Biology

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Video

Materials

• 1 packet of active dry yeast
• A 0.5 L (16.9 fl oz) or smaller plastic bottle
• 1 balloon
• 1 teaspoon measuring spoon (5 mL)
• Sugar
• Water

Short explanation

When yeast eats sugar, the gas carbon dioxide is released. The carbon dioxide takes up a lot of space and stretches the bottle and the balloon. It also appears as bubbles in the liquid.

Long explanation

Yeast is actually a fungus. The reason it does not resemble fungi as we normally know it, is because it is unicellular, just like bacteria. It multiplies by budding, that is, one cell divides into two. This cultivation of yeasts takes place at the factory where yeast is produced. "A packet of yeast" thus contains millions of yeasts. The yeast species that is normally used for baking is Saccharomyces cerevisiae.

Fresh yeast (also known as compressed yest) consists of living yeast cells and residues from the nutrient fluid in which they were grown. The grains in dry yeast consist of lumps of living yeast cells encapsulated by a shell of dead yeast cells and nutrient fluid. When these grains dissolve in water, the living yeast cells are released. Dry yeast has a longer shelf life than fresh yeast, but fresh yeast is considered by many to give a better result in the kitchen.

When yeasts are added to a dough, they begin to eat the sugar in the dough, and in this process, carbon dioxide is formed as a by-product. Lots of small bubbles of carbon dioxide are then formed in the dough, which makes the bread fluffy. In this demonstration, however, there is no dough, only the sugar the yeasts want. In order for the yeasts to feel their best and work the fastest, they need the right temperature. For Saccharomyces cerevisiae this temperature is 40 °C (104 °F).

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• How does the water temperature affect how much carbon dioxide is formed?
• How does the amount of sugar affect how much carbon dioxide is formed?
• What type of yeast (find them in your grocery store) results in the most carbon dioxide?

Gilla:		Dela: