Adapted from Bringing Microbiology to Life, an ASM Educational Publication
- To demonstrate sugar fermentation by bacteria which results in a food product.
- To demonstrate the use of a mixture of bacteria to produce a desired effect on the proteins in milk.
A group of bacteria called "lactic acid bacteria" ferment milk sugars, lactose, to produce lactic acid. The production of acid from the sugar results in a different taste and consistency of the milk. Depending on the age and bacterial activity of the yogurt "starter", the inoculated milk can acidify the sugars enough to cause coagulation of the proteins within 8 to 18 hours. The production of acid may be measured by monitoring the pH of the milk and thus the consistency of the milk. Coagulation of the proteins is evident as a custard-like curd. Because more lactic acid is produced over time, the result of a longer incubation period is that yogurt becomes more tart.
Commercially produced yogurt is a convenient source of starter bacteria. Generally, a mixture of Streptococcus and Lactobacillus have been used to produce yogurt and are present in the starter. Using a simple stain, one may observe typical cocci in chains of Streptococcus and the large rods of Lactobacillus.
Other variables, such as the percentage of butterfat in the milk, the type of starter bacteria used, etc. also affect the quality of the product.
Untreated milk that is allowed to stand without refrigeration will "spoil." Bacteria naturally present in the milk will grow and usually produce highly undesirable results. The spoilage bacteria will breakdown the proteins and lipids in milk as well as the sugars. To prevent the milk from spoiling during yogurt production, most of the spoilage bacteria are killed by heating the milk to 80*C for 30 minutes. Alternatively, one can use commercially available "boxed milk" that has been treated to kill the spoilage organisms. Unopened boxed milk can be stored at room temperature for long periods of time without spoilage.
Materials per group of two students:
- A 100-250 ml of treated milk (skim, low fat, whole, chocolate, etc) in a beaker or cup
- One-fourth teaspoon of yogurt (plain, Acidophilus culture)
- pH strips
- Square of aluminum foil or other top
- A water bath or incubator for maintaining 38-40°C for at least 24 hours
Optional materials to be shared by the class:
- Microscope slides, inoculating loops, Bunsen burners, methylene blue stain, blotting paper
- Determine the pH of the milk.
- Record the type of milk, and pH of milk.
- Cool the milk to 38-40°C.
- Add the quarter-teaspoon of yogurt to the milk. Stir starter thoroughly into milk and cover with foil.
- Incubate at 38-40°C for 8-48 hours. Do not stir.
- Periodically check consistency by inserting a clean spoon or applicator stick. Record observations and check pH.
- Once the curd has formed, determine and record pH.
- Taste and rate the product(s). Record values.
- If microscopes with 100x objectives are available, spread a very thin film of yogurt product on a small area of the slide. Add a tiny amount of methylene blue stain to the area and top with a cover slip.
- View under the microscope. Sketch a representative example of your observations.
1. Type of milk used __________________
2. Initial pH of milk __________________
Final pH of final yogurt________________
Summary of Yogurt Production
|Type of Milk||Initial pH||Final pH
|No Fat Milk