Yogurt Production

MATERIALS

400 mL whole milk

thermometer

600 mL beaker

pan of water

hot plate

incubator

1 teaspoonful plain yogurt culture

methylene blue stain

glass slide

coverslip

microscope

pH paper or pH meter

aluminum foil to cover beaker

PROCEDURE

1. Place the milk in the beaker. (Check the pH using pH paper or pH meter.)

2. Place the beaker in a pan of water and heat the milk to 81 degrees Centigrade.

Maintain this temperature for 5-30 minutes. The longer you hold at 81 degrees Centigrade the more water evaporates and the firmer the curd.

3. Cool milk to 38-40 degrees Centigrade (but not lower).

4. Add yogurt starter culture. Stir in and cover beaker with foil.

5. Place in incubator at 35-40 degrees Centigrade for 5-8 hours. Do not disturb or stir.

6. Periodically check for custard-like firm consistency. Refrigerate.

7. Microscopic examination of product:

a. Spread small sample of yogurt on glass slide in a thin film. Let air dry.

b. Stain with methylene blue dye for 60 seconds.

c. Rinse and blot dry.

d. Examine slide with oil-immersion lens.

8. Taste and rate the product. (Determine the pH.)

9. Fruit or sugar may be added to the yogurt if it is too tart for the palates of students. Liquid flavorings (one cap full) such as vanilla or banana may be added to the milk at Step 4; it will not be changed during the incubation period. Sliced banana, strawberry preserves or other solids may be added at Step 8.