Can you describe what happens when milk is converted into curd or yoghurt, from your understanding of proteins.
Hint: Curd is obtained from the milk by its coagulation through the process called curdling. It can be done with rennet or acidic substances like vinegar or lemon juice that allows it to coagulate.
Complete answer: Proteins are large sized mixed polymers of several types of alpha amino acids that are joined by peptide linkages into linear chains that get folded in various forms. Most proteins are functional in their tertiary structure and they function at a specific temperature and pH called optimum pH and temperature. The increases or decrease in the pH or temperature denatures the tertiary structure of proteins and causes its coagulation.
Milk has a characteristic protein called casein that gives it the characteristic white color. This protein has all the essential amino acids required for the growth of a human body, thus it is of high nutritional value.
Some microorganisms like Lactobacillus, secretes lactic acid that reacts with the casein protein of the milk. It changes the pH of the milk and the protein is denatured or it loses its secondary and tertiary structures which causes coagulation or precipitation of milk proteins and curd formation. So formation of curd from milk is actually a chemical change as it requires the fermentation of milk protein by bacteria like Lactobacillus.
Note: Curd also called dahi in India, is highly nutritious and is eaten in various forms. It can be used to make pudding with raisins, chopped nuts and other ingredients. They can also be eaten fresh without any further additions.