Bitter Shot: Causes & Fixes

Bitterness has more than one real cause depending on your setup: over-extraction and channeling in an espresso machine, a different mechanism entirely in a moka pot, and rancid oil buildup in a machine that's overdue for a backflush. The guides below cover each separately.

Why Is My Espresso Bitter? The real chemistry behind bitterness (caffeine isn't the main culprit), why espresso is inherently more bitter than other brew methods, and the case that most "over-extracted" shots are actually channeling.
Moka Pot Troubleshooting A moka pot has no pump, no motor, and no 9-bar extraction curve, so almost none of the causes in our espresso-machine guides apply. Bialetti's own documentation covers leaking, stalled brews, and a genuinely different cause of bitter coffee.
How Often Should You Backflush an Espresso Machine? Real home-use frequency numbers from a specialty retailer and named La Marzocco engineers, why backflushing isn't the same job as descaling, and the one common mistake that damages more machines than skipping it.