Yeast
Students examine the physiology of a microorganism by growing yeast with sugar and noting pH, alcohol content, and gas production.
Yeast cells producing buds.
Materials Required
• Yeast
• Sugar
• Plastic cup
• Water
• Measuring spoons
• Small glass, cup, or vial
Procedure
1. In a shallow bowl, place a tablespoon of sugar.
2. Add 1 cup of warm, not hot, water.
3. Add some cabbage juice you prepared by chopping up some purple cabbage and placing it in warm water.
4. Measure out 1 teaspoon of yeast.
5. Mix the water, sugar, yeast and cabbage juice together in the bowl.
6. Pour some of the bowl's contents in a small cup, vial, or glass. This is a small plastic portion cup.
7. Fill the small cup to the brim and cover with a lid or some other flat, still surface. This is a canning jar lid:
8. Carefully invert the cup onto the liquid in the bowl:
9. Your test bowl of yeast and sugar are ready for action! Notice the purple color of the cabbage juice at the start and the little cup in the center is full of the liquid mixture. We weren't very careful and allowed some cabbage pieces to fall in as well, but they won't have much of an effect!
10. Place the bowl in a warm environment and check it in regular time intervals. Revisit the solution and build a schedule for data gathering. The warmer the environment (up to about 105 degrees F) the faster the yeast will grow. The progress will be slower in cooler environments.
11. After every 15 minutes, make observations: Color, smell, any activity in the liquid, and what is happening in the small cup.
12. After a designated number of observations, report their findings. Summarize the results from the investigation.
Here's what ours looked like after 45 minutes:
And then, overnight.
Questions:
• What happened to the color of the yeast mixture? (Remember, as the purple turns to red it shows that the pH is decreasing or acid is present)
• What happened inside the upturned cup? (What gas is this?)
• What must the yeast have done to the sugar? (Smell the mixture. If it has a fruity small, it comes from the esters, ketones and alcohol waste products)
• Does the frothy bubbles look appealing? What can you say about the waste products of microbe digestion?
Extension
A. The action of yeasts can be further explored by placing the fermenting mixture in a thermos or insulated cup with an insulated top. Use a thermometer to measure the temperature of the mixture. The temperature is monitored every hour over a 12 hour period or longer. The resulting data can be graphed to show a rise in temperature as yeasts digest and divide and then a drop in temperature when they ultimately die as a result of poisoning themselves in their own wastes.
B. Place yeast and possible foods (sugar, flour, etc.) into labeled plastic baggies. The gas that is produced can be measured to show the volume of carbon dioxide produced. The amount of carbon dioxide indicates the nature of the food for the yeast.
Discussion
Inside the yeast cell, sugar is digested in a process called fermentation. The yeast uses its digestive enzymes to take apart the sugar molecule to extract energy. The yeast stores the energy in specialized molecules that can be readily used to grow more yeast. The byproducts of this process are ethanol (alcohol) and carbon dioxide.
C6H12O6 → 2 C2H5OH + 2 CO2
sugar → ethanol + carbon dioxide
The yeast extracts the energy in this process to help construct new yeasts. Yeasts will digest all the sugar they can and will multiply rapidly. The waste products buildup and create problems. Although the carbon dioxide is a harmless gas, the alcohol can build up and eventually kill the yeasts--they die in their own waste!
Although a distinct disadvantage to the yeast, the waste is of great advantage to food production. Carbon dioxide provides the holes and gaps in bread and gives it the light texture. Gas production in beverages (such as beer) gives a fizz to the drink and it changes the taste by creating an acid (Carbolic acid).
Thus far we have focused on yeasts. There are a number of other kinds of microbes that are also used to modify food. The particular flavors of cheese are the result of different microorganisms that digest the sugars in the milk and leave their particular flavors and wastes behind. Blue Cheese gets its color and flavor from the waste products of the bacteria Penicillium.
Background for the Adult
Although there are many foods that depend on the growth of bacteria and the resulting waste products, most of what we experience from the action of fungi and bacteria is not positive. Molds that consume food are generally destructive as they alter the flavor negatively. Some bacteria not only alter the flavor, they leave waste products that are toxic and even deadly.
A common one of these is the bacteria Clostridium botulinum, or botulism. This bacteria is found in nature and can contaminate food, especially food in the canning or preserving process. If the bacteria are not killed in heating the food and are allowed to grow, they consume the raw materials in the preserved foods and leave a deadly nerve toxin called botulinum. It was originally called “sausage” toxin as it was commonly found in meat preservation. Those who like to can or preserve their own food need to take special precautions to ensure the safety of their hard work.
Although the botulinum toxin is deadly, it has been found to be helpful in affecting the nerves. When injected in tiny quantities in the skin, these “botox” treatments help shape the features of the face by changing and stretching the muscles.