What ends up
in your cup

You already know that grind, temperature and tamping change your espresso. Those are the last three links in a chain of roughly forty causes. Here is the whole chain at once, so the rest of the course has somewhere to hang.

Three ideas that organise everything else

1. Potential, transformation, selection. The farm, the variety and the processing decide which molecules are in the green bean: sugars, acids, lipids, proteins, aroma precursors. That is the potential. Roasting transforms it, turning precursors into roughly a thousand aroma compounds while destroying others. Brewing then selects: water dissolves some compounds faster than others, so how much you extract decides which part of the transformed potential reaches the cup.

2. Strength and extraction are different axes. Strength (TDS, total dissolved solids) is how concentrated the drink is. Extraction yield is how much of the coffee you dissolved. A weak cup can be over-extracted and a strong cup can be under-extracted. Most dial-in confusion comes from mixing these up. Chapter 12 makes this concrete.

3. Evenness beats averages. An espresso at 20 % average extraction can be a sweet, balanced 20 % everywhere. Or it can be 26 % in a channel and 15 % in the rest, which tastes sour and bitter at the same time. The same logic applies inside a roasting bean: surface versus core. Many quality problems are distribution problems, not average problems.

A coffee bean is the seed of a cherry from a tropical shrub. Its job is to feed a seedling, so it packs sugar, protein, fat and defensive chemicals. Those are exactly the raw materials roasting turns into flavor.

Two species dominate the trade

Caffeine and chlorogenic acids are the plant's chemical defence; they are bitter. Robusta lives where pests thrive and makes more of both. More sugar and fat in arabica means more raw material for caramel, Maillard and aroma chemistry in the roaster, and more aroma carried in the oils. That is why Italian espresso blends often add 10–20 % robusta: for crema and body, not for flavor complexity.

Varieties: the grape varieties of coffee

Within arabica, most varieties descend from two lines that left Ethiopia via Yemen: Typica and Bourbon. Ethiopia itself still grows thousands of wild and semi-wild landraces, often sold as "heirloom". Variety affects yield, disease resistance, bean size and flavor.

Origins at a glance

Your friend's green list will be organised by country. These are typical profiles: tendencies shaped by variety, altitude and the dominant process, not rules.

Altitude is really temperature

The air cools by about 0.6 °C for every 100 m you climb. Cooler nights and days slow the plant down: the cherry takes longer to ripen, and the seed has more time to fill with sugars, acids and other compounds. It becomes denser and harder, which matters a lot to the roaster. Latitude does the same job: Brazil grows good coffee at 900–1,200 m because it lies 15–22° south of the equator, while Colombia or Ethiopia need 1,500–2,200 m for the same temperatures.

Ripeness, picking and the farm

A ripe cherry is deep red (or yellow, for some varieties) with roughly 18–24 °Brix of sugar in the pulp. Unripe cherries produce seeds low in sugar that stay pale in the roaster ("quakers"), tasting of peanut, paper and astringency. Selective hand-picking (several passes per season, ripe cherries only) is the main reason specialty coffee costs more than strip-picked or machine-harvested coffee. Farms also float cherries in water: unripe, damaged or insect-eaten cherries are less dense, float, and get removed.

Other farm factors: soil (young volcanic soils are rich in minerals), shade (slows ripening like altitude does and protects biodiversity), rain pattern (rain triggers flowering, so two rainy seasons give two harvests, as in Colombia and Kenya), nutrition and yield (an overloaded tree makes thinner, less sweet seeds) and disease (coffee leaf rust, Hemileia vastatrix, pushes farmers toward resistant varieties).

Between picking and the green bean you buy lie a few weeks in which the fruit is removed and the seed is dried. How that happens, wet or dry, fast or slow, with or without fermentation, can change the flavor as much as the variety does.

Fermentation is chemistry you can taste

As soon as a cherry is picked, wild yeasts and bacteria start eating the sugars in the pulp and mucilage. They produce organic acids (lactic, acetic), alcohols, and esters, the fruity molecules also found in wine and beer. In a washed process, fermentation is used mainly to loosen the mucilage so it can be washed off. In naturals and honeys, the seed dries inside fruit that is fermenting, so more of those products migrate into the seed.

