In everyday chemistry of food, is the statement Formation of curd from milk is a chemical reaction scientifically correct or incorrect?

Introduction / Context:
The transformation of milk into curd is a familiar process in households and an excellent real life example of a chemical change in food chemistry. Understanding whether this change is physical or chemical helps students connect classroom concepts to daily life. A chemical change produces new substances with different properties, while a physical change only alters form or state. This question asks whether the common statement about curd formation being a chemical reaction is scientifically correct.

Given Data / Assumptions:
- The starting material is milk, which contains water, fats, proteins, and lactose. - Curd is obtained from milk by adding a small amount of existing curd or lactic acid bacteria and keeping the mixture warm. - Lactic acid bacteria act on lactose sugar present in milk. - We compare this change with typical physical changes such as melting or boiling.

Concept / Approach:
In a chemical change, the original substances are converted into new substances with different chemical compositions and properties. During curd formation, lactic acid bacteria ferment lactose sugar into lactic acid. Lactic acid decreases the pH of the mixture and causes milk proteins such as casein to coagulate. The resulting curd has a different taste, texture, and acidity compared to milk. These differences come from the formation of new chemical substances, mainly lactic acid and coagulated protein networks, which shows that a chemical reaction has taken place rather than a simple physical change.

Step-by-Step Solution:
Step 1: Identify the main components of milk, especially lactose sugar and casein protein. Step 2: Recognise that lactic acid bacteria present in curd or starter culture break down lactose through fermentation. Step 3: During fermentation, lactose is converted into lactic acid, which is a new chemical substance not present in the same amount in fresh milk. Step 4: The lactic acid lowers the pH, causing casein proteins to denature and coagulate, forming a semi solid curd structure. Step 5: Note that milk cannot be easily recovered from curd by simple physical means like cooling or heating, which is typical of irreversible chemical changes. Step 6: Conclude that formation of curd from milk is indeed a chemical reaction, so the statement is correct.

Verification / Alternative check:
To verify, compare curd formation with a purely physical change such as freezing water into ice. Frozen water can be melted back to liquid water without any change in composition. In contrast, curd retains a sour taste and solidified texture even if warmed or stirred, demonstrating that its composition has changed. Food science texts classify fermentation as a biochemical process involving enzymes and microbes that produce new molecules. This confirms that curd formation from milk is a chemical change based on fermentation and protein coagulation.

Why Other Options Are Wrong:
Option B claims that curd formation is only a physical change, which is incorrect because new substances like lactic acid are formed. Option C suggests that the change is purely like a change of state such as melting, but curd formation is not simply solidification or liquefaction and involves complex biochemical reactions. Option D states that curd formation is reversible and not a chemical reaction, which is wrong because you cannot easily get back the original milk and the process clearly involves fermentation.

Common Pitfalls:
Students sometimes judge changes only by appearance. Since both milk and curd are dairy products and look somewhat similar, learners may think no new substances are formed. Another common mistake is to assume that any solidification is a physical change, forgetting that proteins can chemically denature and coagulate. Always check whether the process produces new substances, changes taste and smell, and is hard to reverse. Such clues strongly indicate a chemical change, as in the conversion of milk to curd.

Final Answer:
The correct answer is: Correct – formation of curd from milk is a chemical change involving new substances.