2. Explain the formation of curd from the milk.

The formation of curd from milk involves a process called "fermentation," which is primarily driven by the action of bacteria known as lactic acid bacteria. The key steps in this process are:

1. Inoculation: To initiate the fermentation process, a small amount of a starter culture containing lactic acid bacteria is added to the milk. Common strains of bacteria used for this purpose include Lactobacillus bulgaricus and Streptococcus thermophilus.

2. Incubation: After inoculation, the milk is left to incubate at a slightly elevated temperature, typically around 40-45°C (104-113°F). This warm environment provides optimal conditions for the growth and activity of the lactic acid bacteria.

3. Fermentation: As the lactic acid bacteria grow and multiply in the milk, they begin to ferment lactose, the natural sugar present in milk. During fermentation, the bacteria produce lactic acid as a byproduct of metabolism. This lactic acid causes the pH of the milk to decrease, leading to the coagulation of milk proteins, primarily casein.

4. Coagulation: The decrease in pH caused by the accumulation of lactic acid destabilizes the casein proteins in the milk, causing them to aggregate and form a gel-like structure. This process is what we observe as the milk transforming into curd or yogurt.

5. Maturation: After the curd has formed, it is typically allowed to undergo further maturation for a period of time, which can vary depending on the desired texture and flavor. During this maturation period, additional metabolic processes may occur, contributing to the development of characteristic flavors and textures in the curd.

The end result of this process is the transformation of liquid milk into a semi-solid curd with a tangy flavor and creamy texture. Curd is a nutritious dairy product rich in protein, calcium, and beneficial bacteria, making it a popular food choice in many cultures around the world.