When baristas first learn about channelling, they’re typically told to look for the familiar sign of a significant break in the puck: watery drips from the spouts at the beginning of the shot or telltale blond streaks in the flow. With a naked portafilter, they might be able to see tiny jets of liquid spraying from the puck or more subtle signs of uneven flow from the bottom of the basket.
The good news is that these kinds of channels are mostly avoidable if you use good equipment, good technique, and properly roasted coffee. The bad news is that these signs may not be the main culprits in the unpleasant flavours we associate with channelling. In a presentation at Prufrock Coffee in London in 2019, Professor Stephen Abbott suggested that these visible channels may not affect the flavour of the espresso significantly, since a channel passes through such a small fraction of the puck. In this case, a channel would merely dilute the espresso by bypassing the puck or, at worst, slightly reduce extraction of the rest of the puck by reducing the amount of water available to it for extraction.
This speculation is borne out by the fact that eliminating these more obvious channels has a limited effect on increasing the extraction ceiling. This implies that the overextraction flavours we associate with channelling come from ‘microchannels’ not visible to us.
Small channels can be detected even if we can’t see them directly. The most definitive way to detect microchannelling is by measuring extraction. If the flow of water through the puck becomes less even, extraction will drop slightly. Uneven flow will also lower the extraction ceiling — the highest extraction you can reach before bitterness and dryness become apparent. Therefore, if a particular technique or piece of equipment fails to achieve a sufficiently high extraction yield or if you find you have to target a lower extraction yield than usual to get good flavour, then microchannelling or uneven extraction is to blame.