Producers now control this deliberately: sealed tanks without oxygen (anaerobic), CO₂-flushed tanks (carbonic maceration, borrowed from Beaujolais winemaking), added yeast cultures, or fruit "co-ferments". These give intense cinnamon, tropical-fruit, boozy or candy-like flavors. Fans love them; critics say they cover up origin character. Either way, they show that processing is a flavor tool.

Drying: slow and even wins

Seeds must go from around 50–60 % moisture to 10–12 %. Too fast (hot patio, strong sun) cracks cell structures and makes the coffee fade quickly in storage. Too slow, or uneven, invites mould and "ferment" defects. Good farms dry on raised beds with airflow, turn the coffee often, and cover it at midday. Washed coffee usually dries in 10–20 days; naturals in 3–6 weeks.

After drying, coffee rests in its parchment for weeks ("reposo") so moisture evens out. Then the dry mill hulls off the parchment and sorts the beans by size (screens), density (gravity tables), color (optical sorters) and by hand. Sorting removes defects, and the more sorting, the cleaner the cup.

Green coffee is a living, slowly aging agricultural product. Before a roaster turns on the burner, they already know a lot from the spec sheet and from a sample roast. These are the numbers that matter.

Defects you can taste

Storage and aging

Green coffee loses quality in months, not years. Oxygen, heat, light and moisture swings drive lipid oxidation and the loss of acids and aromatics. Coffee travels in jute bags with a hermetic liner (GrainPro and similar) or vacuum packs. Roasters keep it cool (ideally below about 20 °C) at stable humidity. Old coffee roasts differently too: it often reaches first crack faster and the cup turns flat, woody and papery. (A few coffees are aged on purpose, like Monsooned Malabar, which is exposed to monsoon winds and tastes low-acid and earthy.)

Price and trade

Commodity arabica is priced off the "C" futures market in New York. Specialty lots trade at premiums, often several times the C price, based on cup score, traceability and relationships ("direct trade"). Importers handle the logistics, financing and sample flow between farms and roasters. A small roastery usually buys from importers who keep stock in European warehouses (Hamburg and Antwerp are big hubs), choosing lots by sample roasting and cupping first.

Decaf

Caffeine is removed from green beans, before roasting. Swiss Water and similar methods use a green-coffee extract saturated with everything except caffeine, so only caffeine diffuses out. Supercritical CO₂ dissolves caffeine selectively under pressure. Ethyl acetate ("sugarcane process") is a natural solvent from fermented cane. Methylene chloride is the efficient, older solvent route. All of them leave the beans more porous and lower in moisture, and darker-looking before they are actually roasted. Decafs roast faster, are hard to judge by color, and degas sooner.

In about ten minutes a dense, grassy seed turns into a brittle, porous, brown bean holding hundreds of aroma molecules that weren't there before. Roasting is cooking. The roaster's job is to control how fast heat gets in and when to stop.

Inside a drum roaster

Your friend most likely has a drum roaster. The parts, in the order the heat flows:

• Burner (gas or electric) under or behind the drum. Its setting is the main lever.
• Rotating drum with internal vanes that tumble the beans. Drum speed changes how much beans touch hot metal and how well they mix.
• Fan / airflow pulls hot air through the drum and out the stack. It carries heat in, and carries steam, smoke and chaff (the silverskin) out.
• Cyclone catches the chaff; many roasteries also run an afterburner or catalyst to burn the smoke.
• BT probe (bean temperature): a thermocouple sitting in the tumbling bean mass. It reads a mix of bean surface and hot air, and it lags behind.
• ET probe (environment or exhaust temperature): measures the air, either in the drum or in the exhaust. Placement differs per machine, so ET numbers don't transfer between roasters.
• Trier (Probenzieher): the little scoop you pull out to look at and smell the beans during the roast.
• Cooling tray with stirring arms and strong suction. It cools the beans to near room temperature within about 3–4 minutes so the roast actually stops.

Three ways heat gets into a bean

The roast, degree by degree

Drag the bean temperature from 20 °C to 245 °C. The bean changes color, swells, cracks and turns oily. The chart shows which chemistry is happening at each moment. Curves are relative (0–100 % of each compound's maximum) and based on typical literature trends, not on one specific coffee.

