tags: [] - coffee/brewing - coffee/science aliases: - Coffee extraction science - Science of coffee extraction - Extraction chemistry and physics

Extraction Science¶

Tags: #coffee/brewing #coffee/science Aliases: Coffee extraction science, Science of coffee extraction, Extraction chemistry and physics Related: Coffee Extraction Fundamentals MOC | Extraction Fundamentals | Extraction Chemistry | Extraction Optimisation | Precision Dialling Status: ✅ Complete

Overview¶

Extraction science is the underlying chemistry and physics of how coffee compounds dissolve into water. It extends beyond the conceptual framework of Extraction Fundamentals into the specific mechanisms, molecular behaviour, and measurement techniques that underpin advanced dialling and recipe development. Understanding extraction at this level allows practitioners to predict how recipe changes will affect specific flavour compounds, rather than relying solely on trial and error.

The Chemistry of Coffee Extraction¶

Coffee contains over 1,000 identified chemical compounds. The subset that dissolves into brew water includes:

Organic acids (~2% of dry weight): - Chlorogenic acids (CGAs): the largest group; converted during roasting to quinic and caffeic acids; astringent and bitter at high concentrations - Citric acid: bright, clean citrus character - Malic acid: soft, apple-like acidity - Acetic acid: vinegar character; positive at very low levels, defect at higher concentrations - Phosphoric acid: bright, mineral quality; particularly prominent in Kenyan coffees - Lactic acid: soft, creamy acidity; produced in fermented-process coffees

Sugars and carbohydrates (~40% of green coffee): Raw sucrose survives into the cup only minimally — most is caramelised or converted during roasting. Caramelisation and Maillard reaction products (melanoidins, furaneol, acetaldehyde) contribute perceived sweetness in the cup.

Bitter compounds: - Caffeine (~1.2% in arabica, ~2.7% in robusta): clean, relatively mild bitterness; extracts at all temperatures - Quinic acid (from CGA breakdown): harsh, dry bitterness; increases with roast level - Diketopiperazines: bitter peptides formed during roasting - Melanoidins: large Maillard products; contribute both colour and bitterness

Lipids and oils: Oils remain in suspension after espresso extraction (emulsified by pressure) but are largely removed by paper filtration in filter methods. Cafestol and kahweol are diterpene lipids that contribute body.

Extraction Kinetics¶

Different compound classes extract at different rates due to molecular size and solubility:

Molecular size and polarity: Smaller, more polar molecules dissolve more readily. Organic acids are small and polar — they extract quickly. Melanoidins are large — they extract more slowly.
Temperature dependence: Most coffee compounds become more soluble at higher temperatures. Some volatile aromatics dissolve more readily at lower temperatures, which contributes to cold brew's distinct flavour profile.
The extraction curve: If concentration of each compound were plotted against time, each would show a different curve. Acids peak early; sweetness compounds dominate the mid-extraction phase; bitter compounds continue rising throughout. The goal is to cut extraction at the balanced midpoint.

Measurement: TDS and Extraction Yield¶

A refractometer measures the refractive index of a liquid — the degree to which light bends when passing through it. More dissolved material yields a higher refractive index, enabling calculation of TDS (Total Dissolved Solids) as a percentage.

Extraction yield (EY) is calculated from TDS:

EY (%) = (Brewed weight × TDS%) ÷ Dose × 100

Example: 36 g espresso yield × 9% TDS ÷ 18 g dose × 100 = 18% EY

Brew method	TDS target	EY target
Espresso	8–12%	18–22%
Filter / pour-over	1.2–1.5%	18–22%
Ristretto	10–14%	15–19%
Lungo	4–7%	18–22%

These are guidelines, not fixed rules — a coffee may taste excellent outside these ranges if the sensory result is balanced.

Roast Level and Extraction Chemistry¶

Roasting fundamentally alters which compounds are available for extraction:

Light roast: Higher chlorogenic acid content (less conversion during roasting); more intact sucrose and sweetness precursors; more volatile aromatics preserved; higher density requiring finer grind or longer extraction to achieve target yield.

Dark roast: Chlorogenic acids largely converted to quinic acid and other bitter compounds; more pyrazines, furans, and phenols (roast-character notes); lower density (extracts more quickly); less acidity but more bitterness and heavier body.

Puck-Level Physics in Espresso¶

In espresso, water does not interact with the coffee grounds evenly. The puck's physical structure determines the flow path of water:

Channelling: If the puck is uneven, water finds a path of least resistance and flows preferentially through that channel. The channel section is over-extracted; the rest under-extracted. The resulting shot combines sourness and bitterness simultaneously. Even tamping, distribution, and matched basket selection all serve to minimise channelling.

Pre-infusion physics: When pressure is first applied, a dry puck absorbs water and swells. Low-pressure pre-infusion allows the puck to expand evenly before extraction pressure begins, improving uniformity and reducing channelling risk.

Fines migration: Very fine particles can migrate toward the basket's bottom under pressure, progressively restricting flow. This explains shots that start well and gradually slow — visible as the stream narrowing and darkening late in extraction.

Advanced Concepts¶

Bypass: In filter brewing, adding a bypass (diluting a stronger extraction with water not passing through the grounds) allows independent adjustment of concentration and extraction yield.

Pressure profiling: Varying pump pressure during espresso extraction changes the extraction rate at different phases. Ramp-up profiles allow pre-infusion; ramp-down profiles reduce channelling risk as the puck compresses. See Equipment Optimisation.

Turbo shots: Coarse-grind, high-dose, short-time espresso recipes (e.g. 18 g in, 36 g out in 15 seconds) exploit the non-linear relationship between grind size and extraction rate to achieve target yield at unconventional parameters.

Key Facts¶

Coffee contains 1,000+ compounds; organic acids, sugars/Maillard products, bitter compounds, and lipids are the primary flavour-active classes
Acids extract first (small, polar, high solubility); sweetness compounds mid-brew; bitter/heavy compounds last
SCA EY target: 18–22% for most methods; calculated as (brewed weight × TDS%) ÷ dose × 100
Light roasts are denser and harder to extract; dark roasts are more porous and extract faster
Channelling in espresso produces simultaneous over and under-extraction; distribution and tamping address it, not grind adjustment alone

References¶

Changelog¶

Date Change

2026-05-03 Compliance review: added frontmatter, metadata block, all required sections; removed _Part of 05_PUBLISHING/Homepage/Coffeepedia... footer; fixed ../Precision Dialling → [Precision Dialling](../coffee-brewing-espresso/precision-dialling.md), ../Water Chemistry Basics → [Water Chemistry Basics](../coffee-brewing/water-chemistry-basics.md), ../Water Treatment → removed (not primary reference); converted ## Related Topics inline group to ## Related Notes bullets; fixed table alignment; added copyright

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