Sugar – Why does adding sugar/salt causes water to release

Salting the water in which you cook starches (pasta, rice, potato) is an effective way of enhancing the flavour of the finished product - boiling starches absorb salt well (which is why adding chunks of potato to an overall salty stew will lessen the apparent saltiness of the dish.

But salt does other things. When I am making roasted potatoes, I parboil them for 5 minutes before drying and roasting them in oil. if you divide them into two batches and boil one half in unsalted water and the other half in well salted water (1tbsp/2 quarts water), the salted potatoes will brown and crisp much better than the unsalted ones. I'm not sure why this is, but I would encourage you to try it because it's amazing to see.

Sugar does not cause sour food to be any less acidic. The difference is purely one of perception; we are wired by evolution to prefer sweet tastes and tend to perceive less of other tastes when a high sugar concentration is present.

Sugar does not ionize - it is not basic or acidic. Acidity itself is a chemical property, and only a base (such as baking soda or trisodium citrate) can actually neutralize it. Two other compounds - miraculin and curculin - actually alter our mouth chemistry and really do cause sour foods to taste sweet, without actually sweetening the food itself. But plain sugar does nothing at all - there is no chemical reaction happening. It's just masking other tastes, not neutralizing them.

The reason sugar doesn't mask bitterness as effectively as sourness is that sweet and sour have roughly similar taste thresholds, while most humans are extremely sensitive to bitter tastes. We can detect quinine (the reference solute for bitterness) in solution at 0.5 ppm, whereas sucrose isn't normally detectable at levels below 5000 ppm. The amount of sugar needed to mask a significant bitter taste is simply not practical.

If you want to neutralize a bitter taste, use salt instead.