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

As we know that some of the foods that we eat generally contain acids. Due to the properties of their aqueous solutions, acids are a distinct class of compounds. Lemons, Vinegar and some candies have a sour taste; they all contain acids, because acids have a sour taste. Aqueous solutions of acids are electrolytes because they conduct electricity.

Complete step by step answer:
We must remember that the milk is a white liquid that is produced by the mammary glands of women and female animals to feed their babies, before they are to digest solid food. It is a nutrient-rich liquid and also contains proteins.
We must remember that the milk gives what is known as an amphoteric reaction, when milk is first taken from a cow's udder. That is milk turns red litmus paper to blue and blue litmus paper to red. From this information, fresh milk is rarely alkaline and is always acid towards phenolphthalein is used as an indicator.
We must know that when milk is first taken from the udder it does not contain acidity. Milk contains lactose, which is a sugar. Lactose contains a harmless bacteria called lactobacillus. It ferments lactose at room temperature and turns into an acid.

The acid present in milk is lactic acid and it gives sour taste to milk.
B. Lactic acid is the correct answer.

Note:
We have to remember that the addition of an acid such as vinegar or lemon juice to milk also gives sour taste. Oxidation of alcohols to render harmless other toxic compounds containing the $OH$ group is used by the human metabolism. During intense activity, muscles produce lactic acid, it is oxidized by enzymes called lactate dehydrogenase to the metabolically useful compound called pyruvic acid.