Pineapple is a food known to contain bromelain, an enzyme that digests protein. The truth is, pineapple hurts to eat because bromelain is digesting the tender skin inside of your mouth.
It is common knowledge that people digest food to get energy. At first glance, this seems to be a one-way process. Curiously enough, there are also some fruits which can have a destructive effect on the tissues of our body. Of course, they cannot digest us the way that we can digest them. But to a certain extent, they can break down the cells of our mucosa.
We all know the burning sensation in the mouth after we eat pineapples. It is hard to get rid of it, even by drinking water or rinsing our mouths. Sometimes, the slight pain can last all day, and our taste perceptions become noticeably worse.
Pineapples contain the enzyme bromelain, which breaks down protein. Bromelain is actually a mixture of two enzymes and several other substances in smaller quantities. Its ability to break down protein is used in cooking, to make meat more tender. Pineapples are not the only fruit containing enzymes. Kiwi fruit, papaya and some other tropical fruits can also be active to protein.