Why does my coffee sometimes taste sour?

Excessively sour flavor in your coffee brew is a likely sign of underextraction, i.e. the coffee has not brewed long enough and has an excess of acids. Acids are extracted early in the brewing process, whereas other balancing flavors are extracted later in the process.

Per Wikipedia:

[The coffee is] "under-extracted", specifically "under-developed" – desirable components have not been sufficiently extracted – and "unbalanced", specifically sour, because acids are extracted early, while balancing sugars (sweetness) and bitter components are extracted later.

Making good coffee involves a multitude of factors, and it can take some trial & error to hit the sweet spot. But let's mention the major ones:

Yields depend primarily on temperature, brew time, and grind size, and in a complex way on method.

For your case, I'll just call out the most obvious thing: Because you're using a French press, you have to use a coarser grind than something like a drip machine requires. Those larger coffee grains are going to require more extraction time.

So it could be that it simply needs to steep a little longer before pushing down the plunger. I personally like to give my coffee a stir for about a minute after pouring in the water to help encourage thorough extraction.

Hopefully some experimentation along those lines will help you achieve the perfect cup. Happy caffeinating!

I am pretty sure this is closely linked with the cleanliness of your apparatus. From experience I have brewed consistently sour coffees using a plunger, despite altering beans, brew temperature, grind settings and brew timings. I then cleaned my plunger thoroughly with detergent and a scrub-brush, pulling it apart to ensure nothing was missed, and was then able to get a non-sour cup of joe going.

Someone once told me that the sourness was due to hot coffee coming in to contact with cold apparatus. I've done some experimenting around this also and have found this isn't true. Specifically testing:
- good hot coffee over ice = delicious/not sour
- good hot coffee in a cold cup = delicious/not sour
- good hot coffee left til it cools for a day = delicious/not sour

Acidity is usually considered desirable in coffee, but maybe that's just not to your taste? Often single-origin beans are given a light roast to preserve acidity, but you can find dark-roasted single-origins. A dark roast will knock out most of the acid.

Over-brewing tends to make for a bitter brew, not a sour one. It's also possible that you're getting flawed beans, but I think it's more likely that you just prefer a dark roast. :)

Well, in my situation I find the difference with the water. I go to my hometown every summer and winter and whenever I make coffee, it doesn't matter how much coffee I use since it tastes pretty good and doesn't have any sour flavor. However, I travel back to the U.S. and again, the coffee tastes way too sour. The only way to drink it is adding cream or milk... and I don't like to do such things. The reason I think it's the water is because I bought coffe in the U.S. tried it there and then on my hometown, i got the same results US = Sour Hometown = good taste

And repeated the same witha coffee from a store in my hometown.. and the same result. The water at my hometown comes from a river below surface that originates at a mountain which means that it comes pretty clean and unadulterated. Different from it, in the US the water I use comes from bottles and the system that distributes it. Not to mention that it's boiled properly. My conclusion is that it might be the quality of the water.

This is just my opinion as a coffee roaster.

Sourness comes from the type of coffee. Brew technique can alter only slight of the sourness, however right roast technique can transform from sourness into more rich mouthfull body. Just as Leon mentioned earlier. That is why roasting coffee is difficult and there are different technique to deal with differen type of coffee.

Washed arabica are usually more 'bright' to sour then the natural. Natural has more body, but there are not much room to play with the aroma and flavour during roasting. That is also why there are coffee blends.

Saying that washed arabica is one of the highest valued coffe on the market. Don't be put off that 'sour' coffee is bad. It is just 'underroasted' in most cases. For example Ethiopean Yirgachee or Washed Rwanda tend to be very bright (sour), but of roasted correctly it can makes most beautiful coffee.

Also for espresso. Sometime a wash arabica need to be roasted longer then let it sit for 2-3 weeks, then the magic will come out. For filter coffee I will suggest to get a Viennese roast profile. Short after the first crack.

A good roaster must learn every beans he has got and do different roast profile for different beans.

I would suggest to talk to the roaster. :)

Sourness beyond what is desirable in coffee usually stems from too long extraction time. The longer the ground beans are steeped in hot water, the more flavors are extracted from the beans. However, the longer you extract the flavor, the more acidity is extracted.

I usually extract for two to three minutes.

I am a roaster and agree with the other roaster that it is under roasted. That is the problem with third wave roasting.It is a fine balance. Also ,it is possible that the green beans are fermenting and or over fermented while processing

usually roasts for filter coffe are less darker then for espresso. darker roasts tend to have less acid, but it depends on the coffe beans used and the conditioning of the beans in the country of origin. a different approch is found here http://www.wolfredo.de/wordpress/ the idea is to roast coffee the "best way possible" and change parameters while brewing. so, changing the degree of ginding, teperature of water, exraction time etc. has a huge influence on the taste. goldfilters filter less oil, so the coffe should tend to taste bitter compared to standard paper filters. if you try beans like monsooned malabar u will notice very little acid, due to the fermentation and cspecial conditioning. to sum it up, the taste of the cup not only depends on the beans and roastng but also on the preparation. changing f.e. the gridner settings changes everything. i often brew espresso roasts like filter coffe and like the taste.

If you grind your beans change the grind setting on your grinder and see if that makes a difference.

This happens to me, I think it is linked with the type of coffee. I bought a Colombian and to me it tastes a bit too tangy or sour. I roast it myself and everything, it's strange. Although a darker roast tends to mellow that out.

I use pour over for my coffee and recently bought Kenyan AA beans. It tasted sourish and I contacted the roaster to confirm it was medium roasted. He suggested I grind it finer to lower the course taste. Typically, Kenyan AA tend to be sourish. But in this instance I suspect it to be under roasted. I took his advise and found it to reduce the sour taste a bit but it is still over sour for my taste. I agree with the earlier comment from a roaster here that brewing may tweak the flavor a tad but not much.

While other answers are perfectly correct, Matt Perger's Coffee Compass is a general "cheatsheet"/tool that can help you adjust your brewing parameters regardless of what's wrong or how you're brewing (except espresso).

1. Make a cup and taste it. Not what you like?
2. Use the map on the left to determine what's wrong. Use that position is the center of the compass on the right.
3. Now use the compass on the right to travel back to the pleasant, green area.
4. Read the text below the compass for available options.

When using the compass, don't mess with your brew temp. And while the compass might suggest you adjust X or adjust water amounts, in some of Matt's Youtube videos he generally recommends keeping the water amount as fixed; it's just fewer parameters to worry about and you probably want the same volume in your cup when you're done anyway...