Chicken – How long is too long to cook chicken

You cooked it at too low a temperature.

Sous vide is intended for meat where you want the protein to remain tender. It shouldn't have any sinews. Think chicken breasts, or the long filet along the spine of a pig. This meat gets nicely cooked at 60-65°C (depends on the animal), and tough and dry above that.

Meat marbled with sinews has to be cooked at a temperature where the sinews (collagen) melt into gelatin. This happens at about 70°C at least, and takes hours. Since the muscle fibres are already toughened at that temperature, there is no reason to hold it low; you can put it at full boil in a normal pot and cook it there, you just have to wait long enough. In theory, you could do it in a sous vide bath too, but you won't get any of the benefits sous vide gives to tender meat.

It depends upon the strength of your brine, for boneless chicken breasts I recommend a 5% brine for 30 minutes to an hour.

Ideally, salt for brining should be measured by weight, especially since volumetric measurements for the same weight of salt will vary depending upon the coarseness of the salt. A 5% salt solution means you should use 20 times by weight the amount of water to salt. That's easy and foolproof if you have a scale. If you must use spoons or cups to measure salt, Morton's Kosher salt is very close to two tablespoons equaling an ounce, so 20 ounces (2.5 cups) of water to 2 Tablespoons of Morton's Kosher Salt would be a 5% solution. Use about 65% of the volume measurement if you must use table salt and measure by volume.

Incidentally, for larger, bone in pieces brine in a 5% solution for 3-5 hours, a whole chicken for 12-16 hours (roughly).

Also, be sure that your chicken does not have water added, as many frozen, bulk pieces do. In effect, those chicken pieces are already brined.