The chemistry, in plain words

Drying (up to about 150 °C)

Water inside the bean heats up and starts to evaporate, and the bean turns from green to pale yellow. "Dry end" is a misleading name: at yellow, the bean still holds roughly half of its original water. What the drying phase really does is heat the water and push the bean to the temperature where chemistry starts. Evaporation absorbs a lot of energy (2,257 kJ per kg of water), which is why the bean's rate of rise falls early in the roast.

Maillard reactions (about 150 °C to first crack)

Amino acids react with reducing sugars (glucose and fructose, which form when sucrose breaks down). The result is hundreds of compounds: pyrazines (nutty, roasty, cocoa), furans (caramel, sweet), pyrroles, thiazoles, and brown polymers called melanoidins (color, body, some bitterness). A side reaction, Strecker degradation, makes aldehydes with malty and chocolate aromas. The beans smell first like bread and toast, then like cake and caramel. Chaff flies off.

Caramelization (from about 170 °C)

Sucrose, 6–9 % of arabica, breaks down on its own: sweet-smelling caramel compounds such as furaneol, plus acids (acetic, formic) and bitter polymers. By a dark roast, almost no sucrose is left. Sweetness in the cup peaks around light to medium roasts; after that, "caramel" turns into "burnt sugar".

First crack (about 196–205 °C)

Water vapour and CO₂ build up pressure inside the cells, estimated at several bar. The cell structure gives way with an audible pop, like popcorn. Steam bursts out, the bean swells, and the overall reactions become net exothermic (they release heat). The roast can suddenly speed up. That is why roasters reduce heat before first crack.

Development (after first crack)

Acids degrade, sugars keep caramelizing, bitterness builds, and the core catches up with the surface. One to three minutes of development is typical for light to medium roasts. Too little gives grassy, sour, cereal flavors and hard-to-extract beans. Too much and origin character is replaced by roast character.

Second crack (about 224–230 °C)

The cell walls themselves start to break down, with a quieter, faster crackle (like Rice Krispies). Oils migrate to the surface, smoke increases, and flavor becomes bittersweet chocolate, smoky, then ashy. Very harsh-tasting phenylindanes form from chlorogenic-acid breakdown products. They are a big part of the dark-roast bite.

What changes physically

Roast degrees by name

Names are fuzzy and temperatures depend on the probe, so roasters usually describe degree relative to the cracks and by color.

Tomorrow you'll watch two lines climb across a screen while your friend turns knobs. This chapter teaches you to read that screen the way a roaster does. The simulator below uses Artisan's own colors and phase defaults.

What's on the screen

Experiments to try in the simulator

1. Batch size. Keep everything else the same and go from 60 % to 100 %. The roast takes longer, the turning point drops, and you need more gas to keep the same timing. This is why roasters adjust charge temperature to batch size.
2. Hot charge, small batch. Charge 235 °C with 35 % batch. Fast, aggressive, scorching warning.
3. Development. Drop at 30 s versus 150 s after first crack. Watch the drop temperature, weight loss, color and DTR.
4. Cut the gas hard at first crack (Maillard 70 %, development 0 %): the RoR crashes. Then leave lots of gas in development (Maillard 75 %, development 60 %): the RoR rises again (a "flick").
5. Airflow. Very low airflow means less convection, a hotter ET and smoke. Very high airflow carries heat out of the stack and slows the roast.

The roaster's levers

Key numbers roasters talk about

• DTR (development time ratio) = development time ÷ total time. Typical: 15–25 %. Light filter roasts often sit around 12–18 %, espresso roasts around 18–25 %. Useful, but only within one machine and one coffee.
• Development time in seconds: often 60–150 s.
• Total time: drum roasters typically land between 8 and 14 minutes.
• Weight loss: a rough but honest measure of roast degree. Light 11–14 %, medium 14–17 %, dark 17–22 %.
• Color: measured on whole beans and ground coffee with a color meter (Agtron, Lighttells, Tonino, ColorTrack). The difference between whole and ground shows how evenly the inside developed compared with the outside.

Roasting for espresso or for filter

Blends. Coffees can be roasted together (pre-roast blending: one profile, simpler) or separately and mixed afterwards (post-roast blending: each component gets its ideal profile, more work). Dense high-grown beans and soft low-grown beans roast at different speeds, so many roasters blend after roasting.

Automation. Artisan can do more than record. With supported machines it can move the burner and airflow itself, follow a background profile with its PID controller, trigger actions by alarms (for example "at 150 °C set burner to 60 %"), and log every event button and slider move so a roast can be repeated exactly.

