The process of curd production from milk is known as

Concept: Biochemical and physical transformations are classified by their defining mechanism: photosynthesis converts light energy into chemical energy using chlorophyll and carbon dioxide; distillation physically separates a liquid mixture's components by differences in boiling point; fermentation is the anaerobic breakdown of sugars by microorganisms into acids, gases, or alcohol; and sterilization eliminates microbial life using heat, pressure, or chemicals.

Application: The step-by-step mechanism by which milk turns into curd is as follows.

1. Milk sours naturally, or a starter culture is added, introducing lactic acid bacteria (e.g., Lactobacillus) into the milk.

2. These bacteria anaerobically metabolize the lactose sugar present in the milk.

3. This metabolic activity produces lactic acid as a byproduct, lowering the pH of the milk.

4. The dropping pH denatures and coagulates the milk protein casein, turning liquid milk into semi-solid curd (dahi).

This bacteria-driven, oxygen-free conversion of a sugar into an acid matches the fermentation mechanism defined above.

Cross-check: The same mechanism underlies other familiar fermentations — yeast converting sugars in dough into carbon dioxide during bread-making, and yeast converting grape sugars into alcohol during wine-making. In every case, a microorganism converts a sugar into a new compound without oxygen, matching the curd-formation mechanism above.

• Photosynthesis needs light and chlorophyll to build sugars from carbon dioxide and water — the milk-to-curd change happens in the dark and starts from an existing sugar, so it does not fit this mechanism.

• Distillation needs heating and vapour condensation to separate liquid components by boiling point — the milk-to-curd change involves no boiling or vapour condensation, so it does not fit this mechanism.

• Sterilization needs heat, pressure, or chemicals to eliminate microbial life — the milk-to-curd change instead depends on live bacteria actively multiplying, so it does not fit this mechanism.

Since curd formation is a microbial, oxygen-free conversion of milk sugar into acid, it is classified as fermentation.