Question

Milk is spoiled/fermented/curdled by
(a)Rhizobium
(b)Lactobacillus
(c)Azotobacter
(d)Clostridium

Answer 1

Hint: Milk is spoiled or fermented or curdled by a bacteria that is also known as LAB. Milk is said to be spoiled when the lactose sugar of the milk is converted into glucose and galactose resulting in the production of lactic acid.

Complete answer:
Lactobacillus is responsible for spoiling/fermenting/curdling of the milk. Lactobacillus is a gram-positive bacteria used commercially in the dairy industry to make many dairy-related products. This is because it converts lactose into lactic acid which is converted to casein which results in the formation of curd from milk. This process is known as curdling or spoiling or fermenting of the milk. But too much curdling of the milk renders it harmful for consumption and thus this process needs to be carefully controlled. One such method to present excessive curdling of milk is pasteurization.

Additional Information: Let us look at the process of pasteurization in detail.
-Milk is preserved by the method of pasteurization which is named after its inventor Pasteur and is done to kill bacteria from milk, thus preserving it for a longer time.
-In this process, foods such as milk and fruit juice are treated with mild heat (less than $100°C$).
- Due to this, pathogens are killed, and also the enzymes responsible for spoiling or curdling milk become denatured.
- It also ensures that pathogenic microorganisms that can cause diseases in humans are also eliminated.
-This is a way of making sure products are safe for a longer time in an inexpensive way.
So, the correct option is ‘Lactobacillus ’.

Note: -Clostridium is a gram-positive genus of bacteria that is responsible for causing botulism and tetanus in humans.
-Rhizobium is a genus of bacteria responsible for nodule formation in leguminous plants through which it fixes nitrogen in the soil.
-Azotobacter is a genus of bacteria that is also responsible for nitrogen fixation.