Sour Shots
Sour espresso means under-extraction. Not enough flavor compounds were dissolved from the grounds during the pull. The acids extracted but the sugars and other compounds did not.
Causes and fixes:
Grind too coarse: water flows through too quickly, not spending enough time with the grounds. Grind finer.
Water too cold: espresso needs 195-205F water. If your machine is not fully warmed up (usually 15-20 minutes for most home machines), the water temperature is too low. Run a blank shot before your pull to stabilize temperature.
Dose too low: not enough coffee in the basket means the puck compresses too loosely and water flows through with less resistance. Check your dose against the recommended basket capacity (usually 18-20 grams for a double basket).
Shot pulled too fast: a double shot should take 25-35 seconds. Under 20 seconds means under-extraction regardless of the cause. Grind finer to slow the pull.
Bitter Shots
Bitter espresso means over-extraction. Too many compounds dissolved, including the harsh, astringent ones that extract last.
Causes and fixes:
Grind too fine: water cannot flow through and over-extracts what it contacts. Grind coarser.
Water too hot: above 205F extracts bitter compounds more aggressively. Let the machine equilibrate. Some machines run hot from the factory. Check your machine's known temperature behavior.
Shot pulled too slow: over 35-40 seconds means over-extraction. Grind slightly coarser.
Stale beans: coffee past 4-6 weeks from roast date extracts differently. Fresh beans have more CO2 which creates back-pressure and slows extraction appropriately. Stale beans have lost this and can over-extract at settings that work fine for fresh beans.
Channeling
Channeling is when water finds a path of least resistance through the puck rather than flowing evenly through all the grounds. You see this as a shot that runs blonde or pale very quickly, often in 10-15 seconds, while the other side of the basket is still dark. The result is a shot that is simultaneously under-extracted (from the channeled areas) and over-extracted (from the slower areas).
Causes and fixes:
Uneven distribution: grounds not distributed evenly in the basket before tamping. Use a distribution tool or WDT (Weiss Distribution Technique) needle to break up clumps. The needle technique involves stirring the grounds in the basket before tamping.
Uneven tamp: a tilt in the tamp compresses one side more than the other. Practice tamping on a level surface. Some tampers have self-leveling designs that reduce this error.
Dosing inconsistency: if the dose varies by more than 0.5 grams, the puck density varies too. Weigh your dose every time.
The Adjustment Framework
Change one variable at a time. If you change grind, dose, and temperature simultaneously, you cannot identify which fix worked. Start with grind size (the most impactful variable). Make small adjustments: half-step on a stepless grinder, or one step on a stepped grinder. Pull a shot, taste it, assess. Repeat until the shot tastes balanced.
A balanced shot has sweetness upfront, a mid-palate with complexity, and a finish that is pleasant rather than harsh. Bright acidity is acceptable in a light-roasted espresso. Sharpness or bitterness that lingers is a problem to fix.
One practical tip: if your machine allows it, pull the shot into a shot glass on a scale and stop the scale when the espresso turns very pale (blonding). This gives you the yield, from which you calculate your brew ratio. A 1:2 ratio (18g in, 36g out) is the standard starting point. A 1:2.5 ratio produces a longer, slightly milder shot. A 1:1.5 ratio is a ristretto, shorter and more concentrated.