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

Souring of milk is caused by the fermentation of sugar present in it. Some bacteria act on sugar and convert it into acid. Hence, this phenomenon is also known as acidification. It gives milk a ‘sour’ or ‘tart’ taste.

Complete answer:
Souring of milk is a phenomenon which is caused as a result of fermentation of sugar present in milk, i.e. lactose. It is a disaccharide made up of glucose and galactose.
Presence of lactic acid bacteria or manual acidification of milk (by adding acid) results in fermentation of milk, as a result of which lactic acid is produced.

As a result of this, pH of milk decreases. This produces a ‘sour’ or ‘tart-like’ flavor in the milk. This phenomenon is also known as acidification of milk.
The other acids given in the options are –
Glycolic acid – It is present in sugarcane
Tartaric acid – It is present in grapes
Citric acid – It is present in citrus fruits

Therefore, the answer is – option (b) – Lactic acid is present in sour milk.

Additional Information:
The microorganisms responsible for the souring of milk are known as ‘Lactic Acid Bacteria’. An example of lactic acid bacteria is Streptococcus lactis. Also, lactose fermenting bacteria are generally facultative anaerobic as they can survive in the absence of oxygen.

Note:
Some people suffer from a disease called ‘Lactose intolerance’. It is caused by a deficiency of an enzyme in the body called ‘lactase’, therefore the body becomes incapable of digesting lactose. People suffering from this can opt for non-dairy alternatives such as soy milk.