Extraction: What Water Is Actually Doing

Brewing is extraction. Understanding what water does to roasted coffee, in what order, at what rate, with what results, gives you a framework for diagnosing and improving every cup.

Every brewing method is a variation on the same fundamental process: hot water dissolving soluble compounds from roasted, ground coffee.

The variables change. The physics do not.

Whether you are pulling an espresso shot, steeping a French press, or running water through a V60, the same sequence of dissolution is occurring, and the same principles determine whether the result is balanced, under-extracted, or over-extracted.

What Is Being Extracted

Roasted coffee grounds contain both soluble and insoluble material. Only the soluble fraction is relevant to the cup.

The soluble compounds include organic acids, citric, malic, acetic, phosphoric, quinic, which extract early and contribute brightness and acidity; sugars and caramelisation products, which extract in the early-to-middle phase and contribute sweetness; aromatic compounds, volatile esters, ketones, aldehydes, many partially water-soluble and extracted throughout, but also volatile and prone to loss through evaporation; caffeine, water-soluble and distributed relatively evenly through the extraction; phenolic compounds, including chlorogenic acid degradation products, which contribute bitterness and extract throughout but become more prominent in later stages; and melanoidins, large Maillard reaction polymers, which extract later and contribute body and the perception of weight and texture.

The insoluble fraction includes cellulose fibres and larger lipid particles, these remain in the grounds, or in a paper filter, and do not end up in a properly filtered cup.

The Extraction Sequence

Soluble compounds do not all dissolve at the same rate. The sequence of extraction roughly follows molecular size and polarity: acids are small, highly soluble molecules that dissolve quickly in the first fraction of the brew; sugars are somewhat larger but still readily soluble, following the acids in early-to-mid extraction; and bitter compounds are larger molecular weight compounds that require more time and energy to dissolve, dominating the later extraction fraction.

This sequence is why extraction timing matters: the beginning of a brew produces an acid-dominant, sweet fraction; the end produces a bitter-dominant fraction. The goal of balanced extraction is to capture enough of the sweet middle fraction while limiting how much of the bitter late fraction enters the cup.

Extraction Yield and TDS

Two measurements define extraction quality.

Extraction Yield

Extraction yield is the percentage of the dry coffee mass that has dissolved into the water. It is calculated as (weight of dissolved coffee) ÷ (weight of dry coffee grounds) × 100.

The Specialty Coffee Association's Brewing Control Chart defines the optimal extraction yield range as 18 - 22%. Below 18% is typically under-extracted, not enough material dissolved. Above 22% is typically over-extracted, too much, including bitter compounds.

TDS (Total Dissolved Solids)

TDS measures the concentration of dissolved coffee in the final cup, how much coffee material per unit of water. The SCA's Brewing Control Chart defines a recommended range for filter coffee of approximately 1.15 - 1.35% TDS, though figures up to 1.45% appear in some versions and publications. The precise upper limit is less important than the principle: too low and the cup tastes thin and weak; too high and it becomes heavy and fatiguing.

TDS and extraction yield are related but not identical. You can have the same extraction yield at different TDS by adjusting the brew ratio: use more coffee with more water and you can achieve the same extraction yield at lower TDS, a weaker brew, or less coffee with less water for higher TDS, a stronger brew.

Under-Extraction

Under-extraction occurs when insufficient soluble material has been dissolved, meaning you have extracted primarily the early, acid-dominant compounds without reaching the sugar and body-contributing compounds in the middle fraction.

Causes include a grind too coarse, which reduces surface area and slows extraction rate so water passes through without adequate contact; water too cool, since lower temperature reduces the solubility of extraction-critical compounds; brew time too short, with insufficient contact time for the required dissolution to occur; insufficient agitation, since in immersion brewing, stirring increases contact and therefore extraction; and a low dose relative to water, where extraction yield may appear adequate but TDS is too low for a satisfying cup.

The flavour profile is sharp, unresolved sourness, acids without the sweetness to balance them; thin, watery body, with insufficient dissolved material for texture; a hollow finish, where the palate registers the sourness but finds nothing behind it; and in severe cases, a salt-like quality, a symptom of early-fraction extraction without subsequent development.

Over-Extraction

Over-extraction occurs when too much has been dissolved, the extraction has moved well into the late fraction, pulling bitter, dry compounds from the grounds in excess.

