Brewing dissolves part of the roasted coffee into water. Two measurements describe the outcome. Total dissolved solids, TDS, is the share of the drink that is dissolved coffee: its concentration, or strength. Filter coffee is a little over one per cent dissolved coffee; espresso is several times that. Extraction yield is the share of the dry coffee's weight that ended up dissolved.
They are linked by simple arithmetic: extraction yield equals the weight of the beverage multiplied by its TDS, divided by the dose. The classic reference for filter coffee, from brewing research going back to the 1950s and carried forward by the Specialty Coffee Association, places most preferred cups at roughly 18 to 22 per cent extraction and a little over one per cent strength.
Those ranges describe the preferences of the tasters studied. More recent sensory research has found that preferences cluster in more than one region of the chart, and that well-made coffee can taste good outside the classic box. Treat the numbers as a map of where most people's taste lies, not a border.
Strength and extraction are two different axes
CoffeeHQ explanatory model — not a measurement
How a cup tends to taste at each combination of strength and extraction
Strength
Under-extracted
Well extracted
Over-extracted
Strong
Intense and sour. A short, fast espresso.
Intense and balanced. Add water if it is too much.
Intense, bitter and drying.
Moderate
Sharp, thin, lacking sweetness.
Balanced. The target for most cups.
Bitter, drying, lingering.
Weak
Watery and sour.
Mild but balanced. Use more coffee if it is too light.
Thin and bitter. A little coffee brewed for far too long.
Read across a row to change extraction: grind, time, agitation. Read down a column to change strength: the ratio.
Basis: The brewing control chart tradition: concentration set mainly by ratio, extraction set mainly by grind, time and agitation. Limits: The descriptions are typical. Roast, coffee and water shift where 'balanced' sits, and a ratio change in a pour-over moves extraction a little as well.
Only about a third of roasted coffee is soluble at all; the rest is the cell structure of the seed. Within the soluble part, compounds differ in how readily they dissolve. Acids and many small aromatic molecules come out quickly. Sugars and the compounds that give sweetness and body take longer. Larger, bitter and astringent compounds come out slowly and keep coming.
A brew therefore changes character as it proceeds. Stop early and the fast compounds dominate: sharp, thin, sour. Go on too long and the slow ones pile up: bitter, drying. Extraction yield is a single number standing in for where along that sequence the brew stopped.
Dissolving happens in two ways. Coffee on the broken surfaces of a particle washes off almost immediately. Coffee inside the particle has to diffuse out through its pores, which is much slower and depends on particle size, temperature and time. Finer grinding creates more surface and shorter paths, which is why it extracts faster.
In immersion brewing, all the water sits with all the coffee: a French press, a cupping bowl, cold brew. As coffee dissolves, the water becomes more concentrated and the difference between the inside of a particle and the liquid around it shrinks, so extraction slows by itself. Immersion is forgiving for that reason. It is hard to over-extract badly, and every particle sees the same liquid.
In percolation, fresh water flows through a bed of coffee: pour-over, batch brewers, espresso. Because the water arriving is always clean, extraction stays fast, and percolation generally extracts more efficiently. The cost is evenness. Water follows the easiest path, so some coffee can be washed heavily while some is bypassed. Most pour-over technique exists to keep the flow even.
Pressure does not extract more by itself. It makes percolation possible through a bed too fine and compact for gravity, in seconds instead of minutes, at a very short ratio. It also emulsifies oils and dissolves carbon dioxide under pressure, which gives espresso its texture and crema.
Agitation — stirring, swirling, turbulent pouring — moves concentrated liquid away from the particles and speeds extraction, and in a filter bed it also moves fine particles, which can clog the paper.
Three ways water meets coffee
CoffeeHQ explanatory model — not a measurement
Immersion, percolation and pressurised percolation compared
Mechanism
How water and coffee meet
What keeps extraction going
Typical risk
Examples
Immersion
All the water sits with all the coffee.
