Science of fruit jellies

Find an excuse for a party, make some party food, and find out why some kinds of fruit work in jelly and others don't!

What you need

What to Do

What may Happen

You should find that most forms of fruit work well, but fresh pineapple, kiwi fruit and papaya will stop the jelly from setting.  However if you use tinned pineapple the jelly will set perfectly.

What is going on?

Jelly is made up of long thin protein molecules.  The reason that jelly sets is that as the gelatin molecules tangle up as they cool creating a huge intertwined tangle which traps the water and makes a flexible solid.

The pineapple contains an enzyme called bromelain and kiwi fruit another enzyme called actinidin - both of these enzymes are proteases, which means that they will chop up protein molecules.  In the same way that the digestive enzymes in your intestines break up proteins to allow you to digest them.

This means that when they are mixed with gelatin, the proteases chop the gelatin protein up into pieces which are far too short to tangle, so the jelly doesn't set.

Why does tinned pineapple make jelly?

In the canning process the pineapple is heated up to kill all the bacteria inside, this means it doesn't decay and can last a long time inside a can.  This heat causes the enzymes to 'denature', meaning they lose their structure and cease to act as a protease, so the jelly can still set.

Why do pineapple and kiwifruit have proteases?

All fruits will have some proteases but pineapple and kiwi fruit have far more.  Exactly why is not known but it is probably to repel pests.  Animals and bacteria are made up of proteins so the proteases will essentially digest any pests that attempt to feed on the fruit.

This is also the reason why your mouth tingles if you eat too much of them.  The proteases start to digest the skin of your mouth.  But don't worry - in small quantities it is quite harmless!

Written by Dave Ansell