Flat water tastes different than regular water because even though the CO2 leaves the water it still leaves a large enough mass of bicarbonate that the water tastes bad (compared to regular water). The key point here is that the dissolved carbonate species (bicarbonate and carbonate) don’t leave solution, just CO2.
Detailed The answers about increasing acidity are not correct. When the bottle is sealed it is supersaturated with CO2 , which forms H2CO3 in water which then forms H+ and HCO3- which forms a miniscule about of H+ and CO32-. This is acidic. When the water goes “flat”, the CO2 leaves causing a shift in the equilibrium to the left, ie causing HCO3- to react with an H+ to form H2CO3. This reduces acidicity and causes the water to be more basic. You are probably tasting bitterness of “basic” ions (bicarbonate), not sourness of acidic ions.
You can try this at home. Take some baking soda (sodium bicarbonate) and add it to water. It does not taste good. Like “flat” water. Or try baking powder if you have that.
This is actually why Guinness made their stout partially infused with nitrogen. They wanted to make the beer less bitter, but keep the same recipe. By replacing some of the CO2with nitrogen, the carbonic acid is reduced, thus lowering the amount of bitterness. They were the first to do this. It also makes for a more silky mouthfeel with the smaller bubbles. Yummy
Swapping out CO2for Nitrogen will not reduce bitterness. Rather, it will reduce sourness. Acids tastes sour to us. Always. Also carbonation bubbles provide a “sharp” mouthfeel. Furthermore, Nitrogen tends to form smaller bubbles than CO2and so given the right protein profile provides a “creamy” mouthfeel.
So by using Nitrogen as a major component in the beer gas we have three desired effects with Guinness:
- less sour tasting
- less sharpness or bite in the mouthfeel
- a creamy mouthfeel
It should also be noted that a Guinness tap has an aerator or mesh on the head that, by providing many nucleation points, causes much of the beer gas to come out of solution and give the abundance of little bubbles and foam we like to have.
Because it is acidic. The CO2reacts with the water creating carbonic acid, which is what gives carbonated drinks their bite. When it goes flat, you just have regular water with the acid but no bubbles.