• Question: in baking do you know how yeast makes cakes raise and what is yeast

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      Asked by Aoife to Aoife, Brian, Conor, Louise, Matthew on 14 Nov 2016.
      • Photo: Louise Mc Grath

        Louise Mc Grath answered on 14 Nov 2016:

        Hi Aoife!

        Yeast are living, single-celled organisms and are found everywhere in the world around us. Varieties of yeast are found on our skin, in the air, and on the skins of fruits and vegetables.

        When you are baking a cake or bread generally flour is mixed with water to form a wet dough mixture. If this was cooked as is then it would taste terrible and be really flat. In order to make the dough rise and become fluffy and tasty, other ingredients must be added. Yeast is important as it makes dough rise.

        When we bake cakes we also add sugar into the dough mixture. When yeast and sugar are in the same mixture, the yeast go crazy! The reason yeast goes crazy is because yeast loves eating sugar, and gets it energy from eating sugar! When yeast eats sugar in warm, wet environments (like in your cake mixture) they start to multiply in numbers and soon hundreds of yeast cells are in the dough.

        As the yeast munch away on their sugar, a process called anaerobic fermentation begins to take place. The by-products of this process are alcohol and carbon dioxide. During this time, the carbon dioxide is trapped by a series of strands of gluten in the cake mixture. This is what causes the dough to rise, or expand on the surface, leaving behind a series of air pockets in the dough. This is how the cake mixture can get fluffy.

        The yeast eventually dies off, from the heat when baking, and any remaining alcohol evaporates. The air pockets left behind are what give cakes their fluffiness and size!

        More information can be found here: http://www.kidsdiscover.com/teacherresources/science-of-yeast-for-kids/
        A simple home experiment to show that yeast produces carbon dioxide gas (CO2) can be found here: https://www.youtube.com/watch?v=qoxY0z8ukUQ

      • Photo: Matthew Kitching

        Matthew Kitching answered on 14 Nov 2016:

        Hi Aoife,

        Louise beat me to it 🙂

        Yeast is a fungus (urrggh i know!) that can turn sugar (its food) into energy (so it can live) and gas (carbon dioxide or CO2). So when you bake bread you can use yeast to generate bubbles of gas (CO2) that make the bread fluffy.
        https://en.wikipedia.org/wiki/Yeast

        Yeast can only do this when the temperature is just right: too cold and they don’t eat the sugar, too hot and the yeast dies. This is why when you make bread you have to leave it to “prove” in a warm place – thats when the yeast are doing their jazz and making bubbles. Needing dough stretches out all the gluten (a chemical made of protein and carbohydrate) making it able to trap the gas (so you have to make sure you need your dough well).

        If you make a cake, or some bread (like soda bread) you don’t use yeast – instead you use baking or bread soda. This is a chemical that when it gets heated gives off CO2. This means it generates CO2 in the oven, and doesn’t need time to “rise” or “prove”

        Hope this answers your question – let me know if you want to know more 🙂

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