I am making a French Press coffee from my pre-ground coffee that I get from my local coffee shop. I store it in an air-tight container and brew it like such
What could be the cause of the coffee being too sour ?
In a comment, you mentioned that the problem seemed to be the beans. Ground coffee doesn't stay fresh very long, even in an air-tight container. But several other things pop out in the question that you could also look at.
People often start with a coarse grind for French Press, then brew it for a very long time to extract the flavor. That can over-extract undesirable flavors. To extract the flavor from inside coarse chunks, the outside of the chunk, and smaller particles, gets over-extracted. Try a slightly finer grind with a shorter brew time.
You're using a 1:20.5 ratio of coffee to water, which is very weak. People tend to use a French Press when they want a full-bodied flavor. If you try to extract a full-bodied flavor from that small amount of coffee, it will be very over-extracted, and you'll get to flavors that don't taste good.
For comparison, a ratio of 1:16 would be closer to normal-strength coffee (it would equate to 14 g of coffee in your recipe); 1:14 would give you strong coffee. I've seen recommendations as high as 1:12 for French Press. At those ratios, you don't need to over-extract. Get out the good-tasting stuff with a shorter brewing time, and leave the bad-tasting stuff in the grounds you throw away.
The extraction time is very long, which may mean that you're over-extracting. If you start with the right ratio of coffee to water and the right grind, a total extraction time of 4-5 minutes should be about right. Try this process as a starting point for timing:
At the end of the 4-5 minutes (or the brewing time you figure out), just end the brewing and pour the coffee. Don't worry about scooping out the sludge (try not doing it and see if you can taste a difference). The sludge is the same coffee grounds, just a higher percentage of fines, and you'll get some of those in your cup, anyway. If you give it a quick stir, most of it will fall to the bottom.
Just do the plunge very gently to minimize forcing fines up through the screen. Some people don't plunge at all, they just leave the screen above the brew and pour through it. From the time the brewing is done, you'll be adding maybe another half a minute of extraction getting the sludge to sink and plunging the screen. This will give you a simpler process, with better control of the timing.
The amount of water sounds like you're brewing a single serving, so I assume you're already pouring all of the coffee rather than letting some sit and continue to extract.
One other thing to try that may improve taste is a water temperature lower than what is typical. With a coarse grind, people tend to use near-boiling water to help with the extraction. If you switch to a slightly finer grind, you could try water well under 195F (try 195F, 185F, and 175F to see which gives you better flavor). Lower temperatures will reduce the amount of harsh-tasting stuff extracted, so it will be a smaller portion of the flavor. If the lower temperature water makes the extraction noticeably weaker, but the flavor is better, increase the extraction time a little to compensate.
One reason could be that your grind is too coarse. Is your coffee pre-ground for french press brewing? Maybe try a different coffee to see if the bean is just not your thing. Your water could also be the problem, if the PH-value is very low (under 7,0).
