What Causes Sour Coffee?

What Causes Sour Coffee?

And how to best avoid it.
by Ever Meister | November 16, 2022


Have you ever taken a sip of coffee and felt your mouth pucker the same way it does when you take a sip of lemonade? No, you’re not imagining it — necessarily: There are a few reasons a coffee might taste sour to you, and we’re going to review them right here in this post.


One of the primary reasons for sour-tasting coffee is under-extraction, or not removing enough of the flavor compounds from the coffee grounds during the brewing process.

It’s the old Goldilocks problem. Some flavor compounds are more easily and quickly dissolved in brewing water, while others dissolve more slowly: In order to create a balanced cup—in other words, not sour or bitter—the extraction needs to be juuuust right.

To this day, much of what we know about extraction and its effect on flavor preference comes from a former M.I.T. scientist (and arctic explorer!) named E. E. Lockhart, who conducted extensive research about American tastes during his time at the head of the Coffee Brewing Institute.

Through his team’s studies, Lockhart developed the Brewing Control Chart. This handy tool shows where most people’s preferences fall in reference to the amount of flavor material extracted from the coffee relative to the strength of that flavor in the finished beverage. He determined that for most people, under-extracted coffee tastes sour, while over-extracted coffee tastes bitter; underdeveloped or weak coffee tastes somewhat papery, while overdeveloped or strong coffee tastes somewhat ashy.

We don’t need to get too in the weeds about the science behind all this (unless you want to, because we’ll definitely go there), but basically, what you need to know is that a consistently sour taste to your brew, regardless of what coffee you’re making, might mean that you’re under-extracting your beans. And even if you sweeten coffee or have thought about what salt in coffee can do to alter the taste, the sour sips may not go away.

Never fear: There are several quick ways to fix under extracted coffee.

Ratio: If you’re under-extracting your coffee, it could mean that there’s too much coffee and not enough water to extract it with. Consider trying a brew with just a bit more water and seeing if it reduces the sour flavor. (We recommend adjusting in 2- to 3-ounce increments while diagnosing your brew until you get the achieve the best flavor compounds.)

Here are more concrete measurements depending on the coffee:

Grind Size:

Coffee grinds against a white background.

Another possible cause for under-extraction is grinding your coffee too coarse. Chunkier coffee pieces are harder for water to penetrate, which makes them harder to extract from. Try grinding your coffee just a little finer to see if that does the trick. (We recommend adjusting in very small amounts while you’re diagnosing your brew—and remember, don’t change both the grind and the ratio at the same time, or you’ll skew your results.)

Water Temperature:

In order to dissolve all the tasty stuff and make a more balanced cup, you need to use properly heated water, which dissolves faster than cooler water. The ideal brewing temperature for coffee is 195°F to 205°F: Any lower and you risk not dissolving everything you want to taste in the cup.

Brewed coffee times may also depend on the type of roast or coffee beans you’re using:

Light Roast vs Dark Roast:

With a light roast, you’ll need soft and low alkaline water (preferably after water is boiled), longer brewing time, a finer grind, and more agitation. As for a dark roast, you need a coarser grind, brew at a lower water temperature, and use more coffee grind than water.

Arabica vs Robusta:

Arabica coffee beans require a coarse grind, an equal ratio of coffee to water, and need to brew in boiling water. For robusta beans, you’ll need more water than coffee, a coarse grind, and a shorter brewing time.

High Acidity

Another reason you might be experiencing a sour taste—not related to your brewing practices this time, thank heavens—is the coffee itself: Certain coffees have very tart, punchy fruit flavors that can be overpowering to certain palates. Especially if you’re accustomed to a smoother, more chocolaty, or nutty tasting coffee, that zesty acidity or brightness might be more intense than you bargained for!

Lighter-roasted coffee also tends to have a higher expression of acidity than darker-roasted coffee since darker-roasted coffee tends to taste a bit like the roast process itself. (Just like wood-oven baked pizza will taste a little charred from its time in the oven—same thing.)

(Remember that coffees that taste fruitier or more tart don’t necessarily have higher pH values than ones that don’t; when we use the term “acidity,” we’re talking more about perceived acidity or how we process the flavors on our taste buds.)

If you find that you’re particularly sensitive to that sour note, you may want to search for coffees that are medium or dark roasted and avoid flavor notes that imply a tart fruit taste: Many coffees have “citric acid” or “citrus” listed in the tasting notes, and those might be ones you’d like to steer clear from. Notes of chocolate, nuts, brown sugar, and toast usually imply a more balanced cup, while less-tart fruits like red apple, watermelon, peach, banana, or blueberry might point toward a fruity coffee flavor without being sour.

Bitterness vs. Sourness

Stumped by the sour flavor? You’re not alone: Lots of folks actually have a hard time tasting the difference between bitter and sour, especially in coffee.

Why? Well, both tastes are so overwhelming we don’t tend to linger over them long enough to really develop a nuanced idea of what’s happening to our taste buds—we just know we want it out!

Knowing that bitter taste is a sign of over-extraction and sour tasting coffee is a sign of under-extraction, it might serve you well to drill your tongue to tell the difference. One easy way to do that is to make a solution of each and taste them comparatively.

To make a bitter solution, you can dissolve a baby aspirin into a ½ cup of water and taste it. To make a sour solution, mix 3 tbs of lemon juice into a ½ cup of water and taste it.

Notice the way your mouth reacts, where it feels dry, where and how it puckers, and notice especially how the aftertaste feels and tastes. (Aftertaste is sometimes your best ally when diagnosing an extraction issue.) Long, lingering aftertaste often accompanies bitterness, while a quick and dry aftertaste often walks hand-in-hand with sour.

Coffee Maker

Moccamaster coffee maker on a countertop.

Your coffee maker can also play a role in your coffee taste. One coffee bean blend might taste great brewed with a French press but taste differently if you use an espresso machine. Whenever you try a new type of coffee, check the package instructions to see if it has a specific brewing method. That way, you can prevent ending up with sour or bitter coffee. Want to learn more about coffee extraction and how to experiment with it? At Trade, we’ve got you covered.

As long as you practice brewing your different coffee bean blends, you can prevent sour coffee tastes. Fortunately, some specialty coffee beans come with package instructions so you can experience their flavor profile.

Whether you’ve been drinking coffee for years or are new to the world of caffeine, take our coffee quiz to know which blend is best for you. Sign up for our coffee subscription to learn more about the different types of coffee and customize brews to your liking.

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