Question

What do you observe when a little amount of curd is added to a bowl containing warm milk and kept for five hours?

Answer 1

Curd has Lactobacillus bacteria present in it which is responsible for this physical change of the milk. Lactose sugar is present in milk. Yoghurt is made by the fermentation of lactose that is present in the milk. Lactobacillus is a rod-shaped bacterium which facilitates this process. It converts lactose into lactic acid.

Complete answer:
When one adds a little amount of curd in warm milk, the entire milk gets converted into curd. This is a type of physical change. It occurs because of the Lactobacillus bacteria present in the curd. These microorganisms convert lactose present in milk into lactic acid by the process of fermentation.
Fermentation is a metabolic process that produces chemical changes with the help of certain enzymes acting on substrates. The study of fermentation is known as zymology. In some living beings, like bacteria, fermentation is a method of producing Adenosine triphosphate or ATP without using oxygen i.e., anaerobically.
Milk consists of globular proteins called casein. The curd forms due to the reