After reading yet another children’s book that identified yeast as a type of plant – an archaic classification, they are really fungi – it’s time to investigate these helpful organisms.

1. First of all, how do scientists know yeast are fungi and not plants? Obtain some baking yeast from a store. Carefully open the packet or jar and look inside. Have some plant seeds handy for comparison.

Consider the characteristics of plants:

• They are multi-celled organisms that obtain their energy from photosynthesis.
• They are green and contain chlorophyll.
• They grow from seeds.

In contrast, fungi:

• are organisms that obtain their energy from food digested externally.
• They are not green, and do not contain chlorophyll.
• They contain chitin, a protein found in animals.
• Make more of themselves via spores or budding.

What color are the yeast particles in the yeast package? Are they green like plants? Even though they are not green, they still might be seeds. How would you tell? What happens when you add water to a seed? It swells up and over time, say a week or so, a small plant emerges.

Try adding a teaspoon of yeast to 1/4 cup of warm water. What happens? Now add a little sugar, to serve as an energy source. What happens? What would happen if these were seeds of a plant?

Note:  Yeast organisms are actually unicellular and would be impossible to see without a microscope, so the baking yeast you examine is a processed form containing many cells.

2. Although we humans use yeast for baking or making beverages, in nature yeast are decomposers. Test the ability of yeasts to decompose common food stuffs. Gather:

• banana (apples or bread will work too).
• plastic bags
• yeast

Cut the banana in half crosswise. Sprinkle 1 tsp of yeast onto one half piece of banana, and then place each half banana into separate bags. Close the bag, and leave them in a warm, dry place. Compare what happens in the banana half treated with yeast and the banana half not treated. Return twice a day and record the appearance of each half over a few days. Would the experiment be less valid if you treated one whole banana and left one whole banana untreated? Why or why not?

Compare the rates of decay to bread and apples treated with yeast to untreated samples. Interesting fact:  fruit flies don’t eat fruit as larvae, but the yeasts that grow on fruit.

Related:

• See our previous post about blowing up a balloon with yeast. You can substitute a few Tablespoons of granulated sugar for the molasses in the experiment.
• Also our previous post on Fungi