Kitchen Science Experiments

Science of fruit jellies

Sat, 17th Jan 2009

Listen Now    Download as mp3 Part 1,2,3 from the show Obesity in Your Genes

What you Need

Jelly cubes

Some jelly (jello for the Americans amongst you)

Chopped Pineapple

Chopped fruit of various types including some pineapple or kiwi

What to do

Chop various types of fruit into small pieces.  We used apple, orange, kiwi, tinned pineapple and fresh pineapple.

Mix up some jelly according to the instructions.

Add the jelly to each fruit in a separate container, and have a further container with just jelly as a control.

Leave in a fridge for a couple of hours to set.

Does all the jelly set properly?

What happens if you use tinned fruit?

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.

Jelly

Orange Jelly

Apple Jelly

The plain jelly set fine

The orange had no effect

Neither did the apple

Kiwi not jelly

Pineapple not jelly

Cooked pinapple jelly

The kiwi fruit jelly wouldn't set...

Neither would the pineapple jelly...

But if you use tinned pineapple it will set fine.

 

 

Why does it happen?

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.

Jelly under a microscope

Jelly under a microscope showing the long gelatin protein molecules

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.

Jelly with protease

Chopped up Gelatin

The proteases in pineapple and kiwi fruit chop up the gelatin molecules...

So the pieces are 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!

Dave Ansell

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