Brew Ratio
Dose-to-yield relationship. 1:1.5 (ristretto territory) is concentrated and heavy; 1:2 is the modern standard; 1:2.5-3 pulls lighter roasts brighter and more open.
Burr Geometry
Flat vs conical burr sets grind particle distribution differently. Conicals trend toward traditional, syrupy shots; flats trend toward clarity and separation. The grinder shapes the cup more than the machine.
Channeling
Water punching an easy path through part of the puck instead of flowing evenly, over-extracting the channel and under-extracting everything else. A bottomless portafilter exposes it instantly.
Cooling Flush
On a heat-exchanger machine, running water through the group to dump superheated water before pulling the shot. A routine you learn by ear and timing.
Crema
The foam of CO2 and oils on top of a fresh shot. An indicator of bean freshness, not of quality — judge the shot by taste, not the foam.
Dose
The weight of dry ground coffee in the basket, measured in grams. The first corner of the dose/yield/time triangle — weigh it to 0.1g, every shot.
Extraction
The percentage of the ground coffee actually dissolved into the cup. Under-extracted shots taste sour and thin; over-extracted shots taste bitter and dry.
Nine Bar
The reference brew pressure for espresso, roughly nine times atmospheric pressure. More is not better — most machines benefit from being turned down to it.
OPV
Over-Pressure Valve. Limits maximum pump pressure. Many entry machines ship at 12+ bar from the factory; adjusting the OPV to ~9 bar is a classic first mod on a Gaggia Classic.
PID
Proportional-Integral-Derivative controller — electronics that hold brew water at a set temperature instead of letting a thermostat swing. The single biggest stability upgrade on small machines.
Pre-Infusion
Wetting the puck at low pressure before full brew pressure hits. Lets the bed settle and swell, reducing channeling — built into dual boilers like the Breville, manual on an E61.
Puck Prep
Everything done to the grounds before the shot — distribution, WDT, tamping. Consistent puck prep is what makes the rest of your variables readable.
Retention
Grams of old grounds a grinder holds back between doses. High retention means your first shot of the day is partly yesterday's stale coffee.
Single-Dosing
Weighing each dose of whole beans before grinding instead of running from a full hopper. Trades convenience for freshness and easy bean-switching; low-retention grinders make it practical.
TDS
Total Dissolved Solids — the strength of the espresso, measured with a refractometer. Combined with dose and yield it gives extraction yield, the number behind the taste.
Temperature Surfing
Timing your shot against a single-boiler's thermostat cycle to catch usable brew temperature. The free workaround a PID makes obsolete.
WDT
Weiss Distribution Technique. Stirring the grounds in the basket with thin needles to break clumps and even out density before tamping. Cheap, boring, and one of the highest-value habits in espresso.
Yield
The weight of liquid espresso in the cup. Expressed against dose as a ratio — 18g in, 36g out is a 1:2 ratio.