Encouraging micro-organisms to produce as much gas as possible can be a tricky task. The conditions have to be perfect. So what effect does temperature have on the amount of gas a microbe like yeast can make?
What you need
Flour
Yeast
Two balloons
Two bowls
Funnel
Measuring cup
Tablespoon and teaspoon measures
Warm water
Fridge
What to do
Combine two cups of flour with two teaspoons of yeast in one of the bowls. Slowly add room-temperature tap water, mixing as you go, until the mixture has the same thickness as custard. Try to remove as many of the lumps as possible.
Stick the funnel into the neck of one of the balloons. Scoop three level tablespoons of the mixture into the funnel. When the mixture has trickled into the balloon, remove the funnel. Squeeze the balloon gently until you have removed as much of the air as you can without spilling its contents. Tie a knot in the neck of the balloon to close it and repeat the process with a second balloon. It is important to have the same amount of yeast/flour mix in each balloon.
Fill the second bowl with warm water from the tap. The water should be about 35 degrees Celsius, which feels lukewarm to the touch. Place one balloon in this bowl of water and the other balloon in the fridge.
Now…we wait. The water in the bowl might need to be replaced every now and then, so check on it to make sure it hasn't gone too cold. Keeping it in the sun will help it stay warm.
After two hours, take out each balloon and compare them. Which one looks the biggest?
What's happening?
Yeast is a type of microscopic fungus. Like us, fungi are living things, meaning they need to take in food – in the form of the complex sugar in the flour – in order to get the energy they need to grow.
Yeast can do this in two ways:
Adding oxygen to the sugar in a process called 'aerobic respiration', which recombines the chemicals to make carbon dioxide and water while producing a heap of energy.
Breaking the sugar down without oxygen, which is called 'anaerobic respiration', which makes carbon dioxide and alcohol.
Either way, both produce carbon dioxide gas and both result in releasing energy for the yeast organism to use.
Breaking the sugar down to get this energy uses proteins called enzymes. They work like tiny machines, physically breaking up and reforming chemicals. Enzymes work best when the conditions are perfect, such as when there are the right vitamins present, the right amount of salt, the right acidity, and (in this experiment) the right temperature. The warmer it is, the faster the enzyme will work. But if it gets too hot, the enzyme will fall apart and not work at all.
In our experiment, the yeast bathing in warm water worked hard at producing energy, releasing lots of carbon dioxide gas which made the balloon expand. However the yeast in the fridge could only work slowly, producing very little gas.
Applications
We use micro-organisms such as yeast and bacteria in many areas. They are useful in making cheese taste good, producing alcohol for beverages or industrial processes or in turning milk into yoghurt. In these situations, we want the enzymes in the microbes to work as hard as possible, so we keep them warm.
Sometimes we don't want bacteria or fungi to work. We keep food we don't want to be spoiled by micro-organisms in the fridge. This doesn't kill them, but it does make their enzymes work slowly, limiting the amount of energy they have to grow and hence make our food go off.
