I prefer fresh pineapples over the canned version but most of the time I can eat more of the latter than the first. I can rarely eat more than three slices of fresh pineapples because my mouth got irritated after eating eating the second slice. And I cannot eat anymore after the third slice. The irritation never happen when eating pineapple in can or any other preserved pineapple.
Pineapples cause mouth and tongue irritation because it contain the substance bromelain. It cause irritation of the mucus membranes, the inside of the mouth or nose. Irritation is one of the many side effects of pineapple. Worse effects are rare.
Bromelain is an enzyme. It is easily destroyed by heat. This explain why canned and other preserved pineapples never cause mouth irritation.
Due to bromelain content, pineapples are also used as meat tenderizer and weight loss diet.