Why does eating pineapple make your tongue feel like it is being bitten?

What happens

After a few pieces of pineapple, your tongue, the roof of your mouth and the inside of your cheeks start to tingle and sting — like being pricked with very tiny needles. Most people blame the sourness, or rough fibres scraping the mouth. Neither is right.

The reason: pineapple carries a pair of protein scissors

Fresh pineapple contains a group of enzymes called bromelain, from the cysteine protease family. They do only one job: cut long proteins into short pieces.

How strong are they? Cooks have known for a long time that if you pour fresh pineapple juice over raw meat and leave it a while, the meat turns tender — because the enzymes chop up the proteins inside it. Factories that make bromelain mostly extract it from the pineapple stem, where there is the most of it.

And here is the trouble: your tongue, the roof of your mouth and your cheeks are covered in a thin layer of mucus that is also made of protein. While you chew, the scissors cannot tell "meat" from "tongue". They quietly shave off a very thin layer of your mouth — and that is the sting you feel.

So why is canned pineapple harmless? Because these enzymes hate heat. Wikipedia puts it plainly: cooked or canned pineapple no longer tenderises meat, because the enzymes are destroyed during cooking. Scientists call this being "heat-labile".

Try it: let pineapple eat your jelly

Buy a box of gelatine jelly powder and mix it as the box says. Split it into two cups: stir two spoons of fresh pineapple juice into one, and two spoons of canned pineapple juice into the other. Leave both in the fridge overnight.

The cup with fresh juice will never set — it stays sloppy, because the enzymes have cut the protein framework of the gelatine. The canned one sets normally.

For a real surprise, try the same test with agar instead: agar is a sugar from seaweed, not a protein, so bromelain cannot touch it, and it sets perfectly. One thing worth remembering: gelatine and agar both look like setting agents, but one is a protein and the other is a sugar.