Coffee: Extraction Chemistry — Yield, TDS, and Solubles
The SCA Brewing Control Chart defines ideal coffee extraction at 18–22% solubles yield and 1.15–1.35% total dissolved solids (TDS) for specialty-grade brewed coffee.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| SCA ideal extraction yield | 18–22 | % of dry coffee mass | Solubles dissolved into final brew as a percentage of original dry grounds weight |
| SCA ideal TDS (brewed coffee) | 1.15–1.35 | % | Total dissolved solids in the cup; specialty standard |
| Under-extraction yield | <18 | % | Results in sour, salty, hollow flavor profile |
| Over-extraction yield | >22 | % | Results in bitter, astringent, drying finish |
| Espresso TDS target | 8–12 | % | Much higher concentration than filter coffee; same yield range applies |
| Soluble fraction of dry coffee mass | ~28–32 | % | Maximum theoretical extractable mass; never fully achieved in practice |
| Typical brew ratio (filter coffee) | 1:15–1:17 | coffee:water (by mass) | 60–67g per liter; SCA standard reference |
| Typical brew ratio (espresso) | 1:2–1:2.5 | coffee:water (by mass) | E.g., 18g dose yields 36–45g espresso |
Coffee extraction is the controlled dissolution of soluble compounds from roasted, ground coffee into water. Of the approximately 28–32% of dry coffee mass that is theoretically soluble, the goal is to dissolve the right fraction — the 18–22% that delivers balanced flavor — and leave the insoluble compounds (cellulose, certain proteins, insoluble fibers) behind.
The Chemistry of Extraction Order
Compounds do not extract simultaneously. Water dissolves different molecular species at different rates, governed by their polarity, molecular weight, and solubility. The general sequence during extraction is:
- Organic acids — acids extract first, rapidly; they are highly water-soluble and small-molecule. This is why short or weak extractions taste sour.
- Caffeine and sugars — extract in the early-to-mid phase; caffeine especially fast (see caffeine-solubility)
- Melanoidins and complex sugars — contribute body and sweetness; extract in the mid-to-late phase
- Bitter phenolic compounds, chlorogenic acid lactones — extract last; these cause the drying bitterness of over-extracted coffee
The practical implication: a little more extraction moves a sour cup toward balance; too much tips into bitterness.
The Brewing Control Chart
The SCA Brewing Control Chart, first developed from E.E. Lockhart’s 1957 Coffee Brewing Institute research and later formalized by the SCAA, maps extraction yield (X-axis) against TDS (Y-axis) to identify the “ideal” brew zone.
| Zone | Extraction Yield | TDS | Flavor Result |
|---|---|---|---|
| Under-extracted | <18% | Any | Sour, salty, hollow, thin |
| Ideal (SCA) | 18–22% | 1.15–1.35% | Balanced, sweet, complex |
| Over-extracted | >22% | Any | Bitter, drying, astringent |
| Weak (low TDS) | 18–22% | <1.15% | Watery, flat despite correct yield |
| Strong (high TDS) | 18–22% | >1.35% | Intense, possibly overwhelming |
| Ideal espresso | 18–22% | 8–12% | Syrupy, concentrated |
Note that espresso targets the same extraction yield range (18–22%) but at much higher TDS (8–12%) due to the low brew ratio (typically 1:2 coffee to water by mass).
Variables That Control Extraction
Extraction yield is determined by multiple interacting variables:
| Variable | Effect on Extraction |
|---|---|
| Grind size | Finer = more surface area = faster and higher extraction |
| Water temperature | Higher temp = faster dissolution of more compounds |
| Contact time | Longer contact = more solubles dissolved |
| Agitation / turbulence | Increases mass transfer; refreshes water at coffee surface |
| Brew ratio | More water per gram of coffee = dilutes but also increases extraction |
| Water chemistry | Minerals (especially Mg²⁺) enhance specific compound extraction |
| Roast level | Darker roasts are more porous, extract faster |
Calculating Extraction Yield
Extraction yield requires a refractometer reading (TDS in %) and a record of your brew:
Yield% = (TDS% × Brewed Beverage Weight in grams) / Dry Grounds Weight in grams
Example: 20g grounds, 300g brewed coffee, 1.4% TDS reading: Yield = (0.014 × 300) / 20 = 4.2 / 20 = 21% — within the SCA ideal range.
This calculation is used by specialty cafes and competition baristas to dial in and replicate consistently excellent cups. For home brewers, a consistent brew ratio (e.g., 1:15 to 1:17 by weight) combined with attention to grind size and temperature will produce results within the ideal zone without requiring a refractometer.
Related Pages
Sources
- Specialty Coffee Association — SCA Brewing Control Chart (scaa.coffee)
- Specialty Coffee Association — Water Quality Standards
- Lockhart EA (1957) — The soluble solids in beverage coffee as an index to the cup quality. Coffee Brewing Institute
- Rao S (2008) — The Professional Barista's Handbook. Scott Rao
Frequently Asked Questions
What does extraction yield mean?
Extraction yield is the percentage of dry coffee grounds that dissolves into the final brewed beverage. If you start with 20g of ground coffee and 4g of dry matter ends up in the cup, your extraction yield is 20%. The SCA defines 18–22% as the ideal specialty range — below this is under-extracted (sour, thin), above it is over-extracted (bitter, drying). Practically, you measure yield by weighing your dry grounds, then measuring TDS with a refractometer and using the formula: Yield% = (TDS% × Brew Weight) / Dry Grounds Weight.
What is TDS and how do I measure it?
TDS (Total Dissolved Solids) is the concentration of dissolved coffee solubles in the final cup, expressed as a percentage by mass. The SCA target for brewed filter coffee is 1.15–1.35%. You measure TDS with a digital refractometer (or a high-quality brix meter calibrated for coffee) — a $50–$300 instrument that reads the refractive index of the liquid. Home brewers can approximate TDS through consistent brew ratios; specialty shops use refractometers for quality control.
Why does my coffee taste sour? How do I fix it?
Sourness indicates under-extraction — you're dissolving fewer than 18% of the coffee grounds' soluble mass. The first compounds to extract are organic acids (citric, malic, chlorogenic), which taste bright and sour. If extraction stops early, these dominate without the balancing sweetness and body that come from longer extraction. Fixes: use a finer grind (increases surface area and extraction rate), increase water temperature (toward 94–96°C), extend contact time, or increase dose relative to water. In espresso, also check pre-infusion and pressure profiling.
What does over-extraction taste like, and how do I fix it?
Over-extraction produces bitter, astringent, drying, and hollow-dry flavors. After sugars and pleasant aromatic compounds extract (at 18–22% yield), continued extraction pulls harsh phenolic compounds, chlorogenic acid lactones, and other bitter constituents. Fixes: coarser grind, lower water temperature (toward 88–92°C for problematic beans), shorter contact time, or lower dose relative to water. In espresso, shorten the shot time or increase grind coarseness.