Why does pineapple stop jelly setting?

When making some jelly last week, I noticed the packet said it wouldn’t set if pineapple, kiwi or papaya were added to it. We decided to find out why pineapple stops jelly setting.

What you need

• Jelly cubes
• Fresh pineapple
• Tinned pineapple
• Any other fruits you wish to test
• Small bowls

Instructions

• Make the jelly up as per the instructions.
• Divide into several small bowls.
• Add a different fruit to each bowl.
• Leave one just as jelly – this is our control

Results

We found only the jelly containing the fresh pineapple did not set. In one bowl I had just two small pieces of pineapple, this was enough to stop it setting.

Why does pineapple stop jelly setting?

Jelly contains gelatine which partially consists of protein molecules. It sets because the protein molecules tangle up as they cool down trapping the water to make a solid. Fresh fruits such a pineapple, kiwi and papaya contain enzymes which break down these protein molecules, making them smaller, so they can’t tangle up, which stops the jelly setting. This is similar to how the enzymes in your stomach break down food.

The tinned pineapple jelly set because as part of the tinning process the pineapple has been heated  to destroy  bacteria. This process has also  destroyed the enzymes ( they are denatured ).

The enzyme in pineapple is called bromelain and the one in kiwi actinidin.

Why does jelly set?

Jelly contains gelatin, which is a processed form of collagen. Collagen is the most common protein found in humans and other mammals. When the gelatin is dissolved in hot water the bonds holding the collagen protein together are broken.

As the gelatin cools new bonds form trapping the water between them.

Why does jelly wobble?

The liquid in the jelly is trapped between gelatin bonds which is why it wobbles!!

More Jelly Science

50 Kitchen Science Activity Ideas

Colour Mixing with Jelly