In food chemistry, the curdling of milk during processes such as souring or cheese making is an example of which type of change in milk proteins?

Introduction / Context:
Curdling of milk is a familiar phenomenon in everyday life, seen when milk turns sour or when cheese and curd are prepared. Chemically, this process involves changes in the structure and physical state of milk proteins, especially casein. The question asks you to identify which specific type of change curdling represents. Understanding this helps link simple kitchen observations to the underlying principles of colloid and protein chemistry.

Given Data / Assumptions:

• Curdling involves formation of a semi solid or solid mass from liquid milk.
• Milk contains proteins, fats, lactose and minerals.
• Acid or enzymes such as rennin can induce curdling.
• The options include inversion, denaturation, hydrolysis, coagulation and sublimation.

Concept / Approach:
Curdling begins with denaturation of milk proteins, where their native structure is disrupted by acid or enzyme action. As denatured proteins lose their original solubility and unfold, they aggregate and form a network that traps water and fat, leading to a gel or solid mass. This aggregation and phase separation is called coagulation. Denaturation is a change in protein structure, and coagulation is the associated formation of a visible solid or gel. The question specifically asks for the process that results in the formation of curds, which is best described as coagulation.

Step-by-Step Solution:
Step 1: When milk is acidified or treated with enzymes, the pH moves closer to the isoelectric point of casein proteins. Step 2: At this point, the electrostatic repulsion between protein molecules decreases and the proteins begin to denature and unfold. Step 3: Denatured casein molecules aggregate and form a three dimensional network, trapping water and fat globules. Step 4: This process results in the formation of curds or a semi solid mass, which is the hallmark of coagulation. Step 5: Therefore, curdling of milk is best classified as coagulation of milk proteins.

Verification / Alternative check:
Dairy science references describe cheese making as involving coagulation of casein micelles. Laboratory experiments show that adding acid to milk leads to a visible separation of curds and whey, confirming that proteins have formed a solid phase. While denaturation is part of the microscopic process, the macroscopic phenomenon observed as curdling is the coagulation of proteins into a gel or solid network. This matches common usage in food technology, where rennet or acid induced changes are described as coagulation.

Why Other Options Are Wrong:
Option A, inversion, refers to conversion of sucrose into glucose and fructose and is not related directly to milk curdling. Option B, denaturation, does occur at the molecular level but does not by itself describe the visible formation of curds; coagulation is the more precise term for the overall process that includes aggregation and solid formation. Option C, hydrolysis, involves breaking chemical bonds, such as in fats, but curdling is more about physical aggregation than complete hydrolysis. Option E, sublimation, is a phase change from solid directly to vapour and has nothing to do with milk. Only option D correctly describes curdling as coagulation of milk proteins into a solid mass.

Common Pitfalls:
A common confusion is between denaturation and coagulation. Denaturation refers to structural change of proteins, while coagulation refers to aggregation and gel formation. In practice, denaturation often leads to coagulation, but the terms are not identical. Exam questions about curdling usually expect the term coagulation because it emphasises the visible separation of solid curds from liquid whey. Remember that many protein rich foods, such as paneer and cheese, are produced by coagulation of milk proteins.

Final Answer:
Curdling of milk is an example of Coagulation of milk proteins into a solid mass, resulting in the formation of curds and whey.