Question

Which of the following acids is present in sour milk?
(A) Glycolic acid
(B) Lactic acid
(C) Citric acid
(D) Tartaric acid

Answer 1

Hint: To answer this question we should know the components present in the milk. Sour milk is nothing but curd. We should focus on the change in composition of the components present in the milk.

Complete step by step solution:
Milk is a liquid food produced in the mammary gland of mammals. The main component that milk contains is nutrition and its primary function is to provide nutrition to enhance the growth in whole. Milk contains nutrition such as calcium, protein and carbohydrates.
Milk consists of globular protein known as casein. The main source of carbohydrates present in the milk is lactose.
-Option A is glycolic acid. Glycolic acid is present in sugarcane not in the milk or curd. Therefore option A is the wrong answer.
-Option B is lactic acid. Milk is converted to sour milk (curd). Milk is converted to curd by the process of fermentation. We know that milk contains casein and lactose. During the process of fermentation, it uses enzymes to produce energy from lactose. The energy is produced as ATP where ATP is adenosine triphosphate, which stores energy or releases energy in cellular processes. During the process of release of energy, lactic acid is obtained as a byproduct. The sour milk (curd) is formed by the chemical reaction of lactic acid and casein. Lactic acid denatures casein present in milk, which destroys the tertiary and quaternary structures of protein and the casein is converted to coagulation of the protein. Therefore sour milk is formed and lactic acid is the main source present in it. Hence, option B is the correct answer.
-Option C is citric acid. Citric acid is present as vitamin C in milk, but in a very small amount. Therefore option C is the wrong answer.
-Option D is Tartaric acid that is present in tamarinds not in milk. Therefore option D is the wrong answer.

Thus, from the above discussion, we can conclude that option B is the correct answer.

Note: Milk is converted to curd by the process of fermentation. The sour milk (curd) is formed by the chemical reaction of lactic acid and casein. Citric acid is present as vitamin C in milk, but in a very small amount. The citric acid is the main source in lemon, orange etc.