The first question to ask about a sour or bitter espresso is which variable to change. A new barista may pull two shots that look similar in the cup but taste different. One shot may be sharp and sour. The other shot may be bitter, dry, and harsh. The coffee may taste different even if both shots are pulled on the same day with the same coffee and machine. However, the shot taste generally shows you something about the extraction, grind, dose, yield, or preparation.
Sour espresso may be under-extracted. The flow rate might be high, the grind size might be too coarse, or the yield may be low. Another cause may be puck preparation that has created channels for water to move through unevenly. The sour taste may be accompanied by a lack of body and an empty aroma. First, taste and time the shot. If it has a fast flow rate and a sharp taste, a slightly finer grind size is a logical first change to try.
Bitter espresso may be over-extracted. The flow rate may be low, the grind size may be too fine, or the yield may be high. The taste may be dry at the back of the mouth with less sweetness and more weight in the texture. Moving the grind size toward a slightly coarser setting, stopping the shot before a higher yield, or both are likely adjustments to try.
Avoid changing three variables at once when a new barista tries a shot: the grinder, the dose, and the yield. You may need to taste several shots to find the right extraction. Keep the variables steady so you can easily read what has happened in the next shot. Keep the dose constant in the basket and keep the tamp flat. Pull a shot into a consistent cup. Try only one or two changes to the grind size or yield and make note of the adjustment. Using a scale and timer may make the process easier because it takes the guess work out of memory.
Puck preparation may cause a taste problem. Sour espresso doesn’t always mean the grind size is too coarse, and a bitter espresso doesn’t always mean the grind size is too fine. If the grounds in the basket aren’t distributed or tamped evenly, water will move through a part of the puck too quickly and leave another part under-extracted. Before deciding if the taste depends on the coffee, examine the espresso basket for clumps, loose coffee on the rim, or an unevenly tamped puck surface. A properly prepared puck will lead to more accurate espresso adjustments.
If you drink lattes and cappuccinos at first, the milk will mask a sour or bitter taste. The milk will reduce the acidity of a sour shot and the bitter espresso of a cappuccino will seem strong and full of flavor. However, taste the espresso first in a small amount to get a better idea of the shot taste. A single espresso shot before the milk can show you whether to adjust the grind size, the yield, or the basket preparation.
The taste of sour and bitter coffee isn’t a failure of the espresso maker. You can turn the taste into your next step by asking this simple question after pulling a shot: Is it possible the water moved through the coffee too quickly, too slowly, or unevenly? Make a small change, pull a few more shots, and taste the new results while you have the information at hand.