Question

Milk can be preserved by adding a few drops of
A) Formic acid solution
B) Formaldehyde solution
C) Acetic acid solution
D) Acetaldehyde solution

Answer 1

Preservatives are used to increase the lifetime of a product. Before adding preservatives we also have to check whether it is damaging the product or increasing the shelf life of the product. Preservatives prevent the growth of microorganisms that can degrade the quality of milk.

Complete step by step answer
Preservatives are the substance which increases the lifespan of a product. It is used to prevent acidification, decomposition, fermentation or any kind of degradation in the product.
Milk is an emulsion. To stabilise milk, preservatives are added as it acts as an emulsifier.
Om adding lemon juice to milk at home, we all know that damage occurs. Lemon contains acid. Thus we can conclude that by adding acids, milk is damaged. Hence we cannot add any sort of acids like formic acid $(HCOOH)$ , acetic acid $(C{H_3}COOH)$ to milk.
Option (a) and (c) are incorrect.
But we can add formaldehyde solution $(HCHO)$ to milk as it acts as an emulsifier. It prevents the growth of bacteria, fungi and different kinds of microorganisms which can degrade or lower the quality of milk.
Acetaldehyde ( $C{H_3}CHO$ ) can also be used as an emulsifier. But it seems that it has some carcinogenic actions when it enters the human body.
Some very popular preservatives which are added in dairy products are sodium benzoate, potassium sorbate and natamycin.
Many milk adulterations are also done with dangerous preservatives like with chemicals, detergents, formalin etc. Nowadays Govt. take strict actions against those dealers.
Hence strictly Acetaldehyde solution cannot be used.
So,the correct option is (B).

Note
Hydrogen peroxide $({H_2}{O_2})$ was very popularly used in the dairy industries as a preservative as it is used to activate enzymes called lacto peroxidase which helps to improve the quality of product. Also it is non-toxic preservative and can be removed easily.