Final answer:
Fresh pineapple contains an enzyme called bromelain, which breaks down proteins and prevents gelatin from solidifying. To make JELL-O set, it cannot be mixed with fruits like pineapple that contain proteases. Canned pineapple does not have this effect since the enzymes are deactivated by heat during the canning process.
Step-by-step explanation:
The reason why the JELL-O did not solidify when fresh pineapple was added is due to a biological enzyme present in the pineapple called bromelain. Bromelain breaks down protein, and since gelatin, the substance that causes JELL-O to set, is made up of protein, the enzyme prevents it from solidifying. For gelatin-based products such as pudding and gelatin mixes to set properly, they must not be mixed with substances that can interfere with the gel-forming process. Other fruits that contain proteases, enzymes similar to bromelain, include papaya (which contains papain) and kiwi. To avoid this issue, one could use canned pineapple instead, as the canning process heats the fruit enough to deactivate the enzymes.