What is casein?

• A. A sugar found in milk
• B. A protein found in milk
• C. A lipid found in milk
• D. A nucleic acid found in milk

Answer: (B)

Casein is the main protein found in milk. It’s a phosphoprotein that makes up most of the protein content in milk and exists as casein micelles stabilized by calcium phosphate, which is why milk can form curds when acidified or treated with rennet. This distinguishes it from the sugar lactose, which is the primary carbohydrate in milk, from the fats that are lipids circulating as milk fat, and from nucleic acids, which are genetic material that isn’t the primary component of milk. Understanding that casein is a protein explains its role in nutrition and in cheesemaking, where its ability to coagulate into curds is essential.