1. NaOH's Role:
* Alkalinity: NaOH is a strong base, making the solution highly alkaline. This high pH disrupts the delicate balance of charges within milk proteins.
* Ionic Strength: NaOH increases the ionic strength of the solution, further disrupting the interactions between protein molecules.
2. Protein Denaturation:
* Protein Structure: Milk proteins like casein and whey proteins have complex folded structures held together by weak bonds (hydrogen bonds, electrostatic interactions).
* Disruption of Bonds: The alkaline environment and increased ionic strength break these weak bonds, causing the proteins to unfold and lose their native structure (denaturation).
3. Coagulation:
* Exposed Hydrophobic Regions: As the proteins unfold, hydrophobic regions (water-repelling) become exposed. These regions tend to clump together to minimize contact with the surrounding water.
* Aggregation: The denatured proteins aggregate, forming large clumps or networks. This aggregation traps water and other components of the milk, forming the observed film.
4. Other Factors:
* Concentration: The concentration of NaOH greatly influences the speed and extent of coagulation. Higher concentrations lead to faster and more pronounced film formation.
* Temperature: Heating accelerates the denaturation and coagulation process.
In summary, adding sodium hydroxide to milk disrupts the structure of milk proteins, causing them to unfold, aggregate, and form a film. This process is driven by the alkaline environment and increased ionic strength created by the NaOH.
Note: This reaction is similar to what happens when making cheese. The addition of rennet (an enzyme) or acids like lemon juice also causes milk proteins to coagulate, forming curds.
