I made two small packets, boiled 2 cups of water, then put in 8 oz of frozen pineapple filled with cold water the rest of the way to the 2 cups mark.
The Jello sat in the fridge overnight and today it wasn't set. I put in a big packet of mix and a cup of boiling water. 4 hours later it is still not set.
What can I do?
As the instructions on the box say, you shouldn't put fresh pineapple (or kiwi-fruit) in the Jell-O. Apparently pineapple has an enzyme called bromelain that breaks up the gelatin into its component amino acids.
You can use canned pineapple instead as the pineapple is cooked during the canning process and this denatures the bromelain.
Generally acids will inhibit the gelling process of gelatin, as I found when I tried to make a lemon jelly once and failed! Pineapple, as well as having its own particular enzymes is also fairly acidic so in your case it was probably this combination of factors. If you are using juice sometimes you can boil it to reduce before adding gelatin and cooling.
