Sweet! What a sensation!

Not all sugars are sweet, and everything that’s sweet aren’t sugars. The sensation of sweet is the interaction between your taste sensors and a particular molecular shape. So generally that sweet sensation in your brain means a particular kind of soluble carbohydrate is in your mouth, and our instincts have been shaped to “like” this taste, a nice genetic inheritance from ancient arboreal primate ancestors. But because of their shapes, not all sugars have the same sweetness; fructose is very sweet, lactose barely sweet at all. Artificial sweeteners and other non-sugar but natural sweet molecules mimic the shape of sugar molecules, thus tasting sweet.
Thus the glycoside in Stevia rebaudiana tastes really, really sweet because of its shape, but since the carbs we associate with sweetness aren’t there, you get a low-calorie sweet treat.
Enter the “miracle fruit” (Richadella dulcifica, sometimes called Synsepalum dulcificum, a taxonomic synonym, Sapotaceae) (This is sort of good to know because actually there are 3 unrelated species with this common name.). You eat this fruit and everything you eat afterwards tastes remarkably sweet. This happens because a protein not only mimics sugars’ shape, but it continues to stimulate your sweet sensors as you eat new foods. It’s effectiveness is pH mediated, so stronger when interacting with sour tastes, but at higher pH (near neutral) the protein changes shape and just blocks your sweet sensors without stimulating them so even naturally sweet foods taste bland. Weird. You can read the whole study here.
So why does a fruit have such a protein? Fleshy fruits are most often a reward for seed dispersers, and the amount of reward is judged in part by size and in part by taste, so a really sweet fruit says it’s packed with sugar. But in this case by making a sweet-tasting protein, the cost of lots of sugar is avoided by fooling the disperser by making only a little protein that tastes like lots of sugar. Bad plant! It’s not nice to fool your seed dispersers! Other sweet tasting proteins are also known, like talin. But since proteins denature in heat sweet-tasting proteins have limited application as sugar substitutes in anything cooked.