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Experimenting with Yeast under the Microscope

For this microscope activity, we will be experimenting on yeast with the use of a compound microscope. Yeast, as most of us are familiar with, are used as one of the ingredients used to make bread. They are the ones responsible for causing the bread to rise or expand. But did you ever wonder how the yeast does this? Or what is it that makes up these microscopic things? By performing this science microscope activity lesson, students or children may find the answers to these questions. We will be using a high power compound microscope for this experiment.

What we need to obtain is a yeast cake. You can buy it from a bakery specialty store or even from the supermarket. Once you are able to obtain one, cut a quarter of the yeast cake and mix it with water. Once the mixture has a pasty texture, add another pint of water with a tablespoon of sugar. Stir well until there are no lumps or the sugar has dissolved completely. This will prevent seeing properties of sugar while examining the yeast under the high power microscope.

Using a water dropper, we place a drop of this mixture on a blank microscope slide and put on the microscope cover slip. Place the sample under the highest power objective of your compound light microscope. This is probably the 100x oil immersion objective, which when used with 10x eyepieces, will yield 1000x total compounded magnification. When we look at the yeast under the high power microscope, we will see that it has numerous microscopic bodies that are shaped like an egg. These bodies that make up the yeast are yeast-plants that have a single cell. They are units of protoplasm that contain nucleus, globules of oil, vacuoles and are enclosed by a cell wall.

As we look at the yeast-plant under the biological compound microscope, we will notice that it does not have any green bodies or chlorophyll that is responsible for its own food manufacture. This might bring us to a conclusion that yeasts might be parasites or plants that live off on other living organisms like fungi. However, yeasts do not thrive on living organisms but instead survive on organic substances.

If you look at the mixture under the high power compound microscope, you will see that there are gas bubbles that rise from the liquid. Let us collect some of this gas and move it into a container that has limewater. It is easy to make your own limewater solution. All you need is a teaspoon on calcium hydroxide and mix it with a gallon of tap water. Shake the jar vigorously then leave it unmoved for about 24 hours. Once the sediments have settled, pour the water through a filter. The end result is the lime water that we will use on our yeast observation under a biological microscope. This limewater solution is essential in order to know the presence of carbon dioxide. So if we transfer these collected gas bubbles in the container with limewater and the water turns turbid, we will know that the gas is carbon dioxide. Now let us try to taste some of the yeast liquid. You will notice that it is not sweet (due to the sugar) but alcoholic. This is because the yeast-plants absorbed the sugar and converted it to alcohol and carbon dioxide. This microbiological and chemical process is called fermentation.

This process explains why the bread expands or rises with the use of yeast. The flour that is mixed with yeast has a certain amount sugar which the yeast absorbs. In absorbing the sugar, the yeast breaks down into its by-products while multiplying rapidly. The air bubbles form and try to permeate the dough which in turn expands, giving the bread its spongy quality. When the bread is baked, the bubbles further expand and the alcohol and water are driven off.

We will also observe how yeast multiply under the high power biological microscope. Yeast multiply through the process called budding. A bulging wall appears to a yeast-plant that is about to reproduce. Protoplasm will flow from these walls until the yeast-plant has two unequal parts. If you look at this process under the student microscope, you will see that at some point, these microscopic parts get separated from each other. However, sometimes there are more buds that grow from the protoplasm even before the buds separate. The budding may go on very rapidly until the attached cells form an irregular colony.

There are other kinds of yeast. The yeast the we have just observed are those that are “domesticated” or cultivated by humans. There are also “wild” yeast that cause some fruits to ferment or decay. Grapes, for example, don’t need any artificial yeast in order to make wine. Learning about yeasts under the compound microscope is a very educational activity especially for students and children. Most of all, it is very easy to perform. That is why it is ideal for high school and elementary students as a science microscope experiment or classroom lesson in microscopy. College level microbiology students will also benefit from examining yeast under the microscope.

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