Time. It slows by itself as the liquid becomes concentrated.
Little: forgiving, but heavy or silty without a fine filter.
French press, cupping, cold brew.
Percolation
Fresh water flows through a bed of coffee under gravity.
A constant supply of clean water.
Uneven flow: some coffee over-extracted, some bypassed.
Pour-over, batch brewer, phin.
Pressurised percolation
Water is forced through a fine, compact bed.
Pressure, which makes a very fine grind and short ratio usable.
Channelling, which pressure makes worse.
Espresso; moka pot at much lower pressure.
Basis: The physical arrangement of each brewing mechanism. Limits: Real brewers mix these: an AeroPress steeps and then percolates under hand pressure, and a siphon steeps and then filters.
No grinder makes particles of one size. Every grind is a distribution: a main population near the target size and a second population of very fine fragments. The fines extract almost instantly and slow the flow; any oversized pieces barely extract. A narrow distribution extracts more evenly, which is what a better grinder buys.
The filter decides what gets from the brewer to the cup besides dissolved coffee. Paper holds back almost all the fine particles and most of the coffee oil. Metal mesh passes both. Published measurements bear this out: unfiltered and metal-filtered coffees contain far more of the oily compounds cafestol and kahweol than paper-filtered coffee does. Oils and suspended fines add body and mute clarity; removing them gives a lighter, more transparent cup. Cloth sits between the two and is the least studied.
Your task. Enter a filter recipe — for example 15 g of coffee and a beverage weight of about 220 g — and find the TDS that gives 20 per cent extraction. Then keep that TDS and halve the beverage weight, as if you had brewed with half the water.
What to notice. The same strength with half the beverage is half the extraction. Strength and extraction are different axes, and you cannot read one from the other without the recipe.
Exercise: Weak is not the same as under-extracted
Objective. Taste a diluted cup beside an under-extracted one, so the two words stop being interchangeable.
You will need
One coffee, an immersion brewer or cupping bowls, a scale, a timer, three cups
Keep the same
Same coffee, dose, water and temperature for every brew
Procedure
Brew a normal cup. This is the reference.
Pour half of it into a second cup and add an equal weight of hot water. This cup is weaker, with the same extraction.
Brew a third cup with the same recipe but stop the steep at about a quarter of the normal time. This cup is under-extracted.
Taste all three, coolest last.
What to notice
Which of the two 'weak' cups still tastes balanced?
Which tastes sour or hollow?
Does the diluted cup taste like a thin version of the reference?
Reading what you found
The diluted cup should taste like the reference with the volume turned down: the same balance, less of it. The short-steeped cup usually tastes different in kind — sharper and less sweet — because the compounds that dissolve quickly dominate. If you can tell these two apart, you can decide whether a cup needs more coffee or more extraction.
What this cannot show
The short steep is also somewhat weaker, so the third cup differs in both respects. The diluted cup is the one that isolates strength.
Without a refractometer these are qualitative judgements.
A protocol for you to run. CoffeeHQ has not run it and reports no result from it.
A French press drinker wants a stronger cup and doubles the steep from four minutes to eight. The coffee is a little heavier and noticeably more bitter and drying. They conclude that strong coffee is bitter coffee.
Reasoning it out
Strength is concentration, which the ratio sets.
Steeping longer raised extraction: more of the slow, bitter compounds dissolved.
In immersion, extraction slows as the liquid becomes concentrated, so doubling the time added only a little strength.
They changed the balance a lot and the strength a little — the opposite of the intention.
What to do
Return to four minutes and add coffee instead: a move from 1:16 to 1:14 raises strength directly and leaves the balance close to where it was. Strong coffee and bitter coffee are different things.
SCA brewing and espresso standards — Specialty Coffee Association (SCA)
UC Davis Coffee Center research — University of California, Davis — Coffee Center
Systematically improving espresso: insights from mathematical modeling and experiment, M. I. Cameron and colleagues — Matter, 2(3)
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