Brewing coffee is extraction

Every time you brew coffee you’re doing an extraction: using hot water as a solvent to dissolve soluble compounds from ground coffee. A roasted coffee bean contains about 30% soluble material — but you don’t want to extract it all. The magic is extracting the right amount of the right compounds.

This isn’t opinion or abstract art — it’s applied chemistry you can measure, control, and replicate.

The two numbers that matter

TDS (Total Dissolved Solids)

It’s the concentration of coffee solubles dissolved in your beverage, measured as a percentage. In simple terms: how “strong” your coffee tastes.

Espresso: between 8-12% TDS (dense, concentrated)
Filter coffee: between 1.15-1.45% TDS (light, transparent)

Measured with a refractometer — a small device that costs between $100-300 USD. It’s not essential to start, but when you want to take your extraction to the next level, it’s the most powerful tool you can have.

Extraction Yield (%)

It’s what percentage of the dry coffee weight you managed to dissolve. Calculated like this:

Extraction (%) = (TDS × beverage weight) / dry coffee weight × 100

The optimal range according to the SCA for filter coffee is 18-22%. For espresso, typically you work between 18-22% as well, though some competition baristas push to 23-24%.

What do these numbers mean in your cup?

Under-extraction (less than 18%): You didn’t pull out enough compounds. The coffee tastes aggressively acidic, salty, thin, with little body and a short aftertaste. Acids dissolve first, so if you stop too early, they dominate.
Optimal extraction (18-22%): You pulled out the right balance of acids, sugars, and bitter compounds. The coffee has pleasant acidity, present sweetness, balanced body, and a clean, lingering aftertaste.
Over-extraction (more than 22%): You pulled out too much. Bitter, astringent, and dry compounds (cellulose, ash) dominate. The coffee tastes bitter, rough, dry in the mouth — like when you leave black tea in hot water too long.

The extraction order

Coffee compounds don’t all dissolve at the same time. There’s a predictable order:

First: acids and salts — Dissolve quickly. If your coffee only tastes sour and acidic, you probably under-extracted.
Then: sugars and Maillard compounds — The sweetness, caramel, chocolate. This is the zone you want to reach.
Finally: bitter and astringent compounds — Some bitterness is desirable (balances acidity), but too much ruins the cup.

This explains something important: a coffee can be simultaneously under-extracted in some particles and over-extracted in others if your grind is inconsistent (many particles of different sizes). This inconsistency in particle size is called bimodal distribution and is the main reason a good grinder matters so much.

The 5 variables you control

1. Grind size

The most powerful variable. Finer grind = more surface exposed = faster and greater extraction. Coarser grind = less surface = slower and lesser extraction.

Rule of thumb: If your coffee tastes acidic/thin → grind finer. If it tastes bitter/rough → grind coarser. Adjust in small increments.

2. Ratio (coffee:water proportion)

How much coffee you use per how much water. Expressed as a ratio. Common standards:

Filter coffee: 1:15 to 1:17 (ex: 15g coffee to 250ml water)
Espresso: 1:1.5 to 1:2.5 (ex: 18g coffee to 36-45ml in cup)

A higher ratio (more water per gram of coffee) produces a more diluted beverage but with greater extraction. A lower ratio produces a more concentrated beverage but with lesser extraction.

3. Water temperature

Hotter water extracts faster and more. The SCA recommended ranges:

Filter coffee: 92-96°C (just below boiling)
Espresso: 88-94°C (depending on roast)

Light roasts generally need higher temperature because they’re denser and harder to extract. Dark roasts need less because they’re more porous and over-extract easily.

4. Contact time

How long the water is in contact with the ground coffee. In immersion methods (French press, Aeropress) you control this directly. In pour-over, you control it indirectly through grind size and pour speed.

Espresso: typically 25-35 seconds
Pour-over: 2:30-4:00 minutes
French press: 4:00 minutes
Aeropress: 1:00-2:00 minutes

5. Agitation

Any movement that accelerates extraction: stirring, pouring with more force, using an agitator. More agitation = faster extraction. The bloom in a pour-over (the first pour that releases CO2) is a form of controlled agitation.

How to use all this in practice

You don’t need a refractometer to start. You need a systematic method:

Fix your ratio (start with 1:16 for filter or 1:2 for espresso)
Fix your temperature (93°C for filter, 93°C for espresso)
Adjust grind as your main variable
Taste and adjust: Acidic/thin? Finer. Bitter/dry? Coarser.
Change only one variable at a time — if you change two, you won’t know which caused the effect

This process of adjusting variables until you find the optimal recipe is called dial-in and it’s probably the most important skill you’ll develop.

What you need for this lesson

A kitchen scale with 0.1g precision (you can find a decent one for less than $20 USD)
A thermometer or temperature-controlled kettle
Your usual brewing method (Chemex, espresso, whatever)
Your tasting journal

If you want to go further: a VST refractometer (~$150-300 USD) lets you measure TDS and calculate exact extraction.

Practical exercise

Brew three cups of the same coffee, changing only the grind size:

Fine grind (finer than you’d normally use)
Medium grind (your usual setting)
Coarse grind (coarser than usual)

Keep everything else the same: ratio, temperature, time, pouring technique. Taste the three and note the differences. Which is acidic? Which is bitter? Which is more balanced? This exercise trains you to connect what you control (grind) with what you perceive (flavor).

Key concepts from this lesson

Extraction = dissolving coffee compounds with hot water
TDS measures strength (concentration); % extraction measures how much of the coffee you dissolved
The optimal extraction range is 18-22% according to the SCA
Acids extract first, then sugars, then bitterness
The 5 variables: grind, ratio, temperature, time, agitation
Change one variable at a time; grind is the most powerful
A good grinder matters more than a good coffee maker

The science of extraction