Coffee science
Brewing has been studied, though less than the confidence of most brewing advice would suggest. These pages set out what the studies found, on what coffee and equipment, and keep that apart from what brewers do by custom.
CoffeeHQ measured and tasted none of it. Where a figure appears it is a paper's own. Where a figure would be a guess, it is left out, which is why you will find no ideal temperature, pressure or extraction here.
Two numbers describe a cup
Most confusion about brewing comes from treating strength and extraction as one thing. They are changed by different controls and go wrong in different ways.
Strength and extraction, side by side
CoffeeHQ explanatory model — not a measurement
| Quantity | What it measures | What mainly changes it | How it shows when it is off |
|---|---|---|---|
| Strength | How much dissolved coffee is in each mouthful | The ratio of coffee to water, above all | Thin and watery, or heavy and overpowering |
| Extraction | How much of the dry coffee was dissolved | Grind, time, temperature and how evenly water met the grounds | Often described as sharp when low and harsh when high, though in one panel study of drip coffee bitterness followed strength and not extraction |
Basis: The two quantities as brewing researchers define them, and one study in which a trained panel tasted drip coffee brewed to planned combinations of the two. Limits: A simplification. The tastes in the last column are how the two are usually described, not a finding that holds for every coffee; the same study found strength had the greater effect on what the panel tasted.
Two ways water meets coffee
Every brewer either steeps the grounds in the water or passes water through them. The two behave differently enough that advice for one often fails on the other.
Steeping and pouring through
CoffeeHQ explanatory model — not a measurement
| Arrangement | What happens | What follows | The control that matters most |
|---|---|---|---|
| Steeping (immersion) | All the water sits with all the grounds | A model tested against measurements found extraction settling at about the same level whatever the ratio | The ratio, which sets strength |
| Pouring through (percolation) | Fresh water keeps arriving and leaving | How much is removed depends on how long water stays and what path it takes | Grind and flow, and how evenly the bed is wetted |
Basis: A published model of full-immersion brewing, tested against its authors' measurements, and the general description of flow through a bed of grounds. Limits: The immersion finding describes a brew left to settle, in one group's experiments. Real brewers mix the two: an AeroPress steeps and then pushes through.
To see how thirteen brewers sit between these two, and what else separates them, use the brewing method comparator.
The physics of brewing, one subject at a time
Each guide names the studies it rests on, says how much of each was read, and ends with what it leaves out.
- Extraction
Strength and extraction as two different things, what studies found about sour and bitter, unevenness, and what a filter holds back.
- What the models say
A fast wash from particle surfaces and a slow diffusion from inside, and what each model was tested on.
- Measuring it
What a refractometer reads, the arithmetic, where the error comes from, and why the famous chart is a convention.
- Grinding
A grinder makes a spread of sizes. What was measured about fines, cold beans and static, and why a setting does not travel.
- Temperature
A panel could barely tell brewing temperatures apart once strength and extraction were held fixed. Cold brew is a different matter.
- Chemistry
What is in the green seed, what roasting breaks down and builds, what reaches the cup, and why knowing the compounds does not yet explain the taste.
- Pressure and flow
Why a machine sets one and the bed decides the other, what changing the flow did, and which rules are convention.
Put it to use
Before the brew
More science guides
Water, coffee chemistry, caffeine, freshness and milk, along with the roasting and processing guides.
- Water chemistry and brewing
Hardness and alkalinity are different properties of water: what a controlled tasting found each does to coffee, and what each does to equipment.
- Milk science: proteins, fat and foam
Why milk foams and steams the way it does, at a level beneath the practical steaming technique.
- Why coffee goes stale
What was measured as roasted coffee ages: gas leaving over weeks, aroma compounds escaping or reacting, and what grinding and packaging change.
- What happens when coffee roasts
Drying, browning, first crack and development — the chemistry behind roast character, at the depth that changes what a roaster does.
- Coffee roasters and heat transfer
How heat reaches the bean by contact, moving air and radiation, how drum and air roasters differ, and what the amount of air has been shown to do.
- Reading a roast profile
What a roast curve records, what rate of rise measures, why curves do not transfer between machines, and how much of the advice about their shape has been tested.
- Coffee roast defects
Baked, scorched and underdeveloped: what one laboratory found when it made roast faults on purpose, and why the names are looser than they sound.
- Coffee fermentation
What ferments when coffee is processed, which microbes do it, and what published trials found when they changed the time or chose the yeast.
- Drying coffee
How coffee is dried after processing: three phases, the moisture and water activity limits in international guidance, surfaces, turning and rewetting.
- Experimental coffee processing: the words explained
Anaerobic, carbonic maceration, thermal shock, co-fermented and infused: what each word is taken to mean, and how little has been published about most of them.
- The coffee plant: roots, flowering and fruit
How a coffee tree roots, what sets off its flowering, how it is pollinated and the stages a cherry passes through between blossom and harvest.
- Where coffee grows, and why
The temperature, rainfall and dry season arabica and robusta are reported to need, what frost and wind do, and why altitude matters without higher being better.
- Shade, spacing and water on a coffee farm
What shade trees are reported to do for coffee and what they cost in yield, why planting density has no single answer, and where irrigation changes the crop.
- How coffee is harvested
Why cherries on one branch ripen at different times, the four ways coffee is picked, and what slope and the cost of labour decide.
- Coffee pests and diseases
Leaf rust, coffee berry disease, the berry borer, wilt and nematodes: what each organism is, what it does to the tree, where it occurs and how it is managed in principle.
- Coffee and climate change
What models project for the land that suits arabica, what has been observed, what is proposed in response, and what is not known.
- Coffee genetics and breeding: from genome to variety
Where arabica came from, why it has so little genetic diversity, what the Timor Hybrid added, why arabica and robusta are bred in opposite ways, and what F1 hybrids, pyramiding and genomic selection have been shown to do.
- Green coffee chemistry: what the raw seed is made of
What an unroasted coffee seed contains and how much: cell-wall polysaccharides, sucrose, oil and its diterpenes, protein, chlorogenic acids and caffeine, what calling something a flavour precursor claims, and what changes in storage.
- Coffee soils and plant nutrition: what five studies found
What soil does for a coffee tree and what coffee does to its soil: acidity under long monoculture, one field trial of nitrogen, phosphorus and the two species, root fungi, nitrogen-fixing shade trees, and what is and is not known about soil and flavour.
- Espresso puck physics: how water moves through the bed
The espresso bed as a porous medium: permeability, why flow stops rising with pressure, wetting and swelling, fines, why extraction falls at very fine grinds, five meanings of channelling, and how little evidence stands behind tamping force.
- Coffee roasting chemistry: what breaks down and what is made
What roasting does to each part of a coffee seed: water, sucrose, cell-wall polysaccharides, chlorogenic acids, trigonelline, caffeine and oil; where the carbon dioxide comes from; and what Maillard, Strecker, caramelisation and pyrolysis each claim.
- Coffee aroma chemistry: why compounds are not flavour notes
The classes of volatile compound in roasted coffee (furans, pyrazines, aldehydes, ketones, sulphur compounds, phenols, pyridines, esters), how the ones that smell are identified, and the steps between a molecule in the cup and a word in a tasting note: thresholds, mixtures, the liquid and the taster.
- Coffee experiments to run at home: nine protocols
Nine controlled comparisons a reader can run with a scale, a timer and a thermometer: water alkalinity, filter material, sieved fines, espresso flow, tamping force, needle distribution, resting, frozen beans and milk temperature, each with its controls, what to record and what the result cannot show.