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If you've ever made gelatin for a picnic or party, maybe you've been warned not to add certain fresh fruits like kiwi or pineapple, and for good reason.  Add these foods to your gelatin and it won't set.  It'll remain a big goopy mess.  The reason why is pretty straightforward, but if you understand it, it can help you learn how to make better desserts and how to up your cooking game in general. 

To understand why these fruits ruin gelatin, you need to know what the stuff is made of.  Once it sets, gelatin is basically a mesh of interconnected collagen mollecules.  Collagen is a long, fibrous protein found in things like skin and bones and yes, the collagen in gelatin does traditionally come from animals.  

Manufacturers boil things like hides or bones until the proteins come out, and then they turn them into a powder you can buy at the store for like, a dollar.  When you dissolve the powder in hot liquid, the collagen separates into individual fibers.  Then as the solution cools, those fibers start to bind to one another and gradually form a mesh, but if you add certain fruits as you're making your gelatin, the last process will never happen.

It's because fresh kiwi, pineapple, and some other fruits contain high concentrations of proteases, or proteolytic enzymes.  These are substances that digest proteins, basically, chopping them into tiny pieces.  If this stuff gets into your gelatin mix, it will cut up those strands of collagen until they're too short to link together, so your dessert will come out looking like a soup. 

If you know why this happens, though, it's easy to find some ways to work around it.  For one, if you cook the fruit, the heat will inactivate those enzymes, so your gelatin will turn out just fine.  This is also why something like canned pineapple doesn't cause issues.  Alternatively, you could make your dessert with agar instead of gelatin.  Agar is made from certain types of red algae, and like gelatin, it forms an interconnected meshwork of fibers, but it's made of carbohydrates, not proteins, so proteases won't digest it.  

Of course, while these fruity enzymes aren't great for gelatin, they do have their uses.  For one, they have some surprising industrial and medical benefits, like they can be used to prep damaged areas for skin grafts if someone gets a severe burn.  The treatment breaks down and clears away dead and damaged tissue, but maybe more importantly to you, you can also use juice or pulp from protease containing fruits in the kitchen to tenderize tough cuts of meat.  The enzymes break down stringy bundles of proteins, making the meat easier to chew.  

So go ahead and save that fresh pineapple for your steak and stick to the canned stuff when it comes to gelatin.  Everyone will thank you.  If you want to learn more science-inspired cooking hacks, we've got an episode with seven more of them for you and as always, thank you for watching this episode of SciShow.

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