Quick answer: Coffee extraction science comes down to two numbers. TDS (total dissolved solids) is how strong the cup is, and extraction yield is the share of the dry coffee that dissolved into it. They are linked by the brew ratio: yield = TDS × beverage weight ÷ dry coffee weight. The SCA’s classic “ideal” zone is 1.15–1.35% TDS at 18–22% extraction. Grind, time, temperature, agitation and water all move you around that chart, and newer UC Davis research shows flavour depends mostly on where you land, not on any single variable.
This is the theory page: what extraction is, how it is measured, what the Coffee Brewing Control Chart really says, and how each brewing variable changes the result. If you want the hands-on version for espresso, go to our practical espresso extraction workflow. If your shot is sour or bitter right now, the espresso dial-in guide tells you what to change next.
Want to put this theory to work on espresso? Our short video course Dial In Espresso by Taste shows you how to taste a shot, tell sour from bitter and change one variable at a time.
What “Extraction” Actually Means
Brewing is a solid–liquid extraction: water dissolves soluble material out of roasted, ground coffee and carries it into the cup. Only part of a coffee bean can dissolve. In their espresso study, Cameron and colleagues note that the experimental upper limit of extraction is roughly 30% of the coffee’s mass, and that around 2,000 different compounds come out of the grounds during brewing (Cameron et al., Matter, 2020).
Those compounds do not all dissolve at the same speed. Research reviewed by the UC Davis Coffee Center shows that many components extract early and then level off, while some flavour compounds extract more slowly. As you keep adding water, the fast extractors get diluted and the slow ones keep accumulating, so the balance of the cup shifts over the brew (Batali et al., Scientific Reports, 2020). Organic acids are among the fast, highly soluble group, which helps explain why short, under-extracted brews tend to taste sour.
The Two Numbers: TDS and Extraction Yield
| Measure | What it tells you | How you get it |
|---|---|---|
| TDS (brew strength) | Percentage of the drink that is dissolved coffee. The rest is water. | A coffee refractometer reading on a cooled, filtered sample |
| Extraction yield (EY, also “percent extraction”) | Percentage of the dry grounds that ended up dissolved in the drink | Calculated: TDS × beverage weight ÷ dry coffee weight |
| Brew ratio | How much water you use per gram of coffee | Set by you on the scale; it links TDS and yield |
Worked example: you brew 15 g of coffee and end up with 250 g of filtered coffee in the carafe. The refractometer reads 1.30% TDS. Extraction yield = 1.30 × 250 ÷ 15 = 21.7%. That lands inside the classic ideal box on both axes.
Two practical notes. First, always weigh the finished beverage rather than the water you poured, because wet grounds keep some liquid back. Second, a refractive index reading depends on temperature, so cool the sample before measuring (Cameron et al., 2020). In the UC Davis studies, samples were cooled to room temperature in sealed containers before being read on a digital refractometer (Batali et al., 2020).
The SCA Brewing Control Chart, Explained
The Coffee Brewing Control Chart comes from biochemist Ernest Lockhart’s work at the Coffee Brewing Institute in the 1950s. It plots TDS on the vertical axis, extraction yield on the horizontal axis and brew ratio as diagonal lines, and it remains the core of the SCA’s brewing education (SCA, “Towards a New Brewing Chart”). Its central “ideal” region spans 1.15–1.35% TDS and 18–22% extraction, often called the SCA Golden Cup Standard (Batali et al., 2020).
| Zone on the classic chart | Range | Label on the chart |
|---|---|---|
| Left side | Extraction below 18% | “Under-developed” (usually read as sour or grassy) |
| Right side | Extraction above 22% | “Bitter” |
| Top band | TDS above 1.35% | “Strong” |
| Bottom band | TDS below 1.15% | “Weak” |
| Centre | 1.15–1.35% TDS, 18–22% EY | “Ideal – optimal balance” |
Because the three quantities are tied together by mass balance, you cannot change one without moving another. Use more water per gram (a longer ratio) and TDS goes down, even if yield stays similar. That is why our brew ratio calculator is the simplest control you have over strength.
What Newer Research Changes
Between 2017 and 2023, the UC Davis Coffee Center ran sensory studies with trained panels, brewing the same coffees to nine target points across the chart (Guinard et al., Journal of Food Science, 2023). The main findings:
- Sourness rose with TDS and fell as extraction increased, peaking at high strength and low yield. That broadly agrees with Lockhart’s “under-developed” label (SCA).
- Bitterness rose with TDS. In the brew-temperature study it was tied to TDS alone, not to extraction yield, which differs from what the classic chart implies (Batali et al., 2020).
- Sweetness was highest at low TDS and low extraction in the roast-level study, so “weak” on the old chart is not simply worse (Frost et al., 2020).
- Preferences vary. A consumer study found that people’s liking for black coffee was spread across a wide range of strengths and yields, so one “ideal” box cannot fit everyone (J. Food Sci., 2020).
The takeaway: the chart is still a useful map, but treat the centre box as a starting point and steer toward the flavours you want.
