Cornflour and Gravy

26 February 2006

Ingredients

A saucepan

A stove

A wooden spoon

Water ~ 250ml

Some Cornflour (Cornstarch) ~ a few teaspoons

Instructions

Put the water in the Saucepan

Add a couple of teaspoons of cornflour to the water

Heat the water until it is simmering

THIS SHOULD BE DONE WITH AN ADULT

Keep stirring until something happens

Add some cornflour to the hot water, how does this behave?

Result

When the mixture gets hot and you stir it, it will get thicker and thicker

If you add the cornflour to the hot water it will turn into a lump covered in a jelly like substance.

We also heated some cornflour particles under a microscope with a hot air blower, which allows you to see what is going on.

Expanding Cornflour Particles 1 Expanding cornflour particles 2
The particles start off small and compact surrounded by cold water. The water can be heated considerably with not much effect on the particles,
Expanding Cornflour Particles 3 Expanded cornflour particles
Over a critical temperature the particles start to swell up . They keep swelling until they almost dissolve and start to become difficult to see.

Here is the same process as a video.

Explanation

Cornflour is made up of tiny starch particles (< 0.01mm across). These are made up of long starch molecules all wrapped up tight, a bit like a ball of wool. In cold water these are stable.

Corflour Particles

If you heat them up in water the starch molecules start to dissolve, they unwrap and swell up hugely.

Cornflour headed up

If you then stir the result the starch molecules from the different particles tangle with each other producing a great big tangled network. This flows slowly because the molecules take a while to untangle, so thickening the liquid. This is how you thicken soups and gravy.

Cornflour Stirred

If you add a lump of cornflour the particles on the outside swell and turn to a jelly like substance, which is actually quite waterproof, protecting the cornflour in the centre from the water, and creating a dry lump. So if you want to thicken gravy, make sure you mix the cornflour with cold water before you heat it up.

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