Which of the following enzymes is used for coagulating milk protein to obtain casein?
A) Rennin
(B) Ptyalin
(C) Trypsin
(D) Pancreatin

Hint:- Enzymes are referred to as biological catalysts which accelerate the rate of the chemical reactions and help in digestion, by converting large organic molecules into simpler ones. They remain unaltered in the whole chemical reaction. Various enzymes are found in the human body that cleaves different substrates, i.e there are different sets of enzymes for carbohydrates digestion, for protein digestion and for fat digestion.

Complete solution:
Casein belongs to the class of molecules called phosphoproteins, commonly found in mammalian milk. In milk, casein forms colloidal casein micelles by undergoing phase separation. There are four major casein molecules which include Alpha-S1, Alpha-S2, Beta and Kappa. The alpha and beta molecules are hydrophobic (water repellant) in nature and are easily precipitated by calcium ions. That's why the calcium content in milk is in excess to precipitate these proteins. However, Kappa casein is not precipitated by calcium and thus forms a major part of milk protein and prevents it from coagulating.
Rennin is a proteolytic enzyme secreted by the chief cells in the stomach of animals. As it is a proteolytic enzyme, it cleaves kappa casein and converts it into para kappa casein. This para kappa casein lacks the ability to maintain the micellar structure. Hence, Rennin has an important role in the coagulation of milk proteins. Ptyalin, also known as salivary amylase, is produced by salivary glands which initiate the digestion of starch molecules. Trypsin is a proteolytic enzyme secreted by pancreas that causes the breakdown of proteins in the small intestine. Pancreatin is a synthetic enzyme which is used as a medication to treat malabsorption syndrome.
So, the correct answer is option (A)- Rennin.

Note:- Adult humans lack the enzyme Rennin, but it is present in infants. In adults, the digestion of milk proteins is carried out by a different enzyme called Pepsin. Another enzyme called Renin is present in every human, may it be an adult or child. This enzyme maintains the blood pressure by a specialised mechanism called the Renin-Angiotensin-Aldosterone System (RAAS).