Extraction Science

Extraction is the process of dissolving soluble compounds from ground coffee into water. Everything about brewing — grind size, temperature, time, agitation — exists in service of controlling which compounds are extracted and in what quantities. Understanding extraction at a technical level doesn't require a chemistry degree, but it does require understanding a handful of concepts: what's being dissolved, how much of it, and what different amounts taste like.

What Extraction Actually Is

A roasted coffee bean is roughly 28–32% soluble in water. The rest — cellulose, fibre, insoluble proteins — stays in the grounds. Of that soluble fraction, not all of it tastes good. Your job as a brewer is to extract enough of the good compounds (organic acids, sugars, aromatic esters, Maillard products) without extracting the bad ones (certain bitter alkaloids, harsh phenolic compounds) that concentrate at high extraction levels.

This is not a binary — there is no clean line between "good extraction" and "bad extraction." It's a spectrum, and the optimal range is narrower than most people assume.

Extraction Yield and TDS

Two numbers define any brew:

Extraction Yield (EY): The percentage of the coffee's dry mass that was dissolved into the water. Calculated as: (brewed coffee weight × TDS%) / dry coffee dose × 100. The SCA's Specialty Coffee Brewing Chart defines the "ideal" range as 18–22%. Under 18% = under-extracted; over 22% = over-extracted.

Total Dissolved Solids (TDS): A measure of how concentrated the brew is — how many grams of dissolved coffee solids per 100g of liquid. Measured with a refractometer.

• Filter coffee: 1.2–1.5% TDS is the SCA target range for balanced strength.
• Espresso: 8–12% TDS, achieved through the higher ratio and pressure.

EY and TDS together tell you whether a problem is a strength issue (TDS too high/low) or an extraction issue (EY too high/low). You can have a brew that hits the correct TDS but is still under-extracted — it will taste thin and sour, but strong. Or correctly extracted but too weak — it will taste pleasant but watery. Separating strength from extraction is the key diagnostic skill.

The Order of Extraction: What Comes Out and When

Not all soluble compounds extract at the same rate. When water first contacts ground coffee, the most water-soluble compounds dissolve first. The extraction order, broadly:

1. Acids (first): Fruity organic acids (citric, malic, acetic) are highly soluble and extract within the first 30–60 seconds of contact. This is why a very short brew or very coarse grind produces a sour, sharp, underdeveloped cup — you're getting mostly acids.

2. Sugars and sweetness compounds: These extract in the middle phase of the brew — the Maillard products, caramelised sugars, and the compounds responsible for sweetness and body. Getting into this zone is the goal. A well-extracted coffee's dominant sensory impression should be sweetness.

3. Bitter compounds (last): Chlorogenic acid breakdown products, certain alkaloids, and high-molecular-weight phenolic compounds extract slowly and late. These are responsible for the harsh, dry, astringent finish of over-extracted coffee. They're also the compounds that accumulate when grind is too fine, water too hot, or contact time too long.

This sequence is why the tasting vocabulary for extraction problems is so consistent:

• Sour = under-extracted (mostly acids, not enough sweetness or balancing compounds)
• Bitter/harsh/astringent = over-extracted (bitter compounds present and dominant)
• Flat/thin = correct extraction but low strength (good EY, low TDS — often a ratio problem)

The Four Variables

Every extraction is controlled by four variables. Changing any one changes the extraction yield.

1. Grind size: The most powerful and responsive variable. Finer grind = larger surface area = faster, higher extraction. Coarser grind = less surface area = slower, lower extraction. This is the primary dialling-in tool for any brew method.

2. Dose (coffee-to-water ratio): Changes strength (TDS) more directly than extraction yield. More coffee with the same water volume increases TDS without necessarily changing EY. However, a very high dose reduces water's ability to extract efficiently (saturation effects), which can lower EY.

3. Water temperature: Higher temperature increases solubility of all compounds, raising extraction yield. Also increases rate of extraction of bitter compounds disproportionately to sugars and acids at very high temperatures (above 96°C). Lower temperatures slow extraction — useful for controlling aggressive naturals or dark roasts.

4. Time (contact time): For immersion methods, longer time = higher extraction. For percolation methods, time is largely a product of grind size and pour rate — you control it indirectly.

Agitation functions as a multiplier on these variables — stirring or turbulence brings fresh water into contact with the coffee bed, accelerating extraction. Gentle agitation helps even extraction; aggressive agitation can cause over-extraction of the outer layer of grounds.

Diagnosing a Bad Brew

| Problem | Tastes like | Most likely cause | |---|---|---| | Under-extracted | Sour, sharp, hollow, weak finish | Grind too coarse, water too cool, too short contact time | | Over-extracted | Bitter, harsh, astringent, dry | Grind too fine, water too hot, too long contact time | | Correct extraction, too weak | Pleasant but watery, thin | Ratio too low (not enough coffee) | | Correct extraction, too strong | Pleasant but intense, heavy | Ratio too high (too much coffee) |

The fix for under or over-extraction is always grind adjustment first. Ratio adjustments fix strength. Temperature adjustments are a secondary tool for fine-tuning.