Causes include a grind too fine, which maximises surface area and accelerates extraction, making it easy to overshoot; water too hot, which increases dissolution rate of all compounds, including bitter ones; brew time too long, with extended contact extracting progressively more bitter compounds; and excessive agitation, particularly in espresso, where over-agitation from channelling or excessive pressure can extract unevenly and create over-extracted zones.

The flavour profile is harsh, unpleasant bitterness, distinct from the pleasant roast bitterness of a balanced cup; drying astringency, a mouth-puckering quality that lingers; muted sweetness, where bitter compounds dominate and mask what sweetness is present; and a hollow or flat aftertaste, where the persistent bitterness overwhelms complexity.

Balanced Extraction: The Target

Balanced extraction sits within the 18–22% extraction yield range and broadly within the 1.15–1.35% TDS range, though the precise upper limit varies by source. These numbers are guidelines, not guarantees. A coffee that measures within these parameters can still taste imbalanced if the distribution of extraction is uneven, with some grounds over-extracted and others under.

Uniform extraction across all grounds requires consistent grind particle size distribution, since a grinder that produces consistent particle sizes with minimal 'fines' and 'boulders' produces more even extraction; even water distribution, flowing evenly through all grounds rather than channelling through paths of least resistance; temperature consistency throughout the brew, since stable temperature ensures consistent extraction rate; and appropriate agitation, enough to ensure water contacts all grounds uniformly.

The goal of extraction is not maximum yield. It is even extraction of the right compounds in the right proportions. Precision, not force.

Grind Size and Its Relationship to Method

Grind size adjusts the extraction rate by changing the surface area of the coffee exposed to water. Every brewing method has a characteristic grind size range because of its contact time: espresso, a 25–30 second extraction, needs a very fine grind to slow water flow and maximise extraction rate within a short contact time; moka pot, 3–5 minutes, needs fine-medium; Aeropress, 1–3 minutes and variable, needs medium-fine to medium; pour over or V60, 2.5–4 minutes, needs medium; Chemex, 4–6 minutes with a thicker filter, needs medium-coarse; French press, 4–8 minutes, needs coarse to allow water to penetrate large particle surfaces during extended steeping; and cold brew, 12–24 hours, needs coarse because the extended time compensates for slow cold-water extraction.

Changing brewing method without adjusting grind size is one of the most common sources of extraction failure. A grind calibrated for French press used in a V60 will be dramatically under-extracted. An espresso grind in a French press will block the plunger and produce a bitter, muddy cup.

Temperature and Its Role

Water temperature directly affects the rate at which compounds dissolve. Higher temperature increases the kinetic energy of water molecules, increasing their ability to break the bonds that hold soluble compounds within the ground coffee matrix.

The SCA recommends a brewing temperature of 90–96°C (194–205°F) for filter coffee. Espresso extraction typically occurs at 90–94°C.

For coffees with less solubility, very light roast, high-density green coffee, temperatures at the higher end of this range help achieve adequate extraction. For darker roasts with higher solubility, lower temperatures can prevent over-extraction of bitter compounds.

Cold brew, using water at or near room temperature, compensates for the dramatically lower extraction rate by extending contact time to 12–24 hours and using a coarse grind to avoid over-extracting bitter compounds despite the long contact time.

Bringing It Together: A Diagnostic Framework

When a cup does not taste as expected, this sequence provides a systematic diagnostic approach: a sour, thin, or weak cup is likely under-extracted, so grind finer, increase temperature, extend brew time, or increase dose; a bitter, astringent, or drying cup is likely over-extracted, so grind coarser, reduce temperature, shorten brew time, or reduce dose; a flat, dull cup without specific sourness or bitterness likely points to a roast issue, baking or underdevelopment, or stale coffee, so check roast date and roast quality; a cup that is acceptable but weak likely has correct extraction yield but low TDS, so increase dose; and a cup that is acceptable but too intense likely has correct extraction yield but high TDS, so increase water volume.

Extraction variables interact, changing one affects others. Adjust one variable at a time, brew, taste, and adjust again. Systematic iteration produces consistent improvement more reliably than dramatic simultaneous changes.

Understanding extraction is ultimately what separates informed brewing from habitual brewing. The equipment and the coffee matter. But the extraction parameters determine whether their potential is reached.

This is the final part currently published in the Foundations of Coffee series. Return to the series index.