Grade Level: 5th - 6th; Type: Biochemistry
The goal of this experiment is to study how yeast grows and determine what conditions favor its growth.
- How do we know that yeast is a living organism?
- What types of nutrients favor the growth of yeast – proteins, carbohydrates, sugars?
- Why do yeast produce gas?
- How is yeast classified?
Yeast – also known as Saccharomyces cerivisiae – is a type of fungus. The reproduction of yeast cells was first observed in 1857 by French scientist Charles Cagniard de la Tour under a microscope.Louis Pasteur later proved that yeast was a living organism when he studied its role in fermentation.
In this experiment, each bottle is treated the same, in that it will contain yeast and some type of nutrient.The only difference is that each bottle will contains a different type of nutrient – gelatin, flour, corn syrup or grape juice.Gelatin is largely composed of protein.Flour is largely carbohydrates. Corn syrup is a processed sugar.Grape juice contains a type of sugar called fructose. Students determine which of these nutrients favors the growth of yeast by measuring the evolution of gas.
As the yeast grows, it produces enzymes, which in turn break down sugar and starch if water is present and the temperature is not too hot or too cold.As the sugar and starch break down, carbon dioxide – also known as CO2 - is given off.This CO2 gets trapped in the balloon. Another product of the breakdown of sugars is alcohol. You may be able to detect the smell of alcohol in the bottles after the experiment is complete.
- Lab book and pencil
- Yeast (found in the baking aisle of the grocery story)
- Four glass bottles(old-fashioned Coca-Cola bottles are great, but any bottle with a narrow mouth that holds roughly ½ -1.0 quart will do.Olive oil, soy sauce, or empty glass quart beer bottles are good choices, too.If possible, use four of the same type of bottle.
- Corn syrup
- White flour (bleached or unbleached)
- Plain gelatin (do not use flavored Jello)
- Grape juice
- Four medium-size balloons
- 2 cup Pyrex measuring cup
- Masking tape or adhesive label for marking the bottles
- Stick an adhesive address label or a piece of masking tape on each of the four bottles.As you prepare each bottle, you will write a number on each bottle and indicate what went into the bottle.
- Make up a yeast solution using three tablespoons of yeast and a cup of lukewarm water.The water should not be overly hot or you will kill your yeast.Stir well.
- In your two-cup Pyrex measuring cup, add ¼ cup maple syrup (or pancake syrup) and four tablespoons of the yeast solution to bottle #1.Stir.Using the funnel, add this mixture to the first bottle.Label the bottle “#1 – Syrup.”Carefully, wash the funnel and the measuring cup.
- Mix ½ cup of floor and ½ cup water into your Pyrex measuring cup.Gently stir your yeast solution, making sure as much of the yeast is dissolved as possible.Add four tablespoons of the yeast solution to the flour and water mixture.Mix well.Using your funnel, transfer this solution to bottle #2. Label this bottle “#2 - Flour.” Carefully wash the funnel and measuring cup to avoid contaminating subsequent mixtures.
- Make up the plain gelatin mixture according to the directions on the package.Generally, the instructions involve adding boiling water to the gelatin. Make sure that the gelatin solution is room temperature before proceeding. Once the solution is room temperature, gently stir the yeast solution.Add four tablespoons of the yeast solution to the third bottle. Label the bottle “#3-Gelatin.” Wash the funnel and measuring cup.
- Gentle stir your yeast solution. Place ½ cup grape juice and four tablespoons of the yeast solution into a Pyrex measuring cup. Using the funnel, transfer this mixture into the fourth bottle.Label the bottle “#4-grape juice.”
- Slip the open end of a balloon over the mouth of each bottle.Using a piece of string, secure the balloon to the neck of the bottle.
- Carefully lay the four bottles on their sides next to each other in a place where they will not be disturbed.Place a heavy book next to the bottles at each end to prevent the bottles from rolling.
- Inspect the bottles at 15 minute intervals over the course of three hours.Document your observations.Observable changes should start happening within a ½ hour.
- After three hours, carefully untie the string and remove the balloon.Carefully sniff the mouth of each bottle and document what you smell.
Terms/Concepts: Carbohydrates, proteins, sugars; Gas evolution; Enzymes; Yeast
Wearing, Judy. Fungi: Mushrooms, Toadstools, Molds, Yeasts, and Other Fungi (A Class of Their Own); Crabtree Publishing Company, 2010.