Which one of the following combinations of organisms are responsible for the formation and flavour of yoghurt

Options

A. Lactobacillus bulgaricus and streptococcus thermophilus
B. Rhizobium melioti and Aztobactor
C. Bacillus subtillis and Escherichia coli
D. Bacillus megathermus and Xanthomonas species

Explanation

To solve the question regarding which combination of organisms is responsible for the formation and flavor of yogurt, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the specific organisms that are involved in the production of yogurt and contribute to its flavor.

2. **Identify Key Organisms**: From the options provided, we need to identify which organisms are known for their role in yogurt production. The options given are:
- A) Lactobacillus bulgaricus and Streptococcus thermophilus
- B) Rhizobium mellioti and Azotobacter
- C) Bacillus subtilis and Escherichia coli
- D) Bacillus megaterium and Xanthomonas species

3. **Recall Yogurt Production Process**: Yogurt is produced through the fermentation of milk. The key bacteria involved in this fermentation process are known to be Lactobacillus bulgaricus and Streptococcus thermophilus.

4. **Temperature and Time for Fermentation**: The fermentation process typically occurs at a temperature of approximately 45 degrees Celsius and takes about 4 hours. This is important to note as it confirms the conditions under which these bacteria thrive.

5. **Flavor Components**: The flavor of yogurt is primarily due to the production of compounds such as acetaldehyde and lactic acid, which are produced by the fermentation activity of the bacteria.

6. **Choose the Correct Option**: Based on the information gathered, the correct combination of organisms responsible for yogurt formation and flavor is:
- **Option A**: Lactobacillus bulgaricus and Streptococcus thermophilus.

### Final Answer:
The correct combination of organisms responsible for the formation and flavor of yogurt is **Lactobacillus bulgaricus and Streptococcus thermophilus** (Option A).

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