Just desserts

You can't make a jelly with pieces of kiwi fruit suspended tantalisingly inside. We learned that lesson yesterday the hard way -- and, after several hours, had only a bowl of sticky runny strawberry gloop with some kiwi pieces floated on top. But here's the reason why, from the helpful people at Planet Science and their fruity-jelly-making tips:

Just don't try fresh pineapple or kiwi fruit!

These fruits contain an enzyme, a molecular machine, which chops up proteins. As gelatine is a protein it is chopped up by the enzyme. As the gelatine fibres are chopped up their net falls to pieces, allowing the water to flow freely and turning the jelly back into a sloppy liquid. If you fancy pineapple in your jelly, then use the tinned sort. As part of the canning process the fruit has been heated up. This destroys the enzyme so that it can no longer chop up the gelatine.

So only tinned pineapple will do.... Damn those fresh fruit!