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.

hot hazard iconfood safety hazard iconSafety: 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

The ingredients for the recipe.

flour, sugar, oats, coconut, butter, golden syrup, bicarb.

The ingredients for the recipe.

  • 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

  1. Turn your oven to 150 °C.
  2. Sift flour, then mix flour, oats, sugar and coconut in a bowl.
    Bicarb and butter undergoing a chemical reaction.

    A brown liquid in a saucepan, foaming wildly.

    Bicarb and butter seem to react quite well!

  3. Melt butter and golden syrup in a microwave.
  4. Mix water and bicarbonate of soda in a cup, then mix with the butter and golden syrup.
  5. Pour the butter mixture in with the dry ingredients and mix it all together.
  6. Put tablespoons of the mixture onto baking trays, leaving 3 or 4 cm gap between biscuits.
  7. Bake for 20 minutes, or until they look done.

What’s happening?

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.

Spaced tablespoons of mixture on a baking tray.

Small round mounds of biscuit dough on a baking tray.

Put well spaced tablespoons of mixture on a baking tray.

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.

Anzac biscuit and spoon.

An Anzac biscuit next to a tablespoon.

The biscuit is a lot bigger than it used to be!

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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One response

  1. May Avatar

    I’ve always wondered about this recipe. If the total mass stayed almost the same before and after mixing, then none of the ingredients disappeared (except a small amount of water evaporating). Mixing compacted all the ingredients yielding a dough that takes up less space than the separate ingredients. Another way of putting it, the finished dough became a compacted mixture of the ingredients. Plus with water and melted butter as ingredients, the liquids have a way of settling into and displacing the air in tiny places so they seem to disappear. Many physical changes take place during baking. The starches burst when they reach a certain temperature and the proteins (mostly gluten in this case because of the absence of eggs) set as well. These events provide structure to the cookie. As the temperature of the dough climbs, water will vaporize causing the structure to expand. Hence your cookies grew larger during baking. Eventually, enough water will escape and allow the temperature of the cookie to climb even higher. The surface becomes so hot that the sugars there will caramelize and give the cookie color and aroma. However, a significant amount of browning occurs at lower temperatures as a result of simple proteins interacting with simple sugars. The large amount of baking soda present in the recipe raises the pH, which further encourages browning to occur.

    What I do not understand is the chemistry that occurs when the bicarbonate is mixed with hot water. Why would we get different results when the water is merely room temperature or cold? Must the butter be melted with the golden syrup? Is the pH in the melted butter and golden syrup low enough to cause gas to develop when the bicarbonate mixture is poured in? Or is the pH high enough so that the fat becomes soap? What is so special about golden syrup? Can another syrup be substituted?

    Still wondering,


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