Question

Milk curdles when lemon juice is added. This is:
A) Evaporation
B) Coagulation
C) Reversible
D) Photochemical change

Answer 1

Hint:We know that milk contains fats, proteins, lactose which is a sugar and water. The proteins that are present in milk are casein proteins. In milk, the casein proteins form micelle which are tiny spheres.

Complete answer
We know that the proteins that are present in milk are casein proteins. One end of casein proteins like to be surrounded by water and hates to be surrounded by fats. The other end likes to be surrounded by fats and hates to be surrounded by water.

In milk, the casein proteins form micelle which are tiny spheres. In the micelle, the water-hating ends face inward while the water loving ends face outward. The end of casein protein that sticks to water grabs an electron and attains a negative charge. Thus, the micelle is negatively charged and repels each other. This prevents the micelle from aggregating.

When lemon juice is added to milk, the citric acid in lemon juice liberates positively charged hydrogen ions in the milk. The positively charged hydrogen ions get attracted towards the negatively charged micelle and get neutralised. All the micelles come closer and aggregate to form gelatinous material called curd.

When lemon juice is added to milk, the milk becomes sour. The bacteria in milk convert the lactose sugar into lactic acid which forms curdles. Thus, the curdling of milk is due to the coagulation of micelles of casein protein. Thus, milk curdles when lemon juice is added. This is coagulation.

Thus, the correct answer is option (B) coagulation.

Note:Curdling of milk occurs by treating the milk with enzymes or by simply letting the milk age for a long time. Chymosin enzyme alters the structure of casein and causes milk to curdle. Protease enzyme breaks up casein thus, disrupting the structure of micelle and causing milk to curdle.