Which milk protein is most difficult to digest?

• A. Whey
• B. Albumin
• C. Lactalbumin
• D. Casein

Answer: (D)

The digestion rate of milk proteins depends on their structure and behavior in the stomach. Casein forms a gel or soft curd when exposed to the acidic environment of the stomach, which slows access of digestive enzymes and prolongs gastric emptying. This coagulated mass is digested more slowly, so casein is harder to break down than other milk proteins. In contrast, whey proteins are soluble and don’t curdle with acid, so they’re rapidly hydrolyzed by enzymes and quickly absorbed. Albumin and lactalbumin are also digested relatively quickly because they remain soluble and accessible to proteases. Thus, among these proteins, casein is the most difficult to digest.