Anzac Day is the time to stick a sprig of rosemary in your hat, watch the dawn service and bake a batch of Anzac biscuits. While I was baking these biscuits, I noticed some strange things going on.
Safety: This activity involves a microwave and an oven. Ask an adult to help. When dealing with food, use clean hands and clean equipment.
First aid: If you burn yourself, put the burn under cool, running water for 20 minutes.
You will need
- 1 cup flour
- 1 cup rolled oats
- 1 cup caster sugar
- 3/4 cup desiccated coconut
- 125 g butter
- 1 tablespoon golden syrup
- 1 teaspoon bicarbonate of soda
- 2 tablespoons boiling water
What to do
- Turn your oven to 150 °C.
- Sift flour, then mix flour, oats, sugar and coconut in a bowl.
- Melt butter and golden syrup in a microwave.
- Mix water and bicarbonate of soda in a cup, then mix with the butter and golden syrup.
- Pour the butter mixture in with the dry ingredients and mix it all together.
- Put tablespoons of the mixture onto baking trays, leaving 3 or 4 cm gap between biscuits.
- Bake for 20 minutes, or until they look done.
There is a lot of really interesting chemistry going on in this recipe, with bicarbonate of soda fizzing, and the wonderful and complicated process of browning. But what I was really amazed by was how everything kept changing size.
The recipe has around 4 cups of ingredients, a grand total of about 1 L. A tablespoon is 20 mL, so I would expect to make 1000 ÷ 20 = 50 biscuits. But I only had enough mixture to make 24. Somewhere in the process, the ingredients had shrunk to half their original size!
When I baked the biscuits, they grew much bigger. A 1 tablespoon biscuit made a circle 7 cm across and 1.5 cm high – a volume of almost 60 mL. I ground one biscuit with a mortar and pestle, and measured 3 tablespoons of crumbs.
I was also keeping an eye on the mass. During the mixing process, it stayed roughly the same at about 720 g. I also weighed a tray of biscuits before and after baking – a tray of 10 biscuits lost about 25 g, about 8% of its pre-baking mass.
A famous person once said, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, ‘hmm… that’s funny…’” So can any young (or not-so-young) scientists help explain what’s going on here?
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