Extraction

The Science Behind Coffee Extraction

Dr. Eleanor Webb · Coffee Scientist — March 2026 — ≈ 5 min read

1. What Extraction Actually Means

When hot water contacts ground coffee, it dissolves and carries away chemical compounds from the cellular structure of the bean. This process — extraction — is not uniform. Different compounds dissolve at different rates, and the sequence matters as much as the total amount extracted.

Acids and fructose dissolve first, within the opening seconds of contact. Sugars and Maillard products follow, contributing sweetness and body. Last to dissolve are the bitter phenolics and chlorogenic acid degradation products. A well-extracted cup captures the middle band and limits the last.

⚡ The SCA's target extraction yield for filter coffee is 18–22%. Below 18% produces sour, thin cups. Above 22% introduces harsh bitterness and a drying finish.

2. The Variables That Control Extraction Rate

Extraction rate is governed by six primary variables. Understanding each one gives you a systematic way to adjust without guessing.

Grind size: Finer grounds expose more surface area, accelerating extraction. Coarser grounds slow it.
Water temperature: Higher temperatures increase solubility and speed. The standard range is 90–96°C (194–205°F).
Contact time: Longer contact extracts more soluble material. Extending time past the sweet spot yields bitterness.
Agitation: Stirring or pouring technique disrupts stagnant water pockets and promotes even extraction.
Water-to-coffee ratio: More water relative to coffee dilutes concentration but can also extend extraction if contact time remains constant.
Pressure: Relevant primarily for espresso, pressure accelerates extraction dramatically — a 25-second shot extracts as much as several minutes of filter contact time.

3. Reading TDS and What the Numbers Mean

Total Dissolved Solids (TDS), measured with a refractometer, gives the concentration of your brew as a percentage. Combined with your brew weight data, TDS allows calculation of extraction yield — the most reliable single metric for cup quality.

A refractometer reading of 1.3–1.45% in a filter cup typically corresponds to an extraction yield of 19–21%, assuming standard brew ratios. Readings below 1.15% indicate a weak brew; readings above 1.55% are often overwhelming.

It is worth noting that TDS alone does not confirm quality — a 1.4% TDS from an over-extracted, bitter cup looks identical on the meter to a 1.4% TDS from a balanced, sweet cup. Always cup alongside measuring.

4. Applying This in Practice

The most consistent workflow is to fix all variables except one, adjust that variable until target TDS is reached, then evaluate the cup sensorially. If the numbers are good but the cup is sour, consider water temperature or distribution uniformity before changing dose.

BeanCraft's brew ratio calculator estimates TDS based on your inputs as a starting reference. It does not replace a physical refractometer, but it narrows the range of variables before your first pour.