The Aeropress is known for easily and quickly brewing a cup of good tasting coffee. So why is your Aeropress brewing sour coffee? Here you can find the most likely reasons why it’s unpleasantly sour and how to fix this.
Sour coffee from an Aeropress is likely the result of under extraction or stale coffee. Provided the coffee is fresh and the right ratio is used, the extraction can be increased by; grinding finer, steeping longer, stirring longer and using a 30 second bloom time. A higher extraction should reduce sourness.
Below you can find out why that works and how to go about it exactly.
1. Under Extracted Coffee
The most likely reason why your Aeropress coffee tastes sour is because it’s not extracted enough. Just like tea, coffee grounds have to be extracted. Almost all the compounds in the coffee grounds provide something towards the taste of the final brew. There are a few factors which really impact how quickly/far you extract the grounds but more on that below.
What does that have to do with the coffee tasting sour though? Doesn’t under extracted coffee just taste weak? Yes and no. there is an element of strength of flavor missing when the coffee is under extracted. However, the taste also changes. Coffee flavor isn’t one single compound in the beans. There are many different compounds in a coffee bean that all contribute a different part to the taste. And most importantly, they don’t all dissolve into the water at the same rate.
And in general, the compounds that are extracted first are the most sour ones. So if you stop extraction too early, that’s the only thing you’ll taste. The compounds that come out later, balance out the taste balance which reduces the overall sourness you perceive.
However, if you go too far, the last compounds are very bitter. Over extracted coffee tastes harshly bitter. Some bitterness in coffee is normal but that harsh edge is not a good thing and doesn’t have to be there.
It’s possible for the coffee to both be sour and harshly bitter at the same time. This happens with uneven extraction. That means some grounds are under extracted while other are over extracted. This is quite unlikely to happen in an Aeropress though.
There are a few things you can do to increase the extraction in an Aeropress;
- Grind Size: The smaller the beans are cut up, the more surface area the grounds have per gram. More surface area means more surface for the water to interact and extract the compounds. The smaller the grounds, the faster the extraction. For an Aeropress a medium-fine (normal tablesalt) is the right size.
- Steep Time: An Aeropress has something many other brew methods don’t; a steep time. That’s the time you wait after pouring in the water and before pushing down the plunger. The longer you steep, the more you extract. 2 minutes is a good starting point for Aeropress steep times.
- Stirring: Agitation increases extraction. This is exactly why a washing machine rotates. Stirring once or twice during brewing will help up the extraction quite a bit.
- Bloom: the bloom time is the time you give the coffee to swell up and release Co2. You do this by pouring just a little bit of water on the grounds (in the Aeropress) and waiting for 30 seconds before pouring in the rest of the water. Co2 repels water so doing this during the main brew time can effectively shorten it. It can also prevent a mess.
- Water Temperature: For an Aeropress, using water that’s just off the boil (94C-96C) is good. Just wait for the water to stop bubbling and pour it in. By the time it hits the grounds, it’ll be around 94-96 degrees Celsius. Lower temperatures extract slower which is usually not necessary in an Aeropress.
- Coffee Roast Level: This isn’t something you can influence during brewing but it’s something to be aware of. Lighter roasted beans are harder to extract. So if you have a light roast, it might need a slightly finer grind, more steep time, etc to get the right taste.
Another piece of the extraction puzzle is the ratio used. You can try to increase extraction through the means listed above but, there is another factor that’s quite important and that’s the ratio of coffee grounds to water.
The more grounds are used, the more extraction has to be done. Water has to do the extraction in the end and if you keep the amount of water the same but increase the grounds, that same water has to extract more. Again, you can do this by using the methods listed above but getting the ratio right in the first place is even more important.
For a full Aeropress (filled up to the top of #4) about 14-17 grams of coffee grounds is good. Measuring with a scale will really help you figure out if you’re in the right range. Scoops are not the most accurate way to measure coffee and a scale is the only surefire way to get exactly the right amount.
3. Stale Coffee Beans
Stale coffee (beans) can also be a cause for sour or just generally nasty tasting coffee. Coffee goes stale over time after the roasting process.
Coffee beans go stale because the compounds in the beans are oxidized over time by oxygen in the air. The most oxygen exposure there is, the faster the oxidation occurs. Oxidized coffee doesn’t brew good tasting coffee.
Whole beans last longer than pre-ground coffee since they have less surface area. Pre-ground coffee is cut up into small pieces so when you open the bag, much more of the coffee is exposed to oxygen. For this reason alone fresh beans often produce better tasting coffee.
It’s also important to store your coffee in an airtight container which is ideally vacuum sealed. There are vacuum storage canisters for coffee which work well to preserve the best taste for longer.
An Aeropress does a good job of extracting the real taste of the coffee. It brews a very ‘clean’ tasting cup because of the paper filter. That means you can pick up on good taste notes earlier but also the bad notes.
Buy a Fresh bag of coffee and see if that helps you brew better coffee. Ideally use freshly ground whole beans.
4. Type Of Beans
There are many different types of beans and all those beans can be treaded in different ways resulting in different tastes. Some beans are much more sour than others. Many people like some fruity acidity in their coffee but this isn’t the same as an unpleasant sourness.
Where this line is, is a matter of personal taste and the quality of the coffee. In low quality coffee, that acidity could taste just sour, especially when the coffee cools down. In high quality coffee, a certain amount of acidity can be delicious and makes the coffee taste fresh and juicy.
If increasing extraction does not work to alleviate the sourness for you, try a different type of coffee next time. If you’re using lower quality coffee, it’s always a good idea to go for something a bit nicer if you can afford it. A local roastery is your best bet since they will be able to advise you on what you need.