A finished cup of coffee is roughly 98–99% water and 1–2% dissolved coffee compounds. That small percentage makes all the difference between a transcendent cup and a disappointing one. Understanding what's in that 2% — and how it gets there — is the foundation of everything in brewing.
The Compounds in Coffee
Coffee contains hundreds of chemical compounds that contribute to flavour, aroma, and body. The main categories:
- Organic acids — chlorogenic acid, citric acid, malic acid. These contribute brightness and perceived sourness. They extract early.
- Sugars and caramelised sugars — contribute sweetness and body. Extract in the middle of the process.
- Melanoidins — formed during roasting, contribute body and some bitterness. Extract later.
- Caffeine — bitter alkaloid. Extracts relatively early and evenly.
- Chlorogenic acid lactones and phenylindanes — the harsh, lingering bitterness of over-extracted coffee. These extract last.
The Extraction Sequence
Compounds don't extract simultaneously — they dissolve at different rates. The sequence is approximately: acids first, then sweetness, then body compounds, then bitter compounds last. This is why the same coffee can taste very different depending on how much you extract:
- Under-extracted (too little extraction): Mostly acids extracted, little sweetness or body → sour, sharp, hollow
- Properly extracted: Balanced acids, sweetness, and body → complex, balanced, pleasant
- Over-extracted (too much extraction): All compounds extracted including harsh bitters → dry, astringent, unpleasantly bitter
Measuring Extraction: TDS and EY
Professionals measure extraction with two metrics:
- TDS (Total Dissolved Solids): The percentage of dissolved coffee compounds in the final brew. Measured with a refractometer. Specialty coffee typically targets 1.15–1.45% TDS for filter coffee.
- EY (Extraction Yield): The percentage of the original coffee mass that was dissolved into the water. Target range is generally 18–22%.
You don't need to measure these at home. But understanding that they exist explains why recipes specify exact ratios, temperatures, and grind sizes — every variable affects what percentage of the bean ends up in your cup.
What Controls Extraction Rate
- Grind size — finer = more surface area = faster extraction
- Water temperature — hotter = faster extraction
- Contact time — longer = more extraction
- Agitation — more stirring/turbulence = faster extraction
- Water chemistry — certain minerals (particularly magnesium) enhance extraction of flavour compounds
Every brewing decision you make affects one or more of these variables. Understanding the framework lets you troubleshoot problems and adjust with intention rather than guesswork.