refractometers are calibrated for room temperature). Use a syringe to draw a small sample and let it cool.
3. Filter the sample through a syringe filter (0.45 micron) to remove suspended solids. Unfiltered samples read higher because particles scatter light.
4. Zero the refractometer with distilled water.
5. Place 2 to 3 drops on the lens. Close the lid. Wait 10 to 15 seconds for temperature equalization. Read the TDS value.
6. Take 2 to 3 readings and average. Clean the lens between readings with distilled water.

Common Refractometers

• VST CoffeeTools LAB III: The industry reference. Optical refractometer with coffee-specific calibration. Accurate to +/- 0.01% TDS. Used in competition and research. ~$700.
• Atago PAL-COFFEE: Digital refractometer with coffee-specific scale. Several models: PAL-COFFEE (TDS), PAL-COFFEE BX/TDS (dual scale). Accurate, portable, fast. ~$300 to 450.
• DiFluid R2 Extract: Compact digital refractometer with Bluetooth and app. Lower cost (~$100 to 150), good accuracy for the price. Becoming popular in the enthusiast market.

A refractometer is not essential for making good coffee. Your palate is the final judge. But it is essential for understanding why a coffee tastes the way it does and for making systematic, reproducible improvements.

Sour, Sharp, Acidic

Diagnosis: Underextracted. The acids dissolved but the sugars did not.

• Grind finer (first adjustment to try)
• Increase water temperature by 2 to 3C
• Increase brew time (slower pour, longer steep)
• Increase agitation (more stirs, more turbulent pour)
• Check that coffee is not stale (underextraction increases with age as CO2 degasses and the porous structure collapses)
• Check water hardness (very soft water under-extracts)

Bitter, Ashy, Dry

Diagnosis: Overextracted. Too many bitter compounds dissolved.

• Grind coarser
• Decrease water temperature by 2 to 3C
• Decrease brew time (faster pour, shorter steep)
• Reduce agitation
• If espresso: reduce yield (shorter shot, lower ratio)
• Check for channeling (espresso)
• Check water alkalinity (high alkalinity produces flat bitterness)

Flat, Muted, Lifeless

Diagnosis: Often a water issue. High alkalinity suppresses acidity.

• Measure water TDS and alkalinity. If alkalinity is over 80 ppm, switch to softer water or use mineral-controlled water.
• Check freshness. Coffee older than 4 to 6 weeks loses volatile aromatics rapidly.
• Try increasing temperature (more aromatics released)
• Try a finer grind to increase extraction, pushing past the "hollow" zone

Thin, Watery, Weak

Diagnosis: Low concentration (low TDS). The extraction may be fine, but there is not enough coffee relative to water.

• Increase dose (use more coffee)
• Tighten ratio (1:15 instead of 1:17)
• If extraction yield is also low, grind finer to increase both
• Check for bypass (water running around the coffee bed instead of through it)

Sour AND Bitter (Espresso)

Diagnosis: Channeling. Parts of the puck are overextracted (bitter) while others are underextracted (sour).

• Improve puck prep: WDT, even distribution, level tamp
• Use a bottomless portafilter to diagnose visually
• Increase pre-infusion time and decrease main pressure
• Check grinder alignment and burr condition

Papery, Cardboard, Stale

Diagnosis: Old coffee, poor storage, or unrinsed paper filter.

• Check roast date. Use coffee within 7 to 28 days of roasting for filter, 10 to 35 days for espresso.
• Store in an airtight, opaque container at room temperature. Never freeze repeatedly.
• Rinse paper filters with hot water before brewing
• Check grinder retention (old grounds contaminating fresh doses)