Making Yogurt

Introduction

Yogurt production demonstrates fermentation by Streptococcus thermophilum and Lactobacillus bulgaricus. Heated milk is inoculated and maintained at a given temperature causing bacteria to grow and ferment lactose, the sugar in milk. The bacteria produce lactic acid which causes the milk to coagulate and adds a sour flavor.

Be sure to follow aseptic technique and use only food-grade containers for this experiment. You must submit a proposal on what your experimental group(s) will entail before beginning this laboratory work.

Materials:

food-grade containers, washed

food-grade thermometer

hotplate

beaker tongs

2 cups milk

1/3 cup nonfat dry milk

2 tablespoons plain yogurt (Old Home Black Label works well)

pH paper or meter

microscope slides

Bunsen burner

inoculating loop (a toothpick will work)

crystal violet stain

microscope (oil immersion if available)

Procedure:

1. Combine milk with nonfat dry milk and heat in a double boiler to 190 degrees F. Hold at that temperature for 10 to 20 minutes so that the protein in the milk mixture will take up more water and make a better gel. Cool to 115 degrees F (warm) and record the pH of the mixture.
2. Place the plain yogurt in a jar and gradually blend in the warm milk.
3. Cover. Place in a bowl of warm water (115 degrees F), a slightly warm oven or a styrofoam cooler. The temperature within the oven or cooler should be about 110 to 120 degrees F to provide optimum conditions for yogurt culture activity.
4. Allow to stand undisturbed until the mixture is firm when the jar is gently wiggled. This may take as long as 6 to 8 hours. Note the time so that less care will be needed for the next batch. Record the pH of the yogurt.
5. Chill yogurt as soon as it is set. It can be stored in the refrigerator for up to 3 weeks.
6. Place a drop of water on the slide. Use the inoculating loop to mix a little yogurt with the water and spread it around the middle 1/3 of the slide.
7. Let the slide air dry.
8. Quickly pass the slide through the flame of the Bunsen burner 3 to 4 times.
9. Let the slide cool.
10. Cover the slide with 2 to 3 drops of crystal violet stain.
11. After 30 seconds, rinse the slide with water.
12. Examine the slide under the microscope and draw the bacteria.
13. Add 2 tablespoons sugar and 1/4 cup fresh, crushed or frozen fruit to the yogurt.
14. Taste your yogurt. (Do not eat any yogurt that smells or looks bad! If in doubt, throw it out!)

References:

• "Bacteria in Yogurt" pages 44-47 of Laboratory Experiments in Biotechnology and Related Areas Volume III: Experiments with Microorganisms, Dept. of Biological Sciences, Mankato State University, Mankato, MN, 1988
• "Bacteria in Yogurt" pages 5-6 of The BioNet Booklet, Volume 3, Spring 1995, BioNet c/o Rod Johnson, Eau Claire North High School, 2700 Mercury Ave, Eau Claire, WI 54703
• "Homemade Yogurt, Sour Cream and Buttermilk" by M. Wagner, R. Bradleu, and M. Mennes, University of Wisconsin Extension publication B2768, available from Agricultural Bulletin, Rm 245, 30 N. Murray St., Madison, WI, 53715, 608-262-3346

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