Espresso Glossary

The vocabulary of espresso is unusually hostile to newcomers, and a lot of it is used loosely even by people selling equipment. These are the definitions we use across the site.

Extraction and the shot

Dose. The dry weight of ground coffee in the basket, in grams. Measured before, not estimated.

Yield. The weight of liquid espresso in the cup, in grams. Weighed, not read off the cup markings.

Ratio. Yield divided by dose. A 1:2 ratio is 18 g in, 36 g out. Ratio, not volume, is how you communicate a shot to someone else.

Extraction yield. The percentage of the dry coffee that has actually dissolved into the cup. Typically 18 to 22 percent for espresso. Measured with a refractometer, inferred by taste the rest of the time.

Channelling. Water finding a low-resistance path through the puck instead of passing through it evenly. The result is simultaneous over-extraction along the channel and under-extraction everywhere else, which is why a channelled shot can taste sour and bitter at once.

Pre-infusion. A period of low-pressure wetting before full pressure arrives, to settle the puck and reduce channelling.

Blooming. The release of carbon dioxide from fresh coffee on first contact with water. In espresso it shows up as instability in very fresh beans, which is why most coffee is better a week or two after roast.

Grinding

Burr. The pair of cutting surfaces that reduce the bean. Conical burrs are cone-in-cone; flat burrs are two parallel discs. The shape changes the particle distribution, and therefore the taste, more than the marketing changes.

Particle distribution. The spread of particle sizes a grinder produces at one setting. A narrow spread extracts more evenly. It is the single most consequential difference between grinders, and the one least visible on a spec sheet.

Fines. The very small particles produced alongside the target size. Some are necessary for body; too many block flow and cause bitterness.

Boulders. Oversized particles that never fully extract. They contribute sourness and waste coffee.

Retention. Ground coffee that stays inside the grinder instead of coming out. It reappears in the next dose, stale, which is why single-dosing exists.

Stepped and stepless. Stepped grinders click between fixed positions; stepless adjust continuously. Espresso usually wants stepless or very fine steps, because the useful range is narrow.

Alignment. How parallel the burrs are to each other. Poor alignment widens particle distribution regardless of how good the burrs themselves are.

Seasoning. The break-in period during which new burrs cut differently. Expect a grinder to change over its first few kilos.

The machine

Group head. The assembly where the portafilter locks in and water meets coffee. Its thermal mass is a large part of temperature stability.

Portafilter. The handle that holds the basket. A bottomless or naked portafilter has no spout, so you can see channelling as it happens. It is the cheapest diagnostic tool in espresso.

Basket. The perforated cup holding the coffee. Precision baskets have consistent hole size and spacing; stock baskets often do not.

Single boiler, heat exchanger, dual boiler. Three answers to brewing and steaming at different temperatures. A single boiler does one at a time. A heat exchanger passes brew water through the steam boiler. A dual boiler keeps two independent circuits, which is why it costs what it costs.

PID. A controller that holds boiler temperature to a tight band instead of letting a thermostat swing. It stabilises temperature; it does not by itself make the group stable.

Pressure profiling. Varying pressure during the shot rather than holding nine bar throughout. Useful, frequently overstated, and not a substitute for a good grind.

OPV. The over-pressure valve that sets the machine’s maximum pressure. Often set high from the factory and adjustable.

Three-way valve. Releases pressure from the group after the shot, leaving a dry puck. Its absence is why some machines leave a soup in the basket.

Backflush. Running water back through the group against a blind basket to clear the three-way valve and the group. With detergent, weekly. Without, daily.

Blind basket. A basket with no holes, used for backflushing.

Water

Hardness. The dissolved calcium and magnesium in water. These matter for extraction and are also what forms scale.

Alkalinity. The water’s capacity to neutralise acid, usually reported as KH or as carbonate hardness. Too high flattens acidity and makes coffee taste dull; too low makes it taste harsh.

TDS. Total dissolved solids. A crude single number that tells you how much is in the water but not what. Useful for tracking, insufficient for deciding.

Scale. Mineral deposit left inside boilers and lines by hard water. It is the leading cause of expensive espresso machine repairs and the reason descaling schedules exist.

Descaling. Dissolving existing scale with an acid solution. Necessary, occasionally harmful to certain machines, and always secondary to using water that does not form scale in the first place.

Technique and tools

Dialing in. Adjusting grind, dose, and ratio until a given coffee tastes right on a given machine. It is the skill that closes most of the gap people try to close with money.

WDT. Weiss Distribution Technique. Stirring the dry grounds in the basket with fine needles to break up clumps before tamping. Cheap, unglamorous, and one of the few accessories that reliably changes the cup.

Tamping. Compressing the bed flat. Level matters; force above a moderate threshold does not.

Puck screen. A fine mesh disc placed on top of the puck. It keeps the shower screen clean and may help distribute water. Its effect on taste is smaller than its popularity suggests.

Puck preparation. Everything between grinding and locking in: distribution, WDT, tamping, levelling. It is where most inconsistency lives.

Crema. The foam on top of the shot, made of carbon dioxide and emulsified oils. An indicator of freshness and roast, not of quality. Thick crema and a bad shot coexist happily.