How does fresh pineapple tenderize meat?
There are a chemical in pineapple called enzymes that digests protein in meat. Digest means that large chemicals are broken into smaller chemicals.
Bromelain is a protein digesting enzyme found in pineapple. Actinidin is another type of enzyme found in kiwi fruit. These two enzymes are proteases, which means they digest protein. Digesting protein means the protein molecules are chopped up into smaller parts, thus making meat more tender to eat.
Note: Heating pineapple denatures its protein. In other the enzymes in cooked pineapple cannot digest protein. Only fresh pineapple has active meat tenderizing enzymes.
Food and Nutrition for Every Kid has more information about protein digesting enzymes as well as other fun facts and investigations.