Brewing Principles

Coffee Fundamentals

Course 1 · Chapter 6 · Brewing Principles: Ratio, Temperature, Time and Extraction

Four Variables

Wikipedia's Coffee preparation article says that "key variables in coffee preparation include grind size, water temperature, brew time, and the ratio of coffee to water", which together control extraction and flavour. It also names the water itself as a factor. Chapter 5 covered grind size; this chapter covers the rest.

Variable	What the article gives
Grind size	Chapter 5: finer for faster methods, coarser for longer ones
Water temperature	Close to boiling; espresso 91 to 95 °C
Brew time	Method-dependent, from seconds to many hours
Ratio	15 to 18 parts water to 1 part coffee by mass

Water

The article says proper brewing uses water of "balanced mineral composition", and notes that the Specialty Coffee Association (written SCAA in the article) defines a precise composition of water for barista championships. It does not give that composition (check). On temperature, it says aromatic oils are released at 96 °C, just below boiling, and bitter acids are released once the water reaches boiling point. Chapter 1 quoted 91 to 96 °C as the optimal range from the Coffee article; the figures agree in spirit, but the two articles state the range differently.

Ratio

The introduction gives a preferred ratio of 15 to 18 to 1 water to coffee by mass. The article also gives standard brew strengths by volume: 55 g/L (the American standard) up to 63 g/L (the Norwegian standard), which it says yields about 14 to 16 g of coffee for a 240 ml cup. Espresso is an exception: 7 g of grounds for a 25 ml shot (280 g/L), and a typical presentation ratio of 1 to 2 (ground coffee to water). That finally supports the 1:2 espresso figure that chapter 1 marked "check".

Numbers that do not quite agree

55 g/L over 240 ml works out to 13.2 g, not the "14 to 16 g" the article states, while 63 g/L gives 15.1 g. The stated range is therefore a little high at the lower end. The mismatch may come from rounding or from a cup being slightly larger than 240 ml; I could not tell which (check).

Time

Method	Time given
French press	Typically 4 to 7 minutes of steeping
Cold brew	8 to 24 hours (one gallery caption says 12)
Cold drip	About 2 hours
Turkish coffee	Brought to the boil, sometimes two or three brief boils; no time given
Espresso	No brew time given in the part I read (check)

Extraction

Extraction means dissolving flavour out of the grounds. The article says the ideal yield is widely agreed to be 20 ± 2 percent (18 to 22 percent), meaning about a fifth of the dry coffee ends up in the cup. It gives ideal strength targets as well: American 1.25 percent (1.15 to 1.35), Norwegian 1.40 percent (1.30 to 1.50) and European 1.20 to 1.45 percent, though some of these are tagged "citation needed" on the page (check).

Problem	Cause given	Result
Over-extraction	Grounds too fine (too much surface)	Bitter, harsh taste
Under-extraction	Grounds too coarse	Weak coffee unless more is used

Four Basic Methods

Type	How it works	Examples in the article
Decoction	Grounds and water are boiled together	Turkish or Arabic coffee in a cezve, cowboy coffee, kopi tubruk
Infusion	Grounds steep in water, then are separated	French press, AeroPress-style immersion, coffee bags
Gravity	Water flows down through the grounds	Drip brewers, Indian filter coffee, percolators, Neapolitan flip pot
Pressure	Water is forced through the grounds	Espresso, the moka pot (about 1 bar of steam pressure)

Espresso uses water at 91 to 95 °C at 8 to 18 bar, forced through a lightly packed puck of finely ground coffee to make a 30 to 60 ml drink. Chapters 7 and 8 take these families in turn.

Code: Doses, Ratios and Yield

The script below turns the article's strengths and ratios into doses, and applies a yield formula. The formula (yield equals strength percent times drink weight divided by dose) is from my own knowledge, not from the pages I read, and the example input numbers are invented, so treat that line as a worked illustration (check). I ran the script to get the output shown.

def dose_for(volume_ml, grams_per_litre):
    return volume_ml / 1000 * grams_per_litre

for name, gpl in [("American standard", 55), ("Norwegian standard", 63)]:
    print(f"{name}: {gpl} g/L -> {dose_for(240, gpl):.1f} g for 240 ml")

print(f"Espresso: 7 g in 25 ml = {7 / 25 * 1000:.0f} g/L")

for ratio in (15, 16, 18):
    print(f"{ratio}:1 -> {1000 / ratio:.1f} g coffee per litre of water")

def extraction_yield(tds_percent, beverage_g, dose_g):
    return tds_percent * beverage_g / dose_g

print(f"Example yield: {extraction_yield(1.3, 300, 20):.1f}% (target 18-22%)")

# Output
American standard: 55 g/L -> 13.2 g for 240 ml
Norwegian standard: 63 g/L -> 15.1 g for 240 ml
Espresso: 7 g in 25 ml = 280 g/L
15:1 -> 66.7 g coffee per litre of water
16:1 -> 62.5 g coffee per litre of water
18:1 -> 55.6 g coffee per litre of water
Example yield: 19.5% (target 18-22%)

What the output shows

The 15 to 18 to 1 ratio range corresponds to roughly 56 to 67 g of coffee per litre of water, a little different from the 55 to 63 g/L standards, so the two ways of stating a recipe overlap but are not identical. The espresso line reproduces the article's 280 g/L. The example yield lands inside the 18 to 22 percent target.

Not Yet Verified, and Sources

Sources. Wikipedia article Coffee preparation, read as returned (about 100,000 of 110,000 characters; the unread tail was references and navigation), plus the earlier chapters' articles. A second article I tried to fetch did not exist (404).
Conflict. The article's "14 to 16 g for 240 ml" does not match its own 55 to 63 g/L figures (see the box above). Not resolved.
Weak support. Some strength targets are tagged "citation needed" on the page.
Not in the page read. The SCA water composition; a brew time for espresso; how to measure strength at home.
My own knowledge. The extraction-yield formula and the example input numbers (check).

Exercises

Exercise 1

List the four variables of brewing. For each, say what goes wrong at one extreme, using only what the chapter supports and marking anything else "(check)".

→ Solution

Exercise 2

Use the script to find the dose for a 500 ml pot at the American and Norwegian strengths, and the coffee needed for 750 g of water at 16 to 1. Then work out the yield for a 28 g dose that makes 420 g of drink at 1.45 percent strength.

→ Solution

Exercise 3

Sort these into the four basic method families and give a typical brew time or pressure for each where the chapter has one: French press, moka pot, Turkish coffee, drip brewer, espresso, cold brew.

→ Solution

Quick Reference

Variables: grind size, water temperature, brew time, ratio (plus water quality)
Ratio: 15 to 18 parts water to 1 part coffee; 55 to 63 g/L
Espresso: 7 g in 25 ml (280 g/L), about 1:2, 91 to 95 °C, 8 to 18 bar
Ideal yield: 20 ± 2 percent
Families: decoction, infusion, gravity, pressure

Coming next

Coffee Fundamentals 7 looks at filter and immersion methods in practice.