What is coffee extraction? Yield, TDS and why grind matters

How is extraction measured?

Extraction is measured as extraction yield: the percentage of the dry coffee's weight that dissolved into the drink. If 20 g of coffee gives up 4 g of dissolved material, the yield is 20%.

To get that number you need the drink's strength, called total dissolved solids or TDS. Counter Culture describes TDS as the objective measurement of a brew's strength. Professionals read it with a refractometer, then calculate yield from the TDS, the dose and the drink's weight.

Yield and strength are separate things. Strength is how concentrated the cup is. Yield is how much of the coffee you used. The brew ratio connects the two.

What extraction yield should you aim for?

The traditional answer is 18% to 22%. That range comes from the Brewing Control Chart, and Counter Culture pairs it with a strength of 1.15% to 1.35% for U.S. drinkers.

The range is a guide, not a law. Perfect Daily Grind notes that the sweet spot depends on personal preference and the coffee itself. Coffee ad Astra's Jonathan Gagné writes that yields usually fall between 19% and 23%. He adds that more uniform extraction can reach 22% to 24% without pulling too much of the bitter material.

Few home brewers own a refractometer. In practice, you judge extraction by taste and adjust from there.

What happens as coffee extracts?

Different compounds dissolve at different speeds. Perfect Daily Grind gives the usual order: fruity and acidic notes first, then sweetness and balance, then bitterness.

That is why a short or coarse brew tends toward sour, and a long or fine brew tends toward bitter. Gagné adds a caution: the idea that early compounds are mainly sour is reasonable but partly speculative. The broad pattern still matches what most brewers taste.

What controls extraction?

Four things do most of the work: grind size, brew time, water temperature and agitation. Wikipedia's summary of the research lists temperature, brew time, grind size and brewing method as the main factors.

• Grind size. Finer grounds expose more surface area, so they extract faster.
• Time. A longer brew extracts more.
• Temperature. Hotter water extracts more. Perfect Daily Grind recommends 195-205°F (about 91-96°C) for brewing.
• Agitation and bed evenness. Stirring, swirling and even pouring help all the grounds extract at the same rate.

Why does grind size matter so much for extraction?

Grind size is the variable that changes both speed and evenness. A finer grind extracts faster. In pour-over and espresso it also slows the flow, which lengthens the brew.

Evenness matters as much as size. Gagné explains that the inside of a coarse particle extracts more slowly than its surface. A grinder that makes a wide spread of sizes extracts some particles too much and others too little. The grind size distribution is the spec that describes this spread.

That is the practical case for a decent burr grinder. A blade grinder makes boulders and dust in one dose, so no single grind setting gets the whole bed right. See why your grinder matters more than your brewer for the longer argument.

Can a cup be over- and under-extracted at once?

Yes. Average yield can look fine while parts of the bed are off in both directions. Perfect Daily Grind describes this in espresso: channeling lets water rush through weak spots, over-extracting them while denser areas under-extract.

The cup then tastes "bitter, sour, and muddy" at the same time. The fix is usually better puck prep or a more even grind, not a single big change in grind size. For the one-way problems, see under-extraction and over-extraction.