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Why Is Your Coffee Sour and How to Prevent It

1 July 2020
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We’ve had days where our morning brew fell short of expectations. If you are struggling to figure out why your coffee is tasting sour, it could be down to a few factors.

From the coffee bean to the water temperature and brewing technique, we list common mistakes and how to correct them so that you can get back to making a perfect brew.

Sour vs Bitter

There’s a difference in how coffee is produced to create a sour-tasting cup of coffee and one that is bitter. The former causes a biting sensation on your tongue whereas the latter leaves a pungent aftertaste.

As the saying goes, “to each their own”. There isn’t a clear answer to what coffee should taste like – some like it sweeter whereas some prefer a bitter cup. In fact, a sour taste shouldn’t be thought of as a positive or negative attribute as it is sometimes perceived as a desirable characteristic of the coffee bean. But when it gets too sour, you might want to read on.

The next time you drink coffee, use this taste wheel to help you define what your brew tastes like.

Light Coffee Roast

coffee beans and grounds spilling out from a cup
Light coffee roasts might be the cause of your coffee tasting more sour than usual. Photo from Anastasia Zhenina.

Different shades of a roasted coffee bean produce varying tastes and acidity. The longer a bean is roasted, the more time it has to release complex flavours.

While it has subtle flavour notes, light roasts contain the most acidity compared to medium and dark roasts. With a thin coffee body, your taste buds tend to pick up the acidic notes which give a lingering sour aftertaste.

Try getting darker roasts for your next purchase if your current roast is too light.

Under-extraction

espresso being extracted from a coffee machine
Using a fully automatic coffee machine eliminates the risk of an under-extracted cup of coffee. Featuring the De’Longhi Magnifica S. Photo from De’Longhi.

One of the most common explanations to why your coffee is tasting sour is due to under-extraction. When you don’t brew your coffee for long enough, there isn’t sufficient time for the water’s heat to bring out the complex flavours of the bean to balance the acidity present in it.

Depending on the coffee equipment that you use, brewing time varies. Gadgets like the French Press require around 2-4 minutes whereas an Aeropress requires only 30 seconds. Refer to this list of popular coffee makers and how to use them to make the perfect cup.

Make sure that you do not brew beyond the recommended time as it can result in a bitter brew. If you have made that mistake, here’s how to fix the unpleasant bitterness.

Water Temperature

pouring water over coffee grounds
Water temperature is essential in the brewing process as a lower temperature can leave the coffee tasting sour. Photo from Tyler Nix.

Water temperature doesn’t increase nor decrease the extraction time; it affects the rate at which the coffee ground’s compounds dissolve into your brew. When the temperature is too low, these compounds and flavours aren’t fully extracted, resulting in a sour brew.

The optimal water temperature should be 96 degrees Celsius (205 degrees Fahrenheit). If you don’t have a kitchen thermometer, wait for 30 seconds to 1 minute after your water is boiled before pouring over coffee grounds.

Grind Size

coarse coffee grounds
If coffee grounds are too coarse for your brewing equipment, it can result in under-extracted and sour coffee. Photo from Nathan Dumlao.

“One size fits all” does not apply in coffee brewing techniques. The optimal grind size of coffee grounds depends heavily on the brewing method used.

Your coffee might be tasting sour as the grounds used are too coarse. This causes the water to filter through the grounds at a faster rate, leading to uneven and under-extraction. 

If you are using brewing equipment such as the pump-espresso machine where high-pressure water is forced through the grounds, finer grind size is optimal. Use this handy guide to find out the recommended grind size for your brewing method.

Your Beans or Grounds Are Not Fresh

coffee beans in a cereal bowl
Purchase your coffee beans or grounds in small quantities to ensure that they are always fresh for brewing. Photo from Sonny Ravesteijn.

No matter how aromatic your coffee grounds are or how good the coffee bean looks, they don’t have an infinite shelf life. Once the coffee bean is roasted, it starts to lose its freshness immediately.

Although proper storage in an airtight container can slow down the oxidation progress, the beans should be used within 4-5 weeks of its roasting date to ensure that your brew isn’t off-tasting and sour.