Question

Milk is changed into curd by
a) Acetobacter aceti
b) Bacillus megaterium
c) Xanthomonas citri
d) None of the above

Answer 1

Hint:This is a fermentation process in which sugar is converted into acid. Lactose is the milk sugar which is converted into acid.

Complete answer:
The above process is brought about by microorganisms. This is a fermentation process. The name is derived from the word 'ferve' meaning to break.The lactose of milk forms lactic acid which is sour to taste and this is curd. This is brought about by Lactobacillus. (which is not in the above option.)
This is a type of anaerobic respiration. The microbes form various products when the amount of oxygen is less and this is the main mechanism of fermentation. Thus the absence of oxygen and the presence of favorable microorganisms are prerequisites for this process.

Fermentation is of two types:
a) Lactic acid fermentation- In this process, lactose is converted into lactic acid. Lactose is milk sugar. When lactic acid is formed in milk, it imparts sour taste and thus results in the formation of curd. The microorganism involved is Lactobacillus.
b) Ethanol or alcohol fermentation- This process is a biological process in which the sugar (such as sucrose,glucose) is converted into cellular energy which produces ethanol and carbon dioxide as byproducts.

Thus, the correct option is ‘d’.

Note:The fermentation process results in the formation of many substances. These substances are easy to digest and assimilate by the body. This process helps to retain the nutritive value of food also.