Channeling

Channeling happens when pressurized water finds a path of least resistance through the coffee puck instead of flowing evenly through the entire bed. That path might come from a clump of fine particles, an air gap left by a tilted tamp, or a crack that opened when the portafilter was bumped or moved.

Once water starts channeling it drains most of its pressure and volume through that one weak spot, so parts of the puck barely see any water while the channel itself gets flushed hard. The result is a shot that is under extracted and sour from the untouched coffee and over extracted and bitter from the flushed channel, at the same time.

You can often see channeling directly with a bottomless portafilter, watching for a fast, spurting, or off-color stream instead of a steady, even flow. Fixing it usually means better distribution before tamping, using a WDT tool to break up clumps, and tamping level rather than at an angle. A common mistake is blaming the beans or grinder when the real cause is puck preparation.