Friday 9 August 2013

Science: The science behind yeast

I was making bread this afternoon for tomorrows lunch, we love homemade cheesy rolls in this house, when I saw the opportunity for Honi to have a science lesson and learn a bit about micro-organisms.
There are lots of questions thrown my way when making bread among the many of them are: Bread is yummy but how is it so spongy?  Why do we use yeast in bread? What makes yeast grow?  What are the bubbles made of?  So in this experiment we will learn about why, about catalysts and a bit about micro-organisms too.
What you will need for this experiment:
  • 6 glasses (or see though jars, plastic cups etc.)
  • dried yeast
  • water (cold and warm but not hot)
  • sugar
  • Salt
  • ruler
  • a measuring jug and a 15ml measuring spoon
 We got our science experiment journal out for this and wrote a simple table...
 If you want to use your own science journal, first estimate, hypothesise, how high it will grow. 
Now for preparing your experiment...
Then add 15ml of dried yeast to each of your six glasses. 
Add 10ml of sugar to just two of them.  Label one (A) Sugar warm and the other (B) Sugar cold.

Add 10ml of salt to just two of them. Label one (C) salt warm and the other (D) salt cold.
The two cups of yeast remaining label (E) warm just yeast and the other (F) cold just yeast.
Add 80ml of warm water to glass A, C, and E. Using your measuring jug for this, to make this a fair test be as accurate as possible.
Add 80ml of cold water to glass B, D and F.  Again using your measuring jug.
Measure, with a ruler, how high they as soon as you add the water this is so you will be able to measure how much it has grown.  Note this down if you are using your science journal.
Wait 10 to 15 minutes then observe and measure with a ruler how much each has grown.  Again write down the results in your science journal if you are using one.
What do you notice?  There are lots and lots of bubbles on top of the water in glass B and C but not A, right?  These bubbles are full of carbon dioxide.  But why?  You'll have to read on...
So what has gone on? 
When you use dry yeast there is a physical reaction with the water as the water rehydrates the yeast so you will notice the yeast looks a little different. 
However, the reaction between yeast and sugar is a chemical one. The reaction that takes place when yeast and sugar are added together is a large amount of foaming, the yeast is the catalyst in this experiment, it is chemically unchanged during the experiment however it does multiply but this is because it’s a living organism.
Carbon dioxide is released while the foaming occurs which is what we are seeing; what you are observing is fermentation and it is fermentation that causes all that foaming!
Something else to remember is yeast is alive.  It isn't a plant and it isn't an animal, it's a fungus.
When it is fed, on the sugar, it multiplies. 
Like with most living things yeast needs moisture, food, and a hospitable environment. In a moist environment, yeast will grow rapidly.
As the yeast cells feed, they expel carbon dioxide and alcohol.  This is what forms the air bubbles in bread, making it spongy and the alcohol and other excretions gives bread it's unique, yummy yeasty flavour. 
Another amazing fact about yeast is its ability to turn starch into sugar which is how it still works in sugarless bread like baguettes.
So now we know sugar feeds the yeast so it multiplies quicker but why does the warmer water make for a quicker reaction?
With any chemical reaction heat increases the speed of the reaction because the particles move quicker, the quicker they move the more likely they are going to collide with particles and reach the activation energy level!
Saying this it works it's hardest at 78 degrees and 80 degrees. Bakers see this as an ideal environment for yeast growth. Because yeast is very sensitive to temperature, temperature is a major factor in how fast yeast multiplies. Yeast remains dormant and will not grow at 40 degrees or less and grows only slowly at 55 degrees. Another thing to remember is yeast dies instantly at 140 degrees. You could experiment with this but you will need a thermometer.
How do we know it's a chemical change?
If you study chemistry this is important to know, so if you're ever not sure if something is a chemical change or a physical one just think can you make it go back to normal? If you could make it the same as it was before, a reversible change, then it would be a physical change for example if you freeze water that's a physical change because you could melt it back to the way it was before. If it is irreversible, unable to change back like when you cook bake cake then it's a chemical change.
A physical change in a substance doesn't change what the substance is. In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed.
In bread this reaction is sped up with yeast as a catalyst.  Because of the yeast we get more air into our dough.
So a bit more about bread...
We add yeast to the flour in its dormant state and add moisture and keep it at the proper temperature to help it thrive. When the bread is warm, the yeast cells feed on sugar and multiply creating the carbon dioxide. It is the carbon dioxide that gets captured by the gluten structure in the dough and is how lots of pockets of air are created in the bread, making the bread rise. A loaf of bread, ready to go into the oven, may contain millions of yeast cells. 
Most of the time, you will want your bread dough as moist as you can handle without being sticky. A bread dough that is too dry will take a long time to rise because the yeast will not multiply as rapidly and because the dry dough is stronger and more difficult for the yeast to lift.
Salt delays the growth of yeast so you can slow down the rise with salt. Whereas with sugar, you speed up yeast growth.  An extra half teaspoon of salt will significantly slow the rise of the dough.  Another experiment to try.
I recommend to let them touch it afterwards; It's good I think to allow them to explore and it is safe to do this with yeast. 
Also kids get a lot of maths practice out of this with all the measuring and counting and finding the difference. 

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