And how to avoid that flaming hot mouth feeling
There's a scientific reason why fresh pineapple burns your tongue, and no, the answer is not (always) allergies. Many people feel a tingling, sometimes-burning sensation when they bite into fresh pineapple, and lots of them—incorrectly—chalk up the feeling to the fruit's acid content. The real reason why your mouth burns when you eat pineapple is because of the presence of "active protein-digesting enzymes that are used in meat tenderizers, but can cause problems in other prepared dishes," and your own mouth, writes Harold McGee in On Food and Cooking: The Science and Lore of the Kitchen. That's why some people wonder if pineapple is a flesh-eating fruit.
It's not, for the record—and eating too much pineapple won't make you look like a zombie straight out of TheWalking Dead. But bromelain, the main enzyme in pineapple that breaks down proteins, is what causes that burning sensation on your tongue and the roof of your mouth. And it's why pineapple can be dangerous when handled in commercial quantities. As Maria Gloria Lobo writes in Handbook of Pineapple Technology: Production, Postharvest Science, Processing and Nutrition, "extremely high amounts of bromelain and handling can cause skin rashes, loss of fingerprints if gloves are not worn," among other nasty side effects.
The good news is that though bromelain is found in pineapples, it's "not in high enough doses to act as medicine," or really do that significant damage to your fingertips if you're just an average human eating an average amount of pineapple, explain the experts at the University of Maryland Medical Center.
The best way to avoid bromelain and, in turn, the burning sensation you might associate with eating raw pineapple, is to cook your pineapple. Tossing a pineapple ring on the grill will definitely take the bromelain edge off. If you insist on eating your pineapple raw, though, cut the fruit as far from the stem and the core as possible; those are the places where bromelain is most highly concentrated in the fruit.