Experiment 2: Yogurt Preparation

Bacterial fermentation allows milk to be stored safely and consumed much later than would be possible with fresh milk. Fermentation has been used for centuries to provide nutrients for nomadic tribes and other people without ready access to fresh milk. This is an instance where bacteria are used in a beneficial manner to prepare a food product.

Procedure

1. Place sauce pan on stove or hot plate. Add 950 mL (approximately 4 C.) milk to the pan. Reserve a small amount (approximately 1 tablespoon) of the milk.
2. Measure and record the pH of the reserved milk using pH indicator paper.
3. Insert thermometer into milk and gently heat while stirring. Bring the milk to 85 °C.
4. Remove the pan from the heat and allow the milk to cool to 43 °C.
5. Remove a small amount (approximately 1 tablespoon) of the cooled milk.
6. Measure and record the pH of the reserved cooled milk using pH indicator paper.
7. Measure and pour 120 mL plain yogurt into a mixing bowl.
8. Slowly stir in the cooled milk, 120 mL (½ C.) at a time, mixing the ingredients with a plastic fork. Be sure to thoroughly mix the ingredients to create a homogenous solution.
9. After all the cooled milk has been added to the yogurt, remove a small amount (approximately 1 tablespoon) of the mixture.
10. Measure and record the pH of the reserved mixture using the pH indicator paper.
11. Cover the bowl with Parafilm™ and use a plastic fork to punch six holes in the wrap. This allows for moisture and gasses to escape.
12. Incubate the mixture in a warm place (30 - 32 °C) for 5 - 6 hours. If you don't have a warm place for incubating the yogurt, wrap the bowl with a heating pad and wrap the heating pad with 1 - 2 towels. Set the heating pad to low or medium heat and incubate for 3 - 4 hours.
13. Check the yogurt's consistency after the recommended incubation time. It should be “set” and have a consistency similar to store bought yogurt. If it is not yet set, incubate the yogurt for an additional 1 - hours.
14. Remove a small amount (approximately 1 tablespoon) of the set yogurt; measure and record the pH using pH indicator paper.
15. Place the yogurt in a refrigerator for 24 hours. After 24 hours, remove a small amount (approximately 1 Tablespoon) of the yogurt; measure and record the pH using the pH strip and the accompanying key.