Kaffeemacher|29. Aug.
Refractometer explained: Understanding TDS and extraction simply
Basic Principle of the Refractometer
Practical Exercises
A refractometer measures the concentration of dissolved coffee particles (TDS = Total Dissolved Solids) as a percentage. This indicates how much coffee is relative to water in the cup, e.g., 6.18% – the rest is water.
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Typical TDS values of beverages:
- Espresso: usually between 6–8% (or up to 10–12%)
- Coffee cream: about 1.8–5%
- Filter coffee: about 1.2–1.5%
- Ristretto: often above 10%
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The measured TDS value says nothing about quality, only about strength.
- Before measurement, calibrate the refractometer with distilled water (zero button).
- The coffee sample must be well mixed (including crema) and filtered (paper filter) to remove fine particles.
- The sample temperature must match the lens temperature – for espresso, allow to cool if needed.
- Solids on the lens cause inaccurate measurements – keep the lens clean.
Extraction (E) is calculated as:
Extraction (%) = TDS (%) × Beverage weight (g) / Coffee dose (g)
Example from the video:
- TDS: 6.18%
- Beverage: 52 g
- Coffee: 18 g
- Extraction ≈ 17.8% (relatively low; typical target for espresso: 18–22%)
- Plots strength on the Y-axis and extraction on the X-axis.
- Helps categorize brews, e.g., under- or over-extraction.
- Originally developed for filter coffee (Golden Cup Standard: extraction 18–22%, strength 1.2–1.5%).
- Now also flexibly applicable to espresso.
| Model | Price (approx.) | Accuracy | Special Features |
|---|---|---|---|
| VST | ~€1,000 | Very high (±0.02% for filter) | Since 2008, robust, own app |
| Atago | ~€450 | ±0.15% | Much more affordable, good everyday use |
| D-Fluid | ~€250 | Sufficient for daily use | Democratic, app with charting, compatible with Bean Conqueror |
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Same strength, different extractions:
- Log TDS (e.g., 1.35%) and then brew variations with extractions from 14% to 28% – learn taste differences.
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Same extraction, different strengths:
- Aim for 20% extraction, then dilute the espresso with water (to e.g., 8%, 6%, 4%, 1.3%, 0.8%) – understand how strength affects flavor.
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Competition preparation:
- Check consistent brewing patterns: same ratio, same movement – measure TDS → identify and correct deviations.
- The refractometer is a tool for consistency and understanding, not for taste evaluation.
- It helps optimize brew parameters, develop recipes, and train sensory skills.
- Particularly useful in gastronomy, for competitions, or for personal education.
Note: All units are already in the metric system or percentages.
