Puck preparation, channeling, and machine types

An espresso puck is only as good as its internal consistency. Distribution — spreading grounds evenly across the basket before tamping, by hand, tapping, or a dedicated distribution tool — matters more than tamping pressure itself, because an unevenly distributed bed will channel no matter how hard it's tamped. Channeling is water finding a low-resistance path through the puck (often along the basket wall or through a crack) instead of passing evenly through the whole bed — it shows up as a fast, thin, sour shot with a weak or asymmetric crema even when the dose and grind look correct on paper.

WDT (Weiss Distribution Technique), which stirs the dry grounds with fine pins before tamping, is one of the most effective single habits for reducing clumping and channeling. Basket choice also matters: baskets are rated by hole count and taper, and different baskets are tuned for different dose ranges — a basket mismatched to the dose (too much coffee for a shallow basket, too little for a deep one) makes even distribution harder regardless of technique. Pre-infusion — a brief low-pressure soak before full pump pressure engages — gives the puck time to swell evenly and settle before the main extraction, reducing channeling risk, and is a standard feature on many higher-end machines.

Machine types differ mainly in how they manage brew temperature and steam: single-boiler machines share one boiler between brewing and steaming (simple, but can't do both at once); heat-exchanger machines have one boiler heating a separate brew-water line that runs through it (steam and espresso can run close together, but brew temperature stability depends on machine design and "flushing" the line before a shot); dual-boiler machines have fully separate boilers for brewing and steaming, giving the most independent, stable temperature control for high-volume service.