Question

A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of fresh milk from 6 to slightly alkaline?
(b) What do you expect to observe when the milk comes to boil?
(c) Why does this milk take a long time to set as curd?

Answer 1

Hint: Milk is oil in water type of emulsion. The dispersion medium is water and the dispersed phase is fat/oil. By changing the pH, milk can be coagulated.

Complete step by step answer:
 In milk, fat droplets are suspended in water. These fat droplets account for around 4 percent of milk. If milk coagulates, then curd is obtained. Curd is nothing but coagulated milk proteins.

(a) The pH of milk is around 6. At this pH, milk soon becomes spoiled and becomes undrinkable. So it cannot be kept for a longer time. To prevent the spoilage of milk, the milkman adds a very small amount of baking soda to fresh milk. Baking soda is sodium bicarbonate. It is basic in nature. Addition of baking soda increases the pH of milk from 6. The milk is now alkaline. Due to this milk can be kept for a relatively longer time.

(b) The milk containing a very small amount of baking soda will become slightly bitter in taste upon boiling. In other words the taste of milk will be slightly different from the original test. Acids have sour taste whereas bases have bitter taste. Thus, the slightly bitter taste of milk is due to addition of base (baking soda).

(c) For curdling milk, an acidic medium is required. When small quantity of baking soda is added, the medium becomes slightly basic. All the acids present are neutralised with baking soda. Baking soda reacts with acids to produce carbon dioxide gas. In this process, acid is neutralised. Due to this, a long time is required for this milk (containing small amount of baking soda) to set.

Note:
Do remember that the process of curdling occurs in an acidic medium which is why we add a little curd to milk while initiating this process.