# Extraction Science — Cordwell Roastworks brew lab reference sheet

Internal primer we hand every new brewer at the Cordwell Roastworks bar and R&D
bench. It is the "why" behind the recipes: what extraction actually is, the
numbers the Specialty Coffee Association (SCA) publishes, and how grind, time,
temperature, and water push a cup around. Keep it by the scale. (For the
cold-brew specifics, see the separate immersion bench log.)

## What "extraction" actually means

Brewing is dissolving. Roasted coffee is only about 28-30% water-soluble by
mass; the rest is insoluble plant matter (cellulose) that stays behind in the
grounds. Extraction yield is the fraction of the dry coffee that ends up
dissolved in the cup — the dissolved solids in the drink divided by the mass of
dry grounds you started with. Pull too little out and you get only the light,
soluble, sour compounds; push too far and you start dragging out the heavy,
bitter, drying ones that come last.

The order things leave the bean is always the same, and it is the single most
useful thing to hold in your head:

- **First out:** acids and bright, fruity aromatics — the most soluble. An
  under-extracted cup tastes sour, sharp, salty, and hollow.
- **In the middle:** sugars and the round, sweet, balanced flavours. This is the
  target window.
- **Last out:** bitter alkaloids, larger polyphenols, and dry, astringent
  tannins. An over-extracted cup tastes bitter, ashy, and drying.

Sweetness lives in the middle, which is why "dial for sweetness" is the whole
game — you are trying to stop after the sugars and before the bitterness.

## The SCA Golden Cup numbers

The SCA "Golden Cup" target — the balanced-cup standard the whole specialty
world quotes — is two numbers held together:

- **Extraction yield: 18-22%** of the coffee's mass dissolved. Below about 18% is
  under-extracted (sour, weak, salty); above about 22% is over-extracted (bitter,
  astringent, hollow). Roughly 20% is dead centre.
- **Strength: about 1.15-1.35% TDS** in the finished brew by mass — that is,
  roughly 11.5-13.5 grams of dissolved coffee per litre of drink. Some references
  draw the pleasant band a touch wider, up to about 1.45%, but 1.15-1.35% is the
  classic Golden Cup strength.

Strength (how concentrated the drink is) and extraction (how much you pulled out
of the bean) are **independent axes**. You can brew a strong-but-under-extracted
cup (a lot of coffee, steeped briefly) or a weak-but-over-extracted one (a little
coffee, steeped forever). The Golden Cup asks for both to land in their windows
at the same time.

Getting there starts with the ratio. The SCA "golden ratio" is **about 55 g of
coffee per litre of water, plus or minus 10%** (so roughly 50-60 g/L) — for
filter that works out to about a **1:16 to 1:18** coffee-to-water ratio by
weight. More coffee per litre pushes strength up; grind and time then decide
where extraction lands inside that strength. Target water temperature at the bed
is **93 °C, plus or minus 3** (about 200 °F).

## Grind, time, and temperature — the three levers you actually turn

Extraction is surface area times solubility times time. The three knobs:

- **Grind size.** Finer grounds expose more surface and extract faster and more;
  coarser grounds extract slower and less. Grind is the coarse-adjust — one or
  two grind steps moves a cup further than anything else on the bar.
- **Contact time.** Longer contact extracts more. Each method pairs a grind with a
  time so the two land in the window together: espresso is very fine plus about
  25-30 seconds; filter and pour-over are medium plus about 3-4 minutes; French
  press is coarse plus about 4 minutes; cold brew is very coarse plus 12-18 hours.
  Fine-and-long over-extracts (bitter); coarse-and-short under-extracts (sour).
- **Water temperature.** Hotter water dissolves faster and reaches further into
  the bitter end. The SCA brewing range is **about 90.5-96 °C (195-205 °F)** —
  roughly 93 °C is the usual aim. Cooler water under-extracts, so you compensate
  with a finer grind or a longer steep; water at a hard boil scorches the grounds
  and over-extracts. Only cold brew breaks the temperature rule, and it pays for
  the cold with those long hours.

Turn one lever and you usually trade against another: grind finer and you may
need to shorten the pour; brew cooler and you may need to grind finer or steep
longer to keep extraction inside 18-22%.

## Immersion vs. percolation

Two families of brewing, and they behave differently:

- **Immersion** — the grounds sit in a fixed body of water for the whole brew
  (French press, the cupping bowl, the steep phase of an immersion dripper, cold brew).
  The water saturates and extraction slides toward equilibrium, so immersion is
  even and forgiving: every ground sees the same water for the same time. Easy to
  reproduce, hard to wreck.
- **Percolation** — fresh water flows through the coffee bed continuously
  (pour-over, batch drip, espresso). Because clean water keeps arriving, the
  concentration gradient stays steep and percolation can reach a **higher**
  extraction than immersion for the same grind — but it is fussier. Uneven pouring
  or a packed bed lets water carve a fast channel through part of the coffee
  (channelling), over-extracting some grounds and under-extracting the rest in the
  very same brew.

Neither is "better"; they simply want different grinds and different attention.

## Water chemistry — the invisible ingredient

A brewed coffee is about 98.5% water, so the water is an ingredient, not a
solvent detail. Two properties matter most:

- **General hardness (GH)** — the dissolved calcium and magnesium. These minerals
  actively grab flavour compounds and carry them out of the coffee; near-zero
  hardness (distilled or heavily softened water) brews flat, hollow, and thin no
  matter how good the beans are.
- **Carbonate hardness (KH, the alkalinity, also called the buffer)** — mostly
  bicarbonate. KH neutralises coffee's acids. Too much of it flattens brightness
  into a chalky, dull cup; too little and nothing buffers the acidity, so the cup
  reads thin and sour.

The SCA water target is about **150 mg/L (ppm) TDS**, with
roughly **50-175 ppm hardness** (a target near 68 ppm as calcium carbonate) and
**about 40 ppm alkalinity**, at a neutral pH near 7. The usable range is wide —
TDS anywhere from about 75 to 250 ppm brews well — but the two failure modes are
the ends: too-soft water tastes empty, while too-hard, high-alkalinity water
tastes dull and scales up the machine.

## Espresso vs. filter — the two ends of the same dial

Same physics, opposite corners:

- **Filter / pour-over / drip:** about 55-60 g/L (roughly 1:17-1:18), medium
  grind, about 93 °C water, 2.5-4 minutes, finishing near the Golden Cup
  1.15-1.35% TDS. A long, dilute, gravity-fed percolation.
- **Espresso:** a roughly **1:2 brew ratio** (about 18-20 g of fine grounds in,
  about 36-40 g of liquid out), forced through the puck at around **9 bar** of
  pressure — nine times atmospheric — in roughly **25-30 seconds**, at about
  90-96 °C. The result is tiny and intensely concentrated: espresso runs around
  8-12% TDS, an order of magnitude stronger than filter. But because the shot is
  so small, the **total** caffeine in a single espresso (about 63 mg) is actually
  *less* than a full mug of drip (about 95 mg). Concentration is not the same as
  dose — a truth the bar has to explain roughly ten times a day.

Everything else — moka pot, siphon — lives somewhere on the line
between those two corners, trading pressure, grind, and time to land extraction
back inside that same 18-22% window.
