This is a mini-project looking at sweet and sour flavors, sponsored by PAMA Pomegranate Liqueur.
Looking at sweet, sour, and the balance between them, we'll now start with sweet.
Sweetness is sensed on the tongue as one of the four (up to possibly six) basic tastes. We sense sugars as sweet, of course, but also some proteins and amino acids taste sweet to us. Some things we consume are both sweet and something else. An example used in the book Taste Matters is saccharin, which is both sweet and bitter.
The preference for sweetness (and different reactions for sour, salty, bitter, and umami) is present at birth. Breast milk is sweet and we are predisposed to like it as with other sweet things. We inherently don't like bitter things, and this was shown in a study on newborns where literally the first thing they ingested after being a few hours old was quinine and they wrinkled their faces to show displeasure. This makes biological sense in that things that are sweet are typically laden with energy, while bitter things often indicate poison or unripeness.
Interestingly, the "optimally pleasant" preference for pure sweetness (sucrose in water), is 10-12 percent by weight regardless of where in the world one grows up. It would be interesting to see how sweet that is and how sweet cocktails are in comparison...
Sweetness, when found in plants, often indicates the presence of carbohydrates, which is a major source of energy for the human body.
In plants that photosynthesize, sugars increase during the day and decrease at night, so plants that are cut later in the day are sweeter. Herbivorous animals like rabbits prefer sweeter greens (those cut later in the day) and indicate a strong preference for sweetness, while carnivorous animals are apparently indifferent to those changes as they get most of their energy from meat.
Of course, sweetness in the presence of fat (aka snacks and desserts) indicates maximum energy and so we naturally crave it the most. Time for cake.