"My coffee is sour."
"I don't like sour coffee."
Probably two of the most common comments any specialty coffee roaster treading the lighter roast spectrum would hear. What is the deal with sour coffee and why do so many coffee shops continue to serve them despite resistance from customers?
Let us first explore what causes sour flavours in coffee. As you might know, coffee contains a good amount of acids. Although still mostly a mystery, there are well-documented information about a few of the acids found in coffee beverages. The following are some of the acids recorded:
Citric Acid: The same acid found in citrus fruits like lemons and oranges. Makes up a significant percentage of a coffee's total acid content and the final perceived acidity.
Malic Acid: Mainly found in fruits like apples and pears, sweeter and less sour compared to citric acid. Contributes more to "fruity" notes within coffee.
Tartaric Acid: Usually found in grapes and can contribute to more winey flavours.
Acetic Acid: Otherwise known as the main component of vinegar, this can be very undesirable at higher levels. In small concentrations, it can provide a pleasant brightness and lime-like flavour.
Chlorogenic Acid (CGA): A family of six to 12 different isomers of the acid, each with different flavour attributes. Decomposes during the roasting process to form Quinic acid and Caffeic. Research suggests that this is the main culprit for acid reflux in the stomach, not caffeine.
Quinic Acid: Increases with darker roasts. Contributes to astringency and perceived sourness in coffee.
Caffeic Acid: Increases with darker roasts. Contributes to astringency and perceived sourness in coffee.
Phosphoric Acid: An inorganic acid. Commonly found in Cola sodas and is exceptionally potent compared to the other acids.
It stands to reason that the more acidic a particular solution is, the more sour it is.
However, this is not exactly true as there are many other factors that contribute to the perception of sourness. Different acids have different impacts on the perception of sourness, with some acids being more potent than others. Certain acids also illicit a sensory aspect of pungency or astringency which can be mistaken for sourness.
Acidity ≠ Sourness
Some of the other factors influencing the perception of acids include the presence of sugars. Sugar helps to balance sourness. For example, most people don't think of Coca-Cola as sour, yet it is high in carbonic and phosphoric acid. This is because the high sugar content helps to balance out the perception of sourness. Other examples include apples with malic acid and grapes with tartaric acid tasting sweet even though they contain high levels of acid within the fruit.
As Maxwell, UK Barista Champion, puts it: "When marking acidity on coffee we are not marking the sourness of the coffee, even though certain types of acidity can contribute to sourness. We are instead marking a quality that (if one is not familiar with as regards specialty coffee) could be misinterpreted as denoting sourness. Although it’s possible for a coffee to have high acidity and also be sour, this doesn’t mean they are mutually exclusive."
How does roasting impact acids in the coffee?
The longer a coffee bean is roasted or the darker it is roasted, the more organic acids are broken down.
Inorganic acids like phosphoric acid increases as a by-product from the breakdown of CGAs.
Citric being one of the more potent acids in the perception of sourness, is significantly reduced as a coffee continues to roast. Malic acid does not deteriorate as quickly. For roasters trying to enhance the perception of fruity flavours and sweetnes in their coffee, developing the coffee further to lower citric levels will help as it interferes with the perception of malic acid due to citric acid’s higher levels and potency. Here's a chart from the coffee chemistry website:
Acids as a function of roast weight loss (%):
Why some acidity is good
As much as we may not like our food to taste sour, some acidity in our food is still essential in making it taste good. Without any acidity at all, our foods/coffee will taste dull and flat. Acids are commonly used in cooking to enhance flavours in each dish. In fact, most popular items like sodas, fruit juices and even your beloved beers and steak all contain varying amounts of acidity.
Low Acid Alternatives
If you are on still on the look out for a more balanced or sweeter coffee, or just don't want to risk drinking an overly sour coffee, there are some low acid content alternatives you can choose. In general, low-grown coffees from Asian or South American regions tend to exhibit more balanced and bittersweet flavours. Some common examples include coffee from Brazil, Guatemala, Mexico, Indonesia, Thailand and India. You could also try to look for coffees roasted to a medium roast level or darker.
Conclusion: Balance and the customer
So why is it that cafes continue to serve sour coffee? It is probably not the intention of any coffee shop to serve sour coffee. But rather, I believe most roasters are trying their best to retain as much if a coffee's origin flavours whilst balancing all other taste aspects of the coffee. This usually means a lighter roast, aka more acids retained. Ultimately, even a medium roasted coffee or supposedly low acid coffee can exhibit sour flavours if it is not properly brewed and extracted. Underextracted brews can still have unbalanced and sour characteristics.
Similarly, a coffee that is roasted light and brewed well can still taste sweet and balanced with plenty of sugars present in the coffee. Certain coffee origins might also be ill-suited for brewing methods like espresso where flavours are intensified greatly due to the concentration of the beverage. This intensity can be mistaken for sourness and be hard to stomach even for seasoned coffee drinkers.
Barring brewing and roasting faults, most of the time a coffee's flavours is an imprint of the roaster and barista and their presentation of how they believe the coffee should taste and what would do justice to the farmer's hard work. As with all flavours, these presented flavours can be subject to personal preferences and almost definitely won't please everyone.
As always, speaking to the barista is the best way to find out more about your coffee options and letting them know your preferences so they can recommend the right drink for you.
If you'd like to see some of the coffees available at Compound Coffee Co., head on over to our online shop here.
Some extra reading material on acids: