What is Yeast? - Definition & Uses

Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Yeast is a microorganism that belongs to the Fungi kingdom. It is found widely in nature and is used in making some of our favorite foods and drinks. Learn more about this tiny fungus and quiz yourself at the end.


Have you ever found yourself walking past a bakery, only to be suddenly overcome by the aroma of freshly baked bread? It is an unmistakable smell, one that elicits an immediate craving for a fresh slice. This delicious smell is due in large part to yeast, an essential ingredient in bread. It's a single-celled microorganism and a member of the Fungi kingdom. When you hear the word fungus, you may picture yellow toenails and rashes. However, there are hundreds of different kinds of fungus, and yeast happens to be one that typically does more good than harm.

Scientifically speaking, fungi are the decomposers of the natural world. It is their job to break down dead matter and return organic molecules back to the earth. This is a very important process, as atoms and molecules are recycled through decomposition. When yeast decomposes organic matter, several useful chemical reactions take place.

But before we get into chemical reactions, lets zoom in and take a look at this microscopic creature. Yeast is a single-celled organism like bacteria. However, that is about all they have in common. Unlike bacteria, yeast has a nucleus which contains its genetic information and organelles. This makes yeast a complex cell, or eukaryote, whereas bacteria is a simple cell, or prokaryote.

Yeast can be found in a wide variety of places in nature. It can be in the air, in the soil, and on plants. It especially loves food sources such as fruits and flower nectar. Yeast is also found within our bodies. In healthy numbers, we don't notice the yeast inside us. However, if there is a proliferation of yeast for some reason, a person can suffer from a yeast infection.

Yeast consumes carbohydrates as its food source. It loves sugar and has quite a sweet tooth. This is the trait that makes it so important for us in creating some of our favorite foods and beverages. Through a chemical process, yeast creates waste products that we actually find quite useful! But how does this work?

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  • 0:01 Definition
  • 1:57 Uses in Baking
  • 2:42 Uses in Beer and Wine
  • 3:49 Downsides of Yeast
  • 4:15 Lesson Summary
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Uses in Baking

So how exactly does this tiny fungus make such a huge contribution to our world?

Yeast is used a great deal in baking. When a package of yeast is opened, it is in a dry and inactive state. Remember that yeast feeds off carbohydrates such as sugar. When the package of yeast granules are mixed with water and sugar, the yeast comes to life. It begins eating the sugar and creating carbon dioxide byproducts in the form of gas bubbles as a result. The process is called fermentation.

When this bubbly yeast mixture is combined with the rest of the bread ingredients, the bubbles create tiny spaces within the bread, thus making it rise. If you make bread without yeast, it's easy to see the difference when you pull a dense, flat loaf of bread from the oven!

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