Part of the show Catalysts for Cleaner Environments and Future Energy
What you Need
A slice of cheap white bread.
A mouth to put it in.
What to do
What may happen
As you chew the bread, you may have noticed that it slowly tastes sweeter. Particularly the juice that is coming out.
Why does it happen?
Foods like bread, rice and potatoes are foods that are mostly made of starch. Starch is a molecule that plants make in order to store sugar. It consists of long chains of glucose (some of them branch).
In your saliva you have an enzyme called amylase, this will cut up the large starch molecules at random until you are left with sugars made of 1,2 or 3 glucose molecules long (glucose, maltose or dextrin).
Starch doesn't taste sweet, but glucose and maltose do, so as you chew the bread it will slowly become slightly sickly sweet.
Why do plants make starch in the first place?
All living things are made up of cells these are surrounded with membranes which will let water pass through them but not larger molecules such as salt or sugar.
If there are more salt or sugar molecules on one side of the membrane than the other, water is pulled towards the salt/sugar - this is called osmosis. This is why slugs shrivel up if you sprinkle salt on them, all the water is sucked out of them.
Plants convert water and carbon-dioxide into glucose using the energy from the sun. If the plant is doing particularly well it will produce huge amounts of glucose in it's cells. This will cause water to be pulled into the cell. The problem is that this will happen until the cell explodes.
The solution is for the plant to glue all the sugar molecules together into a starch molecule, which just being one molecule rather than thousands will suck in water much less by osmosis, reducing the problem hugely.