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This is one of those things which I've always thought to be obvious and never really noticed as strange.
"Obviously", if you add sugar to something sour, it makes something sweet. Take for instance Bramley apples in apple crumble, or lemons in lemonade. This is just something that people know!
But if you add sugar to something bitter (coffee, say) it doesn't stop tasting bitter! It just tastes like sweet coffee.
Any explanations?
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.
There is no chemical reaction taking place when you add sugar to coffee, so the sweetness of the sugar and the bitterness of the coffee are both preserved. You can try to achieve a balance between different tastes, but in most instances you aren't altering the chemical compounds that we detect with our taste buds. The coffee, though sweetened, doesn't stop tasting bitter because humans detect different tastes by using different receptors, so we can taste multiple things simultaneously.
