In olden days when refrigerators did not exist, it was customary to boil milk twice or thrice a day to prevent it from becoming sour. The reason was

Correct option is D

Ans (d)
Sol. Explanation:
· Boiling milk serves several purposes:
· Denaturation of milk proteins (casein): Boiling causes proteins like casein to denature, preventing the milk from coagulating easily, which helps keep the milk fresh.
· Killing lactobacilli: Boiling kills lactobacilli, the bacteria responsible for fermenting milk and converting it into curd (yogurt). By killing these bacteria, the milk remains in its liquid form.
· Pasteurization of milk: Although the term "pasteurization" refers to a specific, controlled method of heating to kill bacteria (named after Louis Pasteur), boiling the milk also serves a similar purpose by killing harmful microorganisms, thus increasing its shelf life.
Thus, boiling milk in the absence of refrigeration was an effective way to preserve it.