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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.yeast

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.

For more information, try: Science of Yeast at the RedStar Yeast site has .pdf files of experiments and images (Note:  some of the manufacturing sequences use the confusing term "seeding" yeasts.)

Classic yeast blowing up a balloon experiment

Previous post on Fungi

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The cool, wet weather we have been having has brought out some mushrooms. Quick, are mushrooms plants or animals? If you say neither, then you know your stuff. Mushrooms were once thought to be plants, but mushrooms don't make their own food. Modern investigations have shown them to be more closely related to animals. For instance, mushrooms have the protein chitin, which is also found in insects. But mushrooms aren't exactly animals either. For example, as far as we know they don't run around like animals do. In fact mushrooms are so distinct, they are now given their own Kingdom, the Fungi.

What are fungi? Most people recognize that mushrooms are fungi. In addition, yeasts and truffles, molds, mildew and also disease-causing parasites of insects are fungi. Lichens, which grow on rocks in a variety of climates, are a mix of fungi and algae growing together.

Fungi range in size from microscopic to quite possibly the largest organism on earth. Scientists are making the case that a giant fungus found growing in the Malheur National Forest in Oregon could be the world's largest individual organism, because it covers some 2200 acres. Information is still being gathered, and because much of the fungus is hidden from view, it also could be made up of clusters of individuals.

The mushroom part of the fungus we see is the reproductive part, or using older analogies, the fruit of the fungus. The body of the fungus is made up of threads called mycelium, and they are often hard to see because they grow underground or in rotting wood. If you have ever broken open a rotting log and seen white hair-like strands, you have seen mycelium.

If the mushrooms are like fruit, then where are the seeds? It turns out that fungi grow from tiny particles called spores. If you have ever found a mature puffball and stepped on it, the smoky clouds that come out are the spores being released into the air. Here is a picture of a puffball, if you have never seen one. The spores produce new strands called hyphae and then mycelia to make new fungi.

Activities:

Fungi as Food
(Safety Note: Many wild mushrooms are poisonous. Beginners should stick to the grocery store varieties).
Explore edible mushrooms by visiting grocery stores to see all the different kinds that are available. We found shiitake, oysters, white, brown and portabellas. Discuss how they are similar and different. You might want to buy a few, and try the different flavors in your favorite recipes. You could also pick up some yeast and make bread with it. Show the yeast to your children and let them smell it. By the way, there is nothing better than white mushrooms sautéed in butter made into a sandwich between two slices of freshly made bread.

Make Spore Prints
If you don't want to eat the mushrooms, use a few different kinds of mushrooms from the grocery store to make spore prints. Try to find older mushrooms already producing the brown powdery spores. Young mushrooms with pink gills or button mushrooms aren't ready to make spores yet.

Place the mushroom with the frilly gill side against some white paper and then cover it with a glass or bowl. (You'll have to remove the stem first). Leave for a few hours or overnight. If the mushroom is producing spores, it should leave a print when you gently lift it away.

Scientists who study fungi, called mycologists, use spore prints to help figure out what species they have found.

Websites:

Check out an experiment that uses yeast to blow up a balloon.

Older children can visit here to learn more about fungi, starting with the attack of the killer fungus.

Finally, if you have the time, growing mushrooms from a kit can be a great experience for kids. Kits are available with different types of mushrooms, at varying sizes and costs. Check the Internet for sources.

A Few Mushroom and Fungi Books:

Mushroom in the Rain by Mirra Ginsburg. Macmillan Publishing Co, New York. 1974.
A fiction picture book with a cute story.

What Rot! Nature's Mighty Recycler by Elizabeth Ring. Millbrook Press, Brookfield Conn. 1996. (Nonfiction)

Fungi by Mary Kay Larson, Newbridge Educational Publishing, New York. 2003. (Nonfiction)

Plants That Never Bloom by Ruth Heller. (nonfiction). Note: This older book still classifies fungi as plants, but the other information and illustrations are still worthwhile.

What is a Fungus? By D.M. Souza. Franklin Watts, New York. 2002. (Nonfiction)