If taste cake or quick bread (zuchini, banana, etc...) batter I find that it very often tastes quite a bit sweeter than the finished and baked product.
What happend during baking that reduces the sweetness?
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I think that SAJ14SAJ listed some good examples, and that these certainly contribute to the overall taste. But I think there is yet another one, whose effect is strong enough to matter: rising.
Let's say that you coat 1 cm² of your tongue with a foodstuff. The more tastebuds are activated by a sweet molecule, the more is it evaluated as "sweet". First imagine your batter. It is dense, and coats all of the tastebuds on your tongue. Each of them is likely to register a few sugar molecules.
Now imagine the baked cake. It has air bubbles in it. If the plane of a cake cut has X% air bubbles by surface, then only 1-X% of your taste buds come in contact with the sweet cake. And the concentration of sugar molecules in the solid part of the cake surface is the same as the concentration in the batter, so you have the same number of sugar molecules per activated taste buds, but less taste buds activated. As cake volume contains lots of air, you also get lots of air bubble area in a cut. The effect is reduced by chewing, but by the time you have chewed the cake very well, it has also been diluted by lots of saliva, so now you have a much better coating, but less sugar molecules per taste bud.
By the way, this effect also occurs with salt in bread dough vs. baked bread, so I don't think there can be factors uniquely bound to the details of sweetness perception, as salt uses very different chemical pathways of being tasted.
In the end, except for some very minor carameilzation, there is actually no actual reduction in the amount of sugar in the baked good, and actually proportionately more sugar as some of the water is evaporated. Any perceived increased sweetness is in fact a perception issue, not an issue of there being less sugar in the baked product.
This is speculation, but informed speculation:
Two major seemingly relevant things differ between the batter and the final baked product:
Compare this to the baked state:
Therefore, in the batter, there is less starch competing for access to the taste receptors, diluting the ability of the sugar to reach them. At the same time, the sugar is already dissolved and more readily accessible to the taste receptors. Finally, the sugar is not locked away in the structure in the batter, and is more available to taste.
Thus it would be easier to perceive the sweet flavor.
Of course, it could all be psychological as well, but that would be outside the scope of a cooking Q&A.
A theory I've heard of is to consider the fact that while it's baking it makes the whole house smell delicious. All those aromatic flavor molecules you're smelling are no longer in the cake!