A good shot of espresso should taste like dark chocolate – sweet with a bit of acidity. If your espresso tastes sour, it's either due to underextraction or channeling.
To fix sour espresso, make sure you are distributing your coffee evenly, adjust your grind size to a finer setting or increase your dose.
What is Coffee Extraction?
Coffee extraction is the process of dissolving flavor compounds from coffee beans into water.
When coffee is brewed, hot water comes into contact with ground coffee, dissolving flavor compounds in the forms of sugars, acids, and fats*.
Flavor compounds that taste acidic or sour dissolve more easily than compounds that taste sweet and bitter. Therefore, if your espresso tastes overwhelmingly sour or salty, it's likely due to acidic-tasting compounds being dissolved early in the shot, and sweet and bitter tasting compounds left unextracted or underextracted from the coffee.
Is My Coffee Underextracted?
Your coffee may be showing signs of underextraction if the:
- Flavor is excessively sour
- Brew time is too short
- Espresso shot yield is low
Espresso Channeling and its Effect on Extraction
Espresso channeling is another common explanation for a sour-tasting cup of espresso. You can visually observe channeling when coffee flows through only one spout or one side of the portafilter, or when your shot is pulled extremely quickly.
Channeling occurs when water finds its way through select areas of the coffee puck and bypasses others, leading to uneven extraction.
When this happens, water can dissolve too much of one part of the puck and not enough of the other, leading to a shot that’s overly sour.
A channeled coffee puck looks like the cross-section of an anthill. Water flows through one or more tunnels running through the puck, escaping more quickly than if it uniformly made contact with all of the grounds.
While it’s not possible to cross-section the puck while brewing, here are some visual indicators for channeling:
- Uneven Puck Surface
- Espresso Shot is Pulling too Quickly
- Espresso Shot Pulls Unevenly
1. Uneven Puck Surface
If grinds in the espresso puck appear unevenly distributed (sometimes even having a hole), channeling is likely to occur.
2. Espresso Shot is Pulling Too Quickly
If your espresso shot is pulling too quickly, this can be a sign of channeling. The water may be finding a weak point in the coffee puck and flowing too quickly through it.
3. Espresso Shot Pulls Unevenly
If your espresso shot is pulling unevenly, with some parts of the puck appearing lighter or darker than others, this could also be a sign of channeling.
You can prevent channeling by ensuring that your ground coffee is properly distributed in your portafilter, and that your coffee is tamped evenly.
Fixing Coffee Underextraction
Because coffee extraction is a chemical reaction, you can influence extraction by adjusting temperature, pressure, time, grind-size, brew ratio (the amount of coffee to water), and kinetic energy (stirring or agitation).
If you're experiencing underextraction you can:
1. Grind Coffee Finer
If your espresso shot is underextracted, try grinding your coffee beans finer. This will increase the surface area of the coffee, making it easier for the water to extract flavor compounds.
2. Create an Even Coffee Bed
Uniformity prevents channeling and underextraction, resulting in a more balanced-tasting espresso.
3. Increase Water Temperature
Increasing the temperature of your water speeds up the chemical reaction of dissolving compounds out of coffee.
4. Increase Coffee Dose
Increasing the dose of your coffee increases the brew ratio of coffee to water. With more coffee, more compounds can be extracted into your final shot.
5. Increase Contact Time by Pull a Longer Shot
By manually pulling the espresso shot for longer, you allow more water to make contact with the puck, giving more time for compounds to extract out of the coffee.
Terroir and Roast Profile
Sometimes, a coffee plant's natural environment and fermentation process produce flavors that tend to feel more fruity. While a roaster's palette can also dictate how sweet or bitter a roasted coffee might become, the coffee's terroir is the ultimate dictator of flavor in your cup.
Below are a few tips to buying coffee that's less acidic.
Buy Beans that Express a Less Acidic Flavor Profile
When it comes to avoiding underextraction, the type of coffee beans you use can also make a difference. Beans that express a less acidic flavor profile will generally be less likely to produce a sour espresso shot.
Generally speaking, washed coffees from Papua New Guinea and Sumatra coffees from Indonesia exhibit fewer acidic flavors and accentuate more nutty, chocolatier flavors.
Dark Roast vs Light Roast Espresso
Dark roast espresso beans also tend to taste less acidic, due to the extended roasting time, which also can give them a bold, smokier flavor.
In contrast, light roast espresso beans are roasted for a shorter period of time, often resulting in brighter, more complex flavor profiles with higher acidity.
Ultimately, the type of roast you prefer is a matter of personal taste, so it may be worth trying both to see which you prefer.
Sour tasting coffee can be due to underextraction. A common cause of underextraction is channeling, which often occurs during puck preparation. You can prevent channeling by ensuring that coffee is evenly distributed and carefully tamped.
Outside of channeling, underextraction can occur when most of the acidic flavor compounds are dissolved in water before bitter and sweet tasting compounds are dissolved more completely.
You can increase coffee extraction by grinding coffee finer, creating a more even bed, increasing the water temperature, increasing the coffee dose, and increasing the brew/extraction time.
If your espresso is still tasting sour after careful puck preparation and dialing-in, your coffee may simply express itself through fruitier, brighter, and more acidic flavors. This may be a unique feature of the coffee's terroir and processing method itself.