What is the role of adding starter culture to milk when making yogurt?

Starter culture plays a crucial role in transforming milk into yogurt. It acts as the catalyst for the entire process, bringing about the characteristic flavor, texture, and beneficial qualities of yogurt. Here's how:

1. Fermentation: Starter culture contains live bacteria (typically *Lactobacillus* and *Streptococcus* strains). When added to milk, these bacteria consume lactose (milk sugar) and produce lactic acid as a byproduct. This process is called fermentation.

2. Acidification: The lactic acid produced by the bacteria lowers the pH of the milk, causing it to thicken and solidify. This is the key step in transforming liquid milk into a thick, creamy yogurt.

3. Flavor Development: The fermentation process also contributes to the tangy and slightly sour flavor that is characteristic of yogurt.

4. Probiotic Benefits: Many starter cultures contain probiotics, which are live bacteria that can benefit gut health. These bacteria survive the fermentation process and are present in the final yogurt product.

5. Preservation: The acidic environment created by lactic acid also acts as a natural preservative, inhibiting the growth of harmful bacteria and extending the shelf life of yogurt.

In essence, starter culture is the key ingredient that makes yogurt possible. Without it, milk would simply remain milk. The specific strains of bacteria in the starter culture determine the unique flavor, texture, and potential probiotic benefits of the yogurt.