The Variables That Move Extraction
| Variable | Main effect | What the evidence says |
|---|---|---|
| Brew ratio | Mostly strength (TDS) | Fixed by mass balance: more water per gram lowers TDS |
| Grind size | Speed of extraction and flow | Finer grinds expose more surface and slow flow, but in espresso grinding too fine caused uneven flow and lower, more variable yields (Cameron et al., 2020) |
| Contact time / flow rate | Extraction yield | Longer contact gives slow-extracting compounds more time; UC Davis reached higher yields with longer brews (Batali et al., 2020) |
| Water temperature | Speed of extraction | At equal TDS and yield, 87–93 °C made little sensory difference in drip coffee (Batali et al., 2020) |
| Water minerals | What dissolves and how acidity reads | See our water chemistry guide |
| Bed evenness / agitation | Uniformity of extraction | Uneven flow over- and under-extracts parts of the bed at once (Cameron et al., 2020) |
Grind size and surface area
Grinding breaks the bean’s cell structure and exposes soluble material. Smaller particles have more surface per gram and a shorter path for solubles to travel out, so they extract faster. Grinders do not make one size: Cameron and colleagues measured a two-part distribution of “boulders” set by the burr gap and much smaller “fines” (2020). A more uniform grind makes extraction more predictable.
Temperature
Hot water speeds up dissolution, and the SCA’s brewer certification requires machines to reach 92 °C within the first minute and stay at or below 96 °C. However, when the UC Davis team adjusted grind and flow to hit the same TDS and yield at 87, 90 and 93 °C, trained tasters found almost no difference (Batali et al., 2020). In practice, temperature is one more lever for reaching a target, not a magic setting.
Flow and channeling
Water takes the easiest path through a coffee bed. Where it runs fast, grounds over-extract; where it barely reaches, they under-extract. The averaged cup can then taste sour and bitter at once. In espresso, Cameron and colleagues found that past a certain fineness, extraction yield actually dropped because parts of the puck were effectively not extracted (2020).
Immersion vs Percolation
In immersion brewing (French press, AeroPress, cupping), all the water sits with all the grounds. As the liquid fills up with dissolved coffee, the concentration difference that drives extraction shrinks, so extraction slows on its own. That makes immersion forgiving.
In percolation (pour-over, batch brew, espresso), fresh water keeps arriving at the top of the bed, which keeps the concentration difference high and extraction efficient. The trade-off is sensitivity: any unevenness in the bed shows up as channeling. Our guides to Chemex filter physics and moka pot extraction look at two percolation brewers in detail.
Bypass: Changing Strength Without Changing Extraction
Bypass means adding water that never touches the grounds. It lowers TDS but leaves extraction yield unchanged, because no extra coffee dissolves. Example: 20 g of coffee yields 250 g of brew at 1.60% TDS, an extraction of 1.60 × 250 ÷ 20 = 20%. Add 60 g of hot water and you have 310 g at about 1.29% TDS, still at 20% extraction. Many batch brewers and iced-coffee recipes use this idea, and it is the same logic as diluting cold brew concentrate in our cold brew guide.
Putting the Science to Work
- Fix your dose and brew ratio and write them down.
- Brew, weigh the beverage, and (if you have one) measure TDS on a cooled sample.
- Calculate extraction yield and note where you land on the chart.
- Taste. Sour and thin usually means push extraction up (finer grind or longer contact). Harsh and drying usually means bring it down.
- Change one variable at a time so you know what caused the difference.
For espresso, the same principles apply with much less margin for error. That is where the espresso extraction workflow picks up.
Frequently Asked Questions
What is a good extraction yield for coffee?
The classic SCA Golden Cup range is 18–22% extraction yield, paired with 1.15–1.35% TDS for filter coffee. Treat it as a starting point: UC Davis research found that flavour changes smoothly across the chart and that consumer preferences are spread over a wide range of strengths and yields.
How do you calculate extraction yield?
Extraction yield (%) = TDS (%) × weight of the brewed coffee (g) ÷ weight of the dry coffee (g). Weigh the finished beverage, not the water you poured, because the spent grounds hold some water back.
Do I need a refractometer to use extraction science?
No. A scale and your palate get you most of the way: keep the brew ratio fixed and adjust grind, then taste. A refractometer adds a number for TDS, which makes changes easier to compare and record, but it cannot tell you whether you like the cup.
Does water temperature matter for extraction?
Hotter water extracts faster, so temperature changes how quickly you reach a given TDS and yield. But a 2020 UC Davis study found that when TDS and extraction yield were held constant, brewing at 87, 90 or 93 °C made little difference to how drip coffee tasted.
Why can coffee taste sour and bitter at the same time?
Usually because water flowed unevenly through the bed (channeling). Some grounds were over-extracted and others barely touched, so the cup averages two problems. Better distribution and a more even grind are the fix. Our guide to espresso channeling shows how to spot it and correct it.
Keep Learning
- Espresso extraction workflow: a practical, shot-by-shot method
- Espresso dial-in guide: fix sour, bitter, fast or slow shots
- Water chemistry for better extraction
- Brew ratio calculator
Sources
- Batali, M. E., Ristenpart, W. D. & Guinard, J.-X. (2020). Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee. Scientific Reports 10, 16450.
- Frost, S. C., Ristenpart, W. D. & Guinard, J.-X. (2020). Effects of brew strength, brew yield, and roast on the sensory quality of drip brewed coffee. Journal of Food Science 85, 2530.
- Guinard, J.-X. et al. (2023). A new Coffee Brewing Control Chart relating sensory properties and consumer liking to brew strength, extraction yield, and brew ratio. Journal of Food Science.
- Consumer preferences for black coffee are spread over a wide range of brew strengths and extraction yields. Journal of Food Science (2020).
- Specialty Coffee Association. Towards a New Brewing Chart. 25 Magazine, Issue 13.
- Cameron, M. I. et al. (2020). Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter 2, 631–648.
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