Roast defects and their curve signatures

Artisan features worth knowing

• Roast › Background… loads a previous roast as a reference behind the live curve.
• Roast › Properties… holds green and roasted weight (so Artisan calculates weight loss), color, density, moisture and notes.
• Config › Phases… sets the dry end and phase limits (default 150 °C; Maillard until first crack).
• Config › Curves… controls RoR smoothing and the delta span. More smoothing means a calmer line but more lag.
• Tools › Designer lets you draw a target profile. Tools › Simulator replays a profile. Tools › Comparator overlays many roasts.
• Roast › Cup Profile… records cupping scores as a flavor wheel.

Fresh-roasted coffee isn't at its best straight out of the cooler. It is full of CO₂ that gets in the way of extraction. A few days to weeks later it peaks, and then it slowly goes stale.

CO₂: trapped gas

Roasting creates CO₂, mostly from Maillard and Strecker reactions and sugar breakdown, and darker roasts create more. Much of it stays trapped in the bean's porous structure and slowly diffuses out over days and weeks. During brewing, escaping gas pushes water away from the grounds. In filter coffee you see that as the bloom. In espresso, too much gas makes uneven, gassy shots with big bubbly crema and sour flavors. Some CO₂ is good, though: it forms crema.

Gas leaves a ground particle far faster than a whole bean. Diffusion time grows with the square of the distance, and a 0.3 mm particle has about a twentieth of a 6 mm bean's escape distance, so it degasses roughly 400 times faster. Ground coffee loses a large share of its CO₂, and of its aroma, within minutes. Grind just before brewing.

Staling

Staling is chemistry, not just gas loss. Oxygen attacks the coffee's oils (lipid oxidation gives cardboard and rancid notes). Delicate aroma molecules evaporate, and some react with each other or with oxygen. The bean also absorbs moisture and smells from its surroundings. Heat speeds up all of it: as a rough rule, chemical reaction rates about double for every 10 °C warmer.

Water can only dissolve what it can reach. Grinding decides two things: how much surface the water touches, and how far dissolved molecules have to travel from inside a particle to escape. Both follow simple geometry.

The geometry

A coffee bean is built from cells about 20–50 µm across (a hair is about 70 µm). When you grind, cells at a particle's surface are smashed open; their contents dissolve almost instantly. Cells inside the particle stay intact, so their solubles must diffuse out through cell walls and pores. That is slow.

Nobody grinds one size

Every grinder makes a particle size distribution: a main peak around the size you set, plus a population of fines (under about 100 µm, mostly cell-wall fragments) and some boulders. Fines extract almost instantly and add body, but they also clog the puck and migrate to the bottom. Boulders under-extract. The shape of the distribution is why grinder quality matters so much.

Burrs, alignment and other grinder physics

• Conical burrs tend to make a wider, more bimodal distribution (more fines): more body and texture, more forgiving, less clarity.
• Flat burrs tend toward a narrower, more unimodal distribution: more clarity and separation of flavors, often higher extraction, less body. Big flat burrs (64–98 mm) are popular for light-roast espresso.
• Alignment: if the burrs aren't parallel, one side grinds finer than the other. Distribution widens.
• Retention and static: old grounds stuck in the grinder end up in your next shot. A drop of water on the beans before grinding (RDT, the Ross droplet technique) kills static and clumping.
• Roast degree: darker beans are brittle and shatter into more fines. Light beans are tough, so you usually need to grind them finer and expect a tighter distribution.
• Heat: grinding warms the grounds slightly and drives off some aromatics. Single-dosing cold beans is a trick to get a tighter distribution.

An espresso is about 90 % water, a filter coffee about 98.7 %. The minerals dissolved in it change how well coffee compounds are extracted and how acidity tastes, and decide whether your machine scales up or corrodes.

Two numbers matter most

Hardness is the amount of calcium (Ca²⁺) and magnesium (Mg²⁺). These positively charged ions bind to many flavor molecules and help pull them out of the coffee. Research (Hendon et al., 2014) found that magnesium binds more strongly than calcium to many coffee compounds and tends to extract more fruity and acidic notes. With very soft water, extraction tastes hollow.

Alkalinity (buffer, mostly bicarbonate HCO₃⁻; on German water reports it appears as Karbonathärte, KH) neutralises acids. A little keeps acidity pleasant. Too much flattens it into a dull, chalky cup. Too little and acidity becomes sharp and sour.

