Number 16: Cinder toffee

No, I haven't turn this into a cookery blog. There's actually loads of cool science involved in cooking and cinder toffee is a particularly interesting example, plus as an added bonus it tastes great.

You'll need:

• Sugar
• Golden syrup
• A jam/jelly thermometer
• Bicarbonate of soda
• Grease proof paper
• A baking tray
• A saucepan

Safety:
The toffee mix gets very hot, be careful when handling in and make sure there's an adult helping.


What to do:

1. Weigh out 100grams (3.5 oz) of sugar into the saucepan.

2. Add 3 tablespoons of syrup

3. Heat the mixture on a stove whilst stirring it.

4. Check the temperature of the mixture.
5. Carry on heating until it reaches 145-150^oC (293-302).

6. Quickly stir in 1 teaspoon of bicarb. It will suddenly bubble up. 

7. Now pour it into the baking tray, lined with grease proof paper.

8. Leave it to cool. 

9. Break it all up (best done with a hammer) and enjoy!

What's going on?


So that's a nice simple recipe for a tasty treat but where is the science?

First off there's the sugar and syrup. There are actually loads of different types of sugars, the stuff you put in your coffee and the granulated sugar used here is sucrose. It looks like this:

Sucrose

Golden syrup is a mixture of water, sucrose and two other sugars called fructose and glucose. They look like this:

Fructose

Glucose

Sucrose is actually made up of a fructose and glucose molecule that have been joined together.

So why do we need these three sugars to make the toffee? Well, when they are mixed all together they interfere with crystal formation. To explain how this works let's represent each of the sugars with a different shape. 

If we have one type of sugar then the molecules can pack together nice and neatly, like in the diagram. And that is exactly what happens in a crystal. But if you mix them all together they can't form ordered patterns and so you don't get crystals forming.

So if we tried to make the toffee with just one type of sugar then we'd end up with crystals forming which make for hard dense toffee (more like a boiled sweet). But by using 3 different sugars the crystals don't form and instead you end up with a brittle, crunchy, glass like toffee.

Then there's the bicarbonate of soda. You normally put this in cakes to make them rise. That's because when you heat up the bicarb it turns to carbon dioxide gas (hence the bubbles in your cakes). The same thing happens here (and its also almost the same reaction as in the film canister rockets). When you spoon the bicarb into the hot sugar it almost instantly gets converted to carbon dioxide and causes the mixture to foam up.

Hope you enjoy the toffee and whilst you do you can find out more about the science of cinder toffer here.