You may think Jello with fruit, similar to boiling eggs, is just about the easiest dish anyone can make. You are half correct. Boiling eggs is easy. Jello with fruit is easy only if you add the right fruits. Here is what NOT to add.
When Jello Doesn't Work
Gelaltin is made from collagen. As its proteins tangle, they form little pouches that trap in the fruit. When gelatin solidifies you have a wiggly mound of goodness that is fun to eat.
Just follow the simple directions on the packet to add your cups of hot water, gelatin powder, cold water and pieces of fruit. Stir and set in the fridge until it all gels. Jello experts advise you to be sure to stir until you get a thick 'soup' of jello. Otherwise, the fruit will settle only at the top of the Jello and not inside. Also, pat your bits of fruit dry with a towel before placing the fruit inside the wet mixture. That would not be an issue with bananas, but with other fruits, excess liquid might prevent a proper gel.
Fruits that work nicely with Jello are fresh bananas, blueberries, apples and pears. If you were a child whose mom let you watch her prepare Jello with fruit, then you probably saw her use canned fruit sometimes, such as fruit cocktail. What you may not remember, though, is what she did to make sure the canned fruit did not make the Jello too runny. She no doubt drained off all the juices carefully before adding the canned fruit.
What NOT to Add
The scenario is familiar to everyone who has failed at Jello at least once. You go to the fridge, looking forward to your festive ring of cherry jello with colorful bits of fruit, only to discover a soupy ring of a watery mess. You were careful to add just enough hot water and just enough cold water and the entire packet of powder as directed. What went wrong? The answer is probably in the type of fruit that you chose. Gelatin has its own finicky chemistry.
Pineapple, guava, mango, fig and kiwi are the culprits that prevent Jello from becoming solid. They all have an enzyme, bromelain, that interferes with the ability of gelatin to thicken to a more solid gel. The general scientific explanation is that the bromelain in these fruits cuts away at strands of protein, and the strands do not join together. The protein is degraded and the gelatin simply never gets the chance to become more solid.
Don't Add More Water
Adding the wrong fruit is not the only mistake that can ruin Jello. Other disasters are rubbery jello and lumpy jello, because the mixture was not thoroughly stirred. In the Don't Add category, though, a common error is to add too much water. Your best bet is to work with measuring cups and pour in just the amount of hot water and cold water that the packet instructions or your more elaborate recipe book call for. Jello is not only finicky about the fruits added to it, but the water used. Adding too much water will also give you a soupy mess.