On top of taste: hot water with lots of calcium and bicarbonate precipitates limescale (Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O), the reaction that clogs boilers. Very soft or chloride-rich water corrodes metal instead.

Make your own water

Coffee nerds build water from distilled or reverse-osmosis water plus two concentrates. Epsom salt (magnesium sulfate heptahydrate) supplies hardness; baking soda (sodium bicarbonate) supplies buffer.

Filters

• Jug or inline cartridge filters (ion exchange): swap calcium for hydrogen or magnesium and remove some bicarbonate. They lower KH, which is great for taste and scale. Change them on time, or they release what they collected.
• Reverse osmosis: removes almost everything; you must remineralise, or you get flat coffee and corrosion.
• Activated carbon: removes chlorine and off-flavors but not hardness.

An espresso machine has two jobs: deliver water at a stable temperature, and push it through a compacted bed of coffee at high pressure. Each design solves those two jobs differently, and the differences show up in the cup.

Pumps and pressure

Vibration pumps (small, cheap, buzzing) can make up to about 15 bar; an over-pressure valve (OPV) bleeds the excess so the puck sees about 9–10 bar. Rotary pumps are quieter and steadier and can be plumbed in. Newer machines control pressure or flow electronically (pressure profiling, flow profiling).

Why around 9 bar? Partly tradition, partly physics: enough pressure to push water through a fine, dense bed in 25–35 s and to emulsify oils and dissolve CO₂ for crema. More pressure is not simply better. The puck compresses harder and gets less permeable, and channels form more easily. Several studies and many modern recipes use 6–9 bar.

Temperature stability

Brew temperature is usually 90–96 °C. A degree or two makes a noticeable difference: hotter dissolves and diffuses faster, so you extract more, which is often good for light roasts and harsh for dark ones. What matters most is that the temperature is repeatable. A PID controller (a feedback algorithm, the same principle Artisan uses for roaster control) holds a boiler within about ±0.5 °C, while an old pressure switch swings several degrees.

The group head matters too. The famous E61 group (1961) is a heavy chromed brass lump heated by a thermosiphon: hot water circulates from the boiler through the group by convection. It is stable once hot but needs 20–30 minutes to warm up. Saturated groups (directly attached to the boiler) and electrically heated groups warm up faster and hold temperature tighter.

Baskets

Many home machines ship with pressurised (double-wall) baskets. They have a single tiny exit hole that builds pressure regardless of grind, so pre-ground coffee produces a foamy imitation crema. They are forgiving but make real dial-in impossible: grind size barely changes the shot. For everything in this course you need a standard single-wall basket, ideally a precision one, matched to your dose (an "18 g basket" for about 17–19 g).

Cleanliness is flavor

Coffee oils polymerise and go rancid on the shower screen, in the group and in the basket. Backflush with a blind basket and espresso-machine detergent regularly, soak baskets and portafilters, change the group gasket when it hardens, and descale according to your water. A dirty machine adds bitter, rancid notes that no grind change can fix.

This is where the physics you have been tasting becomes visible. The puck is a porous filter bed, and water flowing through it follows a law worked out in 1856 for the town fountains of Dijon.

How hard should you tamp?

At 9 bar on a 58 mm basket the water presses on the coffee with about 240 kg-force, more than ten times a firm tamp. So the job of tamping is not to "compress the coffee enough". It is to remove air gaps and create a level, uniform bed before the water arrives. Past full compaction (roughly 10–15 kg is plenty), extra force changes very little. A tilted or uneven tamp, though, creates a thin side that water will prefer.

Darcy's law: why grind size dominates flow

Flow is proportional to pressure, but permeability depends on particle size squared (and on porosity, very steeply). That's the Kozeny–Carman relation: k ∝ d² · ε³ / (1 − ε)². Small grind changes therefore make large flow changes. The particles that control permeability are mostly the fines, because they fill the gaps.

Channeling, the hidden killer

A real puck is never perfectly uniform. Where it is slightly looser, water flows a bit faster. Faster flow extracts that region faster; extracted coffee loses mass and gets more permeable; so even more water goes there. That is a positive feedback loop, and it ends in a channel. A few channels carry most of the water and get over-extracted (bitter, astringent), while the rest of the puck barely gets wet (sour). Average extraction falls, and the shot tastes sour and bitter at once.

