If you’re new to making coffee at home, or even if you’re experienced at the process, you might not know what can make your coffee taste bad. There’s a lot of things that can go wrong during the brewing process, and there’s a lot of ways you can create those less than ideal results. Today we’re going to discuss one key flavor faux-pas for your at-home brew: Sour coffee. How does it happen? And more importantly, how can you fix it?

Why Is My Coffee Sour?

Getting a good coffee flavor means you have properly extracted the coffee from the grounds. If your coffee tastes bad, usually it’s because of improper extraction. Extraction refers to the chemical process of coffee bonding with water as it passes through the grounds and whatever filter your brew method calls for. Believe it or not, this issue is brew method agnostic. Whether you’re pulling an espresso shot, making a batch of cold brew, or anything in between, sour tasting coffee comes from the same issue: under extraction.

Under-extracted coffee can be caused in a few ways, but no matter what, this is the reason you’ve got a sour cup or shot. This means that your water didn’t properly bond with the coffee grounds, and so it came out tasting sour. This is distinct from a weak cup, which may just taste milder, and be to your preference! Making a weaker cup usually comes from using less grounds or more water, but still getting proper extraction. So why does under-extraction happen?

How Do I Fix It?

Under-Extraction happens a few different ways, and it all comes down to heat, grind size, and time (which is often regulated by grind size, but can be a pump issue in espresso). First up is your water heat. You should be brewing coffee with water in the 195-205 Fahrenheit range. If your water isn’t hot enough, the chemical process that bonds it with the coffee grounds won’t properly complete, and you’ll get that sour tasting cup. If you have a decent espresso machine or drip brewer then you shouldn’t have to worry about this issue, but if you solve everything else it could mean that your heating element isn’t up to snuff. For pour over, you’ll want to invest in an adjustable temp kettle to get that perfect temp. Finally, for cold brew, you’re obviously using room temperature water, and generally won’t have to worry about this issue since the extensive brew time makes up for the heat.

Next is grind size, which is closely related to flow rate and time. Generally, if your brew method has more time in contact with your coffee grounds, you’ll want a coarser grind. This is because the water has a longer time to bond with the larger surface area of coarse grounds. To translate this to the topic at hand - your coffee might be too coarse for your brew method if you’re getting sour cups or shots. You can think about it this way: if the water flows quickly through the coffee, giving it too little time to bond, then you’ll get that same under-extraction that you get with cool water. You need a finer grind as pressure increases, as the finer coffee will help to slow the water passing through your filter. This is why we use an almost powdery grind for espresso, which is brewed with a pressurized pump.

Now you can see how time fits into this equation. You need to make sure that your water has time to bond with the coffee molecules, which it does faster at increased temperatures.

So how then, do we fix it? It’s dependent on the brew method. For espresso, consider fining up your grind to get your shot time to around 25 seconds. This is usually a good baseline for proper extraction. For drip and pour over, consider doing the same thing to reduce flow rate and ensure proper time for extraction. For press and cold brew, you could try fining up your grind, or increasing the brew time to make up for the increased surface size of the grounds. All of this assumes that you’re brewing at that proper water temperature too!

Now you should be able to diagnose that sour shot or cup of joe!