Puck preparation, in order

1. Dose by weight (±0.1 g). Match the dose to the basket: too much and the puck touches the shower screen; too little and there is a wet, sloppy headspace.
2. Distribute with WDT (Weiss distribution technique): stir with thin needles (0.3–0.4 mm) to break clumps and even out density. This has the biggest effect of all the steps.
3. Level the bed (tap or use a distributor) so there are no hills or slopes.
4. Tamp straight and level, firmly enough to fully compact. Don't twist hard or knock the portafilter afterwards; that breaks the seal at the edge.
5. Optional puck screen on top: spreads incoming water and keeps the shower screen clean.
6. Lock in and start immediately, so the hot group doesn't pre-cook the dry coffee.

About 28–30 % of a roasted bean's mass can be dissolved in water. The rest is cell-wall material that never dissolves. Good coffee dissolves roughly 18–22 %, the part that tastes good, in the right concentration.

TDS (total dissolved solids, strength) is measured with a refractometer: dissolved coffee bends light, and the instrument converts that to concentration. Espresso is usually 7–12 % TDS; filter coffee 1.15–1.45 %. EY is calculated from TDS, dose and yield.

Compounds dissolve at different speeds

Small, very soluble molecules come out first: acids, salts, caffeine, simple sugars. Larger, slower ones follow: more sugars and melanoidins (sweetness, body), then bigger polyphenol and melanoidin fractions that taste bitter and drying. The cup concentrates the early compounds; as the shot runs, the later compounds gain weight in the mix. That is why yield (ratio) is such a powerful taste control.

The brewing control chart

Plot strength against extraction and you get the chart coffee professionals use. Its axes are the two independent dials from chapter 1. Ratio lines show where a recipe can possibly land.

Ratios, drink styles and other brew methods

Percolation (water flowing through) keeps a high concentration gradient, because fresh water keeps meeting the grounds, so it extracts efficiently. In immersion the water saturates as the brew gets stronger, so extraction slows down by itself. That is why immersion is forgiving and percolation is sensitive to evenness.

Crema

Under 9 bar, hot water dissolves far more CO₂ than at normal pressure. When the coffee leaves the basket, the pressure drops and the gas comes out of solution as tiny bubbles, like opening a soda. Melanoidins and proteins act as surfactants that stabilise the bubble walls, and emulsified oils add color and texture. Crema tells you the coffee is fresh and was brewed under pressure. It doesn't tell you the shot is good, and on its own it tastes bitter and ashy. Robusta and very fresh beans make more of it; old beans make little.

What else is in the cup

• Caffeine: a double espresso has very roughly 80–150 mg, a large filter cup 100–200 mg. Caffeine extracts quickly. Its half-life in the body is about 5 hours (3–7 h, varying from person to person).
• Diterpenes (cafestol, kahweol): oily compounds that can raise LDL cholesterol. Paper filters remove almost all of them; espresso, French press and boiled coffee keep much more.
• Chlorogenic acids: antioxidants and a source of bitterness and acidity. Lighter roasts keep more.
• Acrylamide: forms early in roasting from the amino acid asparagine and sugars (a Maillard side product), then partly breaks down again. Lighter roasts contain somewhat more than dark ones.

Temperature and profiles

• Hotter speeds up dissolution and diffusion and lowers viscosity (faster flow). Light roasts: 93–96 °C. Dark roasts: 88–92 °C to tame bitterness.
• Pre-infusion (1–4 bar for 5–15 s) wets the puck evenly before full pressure, so there is less channeling.
• Declining pressure (lever-style, 9 → 6 bar) counters the puck losing resistance as it erodes, keeping flow steadier.
• Flow profiling controls the flow rate directly, for example a slow start, a faster middle and a gentle finish.

Dialing in is running experiments: you change one variable, taste, and reason backwards from the taste to the cause. Everything in the earlier chapters exists to make that reasoning faster.

A reliable routine

1. Fix the dose for your basket (for example 18 g in an 18 g basket). Leave it alone.
2. Choose a ratio for the roast: about 1 : 2 for medium, 1 : 2.2–2.5 or more for light, 1 : 1.5–2 for dark.
3. Grind to hit a time window (about 25–35 s including pre-infusion) at that ratio. Time is a guide, not the goal.
4. Taste. Then adjust ratio (first), grind (second) or temperature (third).
5. Change one thing at a time, and write it down: dose, yield, time, grind setting, taste.

Steaming does two things: it heats the milk with condensing steam, and it whips air into it. Proteins hold the bubbles, fat makes them creamy, and temperature decides sweetness.

• Stretching (tip just under the surface, a gentle hiss) draws in air. Do it early, while the milk is cold (below about 35–40 °C), because cold proteins build foam better.
• Texturing (tip deeper, a whirlpool) breaks big bubbles into microfoam that is glossy, with invisible bubbles, like wet paint.
• Proteins (casein and whey; about 3.3 % of milk) unfold at the air–water surface and form a skin around each bubble.
• Fat makes foam creamier but less stable; skim milk foams big and dry.
• Lactose (about 4.8 %) tastes sweeter when warm. Perceived sweetness peaks around 55–65 °C.

Plant milks: barista versions add oil (for texture), and acidity regulators such as dipotassium phosphate, so the hot, acidic coffee doesn't make the proteins clump (curdling). Oat foams most like dairy.

Your tongue detects five basic tastes. Almost everything else you call "flavor" is smell, coming up from the back of your mouth into your nose (retronasal aroma). That is why slurping matters and why coffee tastes of nothing when you have a cold.

The parts of a taste

The flavor wheel

A few key molecules

Over a thousand volatile compounds have been found in roasted coffee, but a few dozen carry most of the aroma. Some to know by name:

Cupping: how roasters taste

Cupping is the industry's standard tasting method. It is immersion brewing in bowls, designed to be repeatable so many coffees can be compared side by side. Your friend will probably cup the roasts.

1. Ratio: 8.25 g of coffee per 150 ml of water (about 1 : 18), ground slightly coarser than filter.
2. Fragrance: smell the dry grounds.
3. Pour water at about 93 °C, filling to the top. Start the timer.
4. Break the crust after about 4 minutes: push the floating layer of grounds back with a spoon and smell the burst of aroma.
5. Skim off the remaining foam and grounds.
6. Slurp from a spoon, hard, so the coffee sprays across your whole palate and aroma reaches your nose. Taste repeatedly as the coffee cools from about 70 °C to room temperature. Defects and acidity show more clearly when it is cooler.
7. Score each attribute and note descriptors. Rinse the spoon between bowls.

A field guide for the visit. Tick things off as you see them; ticks are saved in this browser.

Follow one roast from start to finish

Questions to ask

Things to try if you can

• Call first crack by ear before the roaster does.
• Pull the trier every 30 s and line the samples up, to see the color progression.
• Smell the green coffee, then the beans at yellow, at first crack and at drop.
• Cup two development times of the same coffee side by side.
• Bite a light-roast bean and a dark-roast bean (crunchy versus brittle).
• Watch the cooling tray: how quickly do the beans reach room temperature?
• Ask about safety: chaff fires, cleaning the exhaust, the afterburner.

Everything to remember, on one page

Collected from the "Remember" boxes of every chapter.

Answer, then read why. Questions you get wrong point to the chapter to re-read before tomorrow.

Your friend will likely use the German terms, and Artisan might be set to either language.

Sources and further reading

• Scott Rao, The Coffee Roaster's Companion (2014): the source of most RoR and development heuristics.
• James Hoffmann, The World Atlas of Coffee (2nd ed., 2018): origins, varieties, processing.
• Andrea Illy & Rinantonio Viani (eds.), Espresso Coffee: The Science of Quality (2005): the scientific reference for espresso.
• Jonathan Gagné, The Physics of Filter Coffee (2021) and his blog Coffee ad Astra: grinding, extraction and water physics.
• M. I. Cameron et al., "Systematically improving espresso: insights from mathematical modeling and experiment", Matter 2 (2020).
• C. H. Hendon, L. Colonna-Dashwood, M. Colonna-Dashwood, "The role of dissolved cations in coffee extraction", J. Agric. Food Chem. 62 (2014).
• Specialty Coffee Association: Water Quality Handbook, cupping protocols, brewing control chart; SCA / World Coffee Research Coffee Taster's Flavor Wheel (2016).
• Artisan documentation: artisan-scope.org/docs and the blog at artisan-roasterscope.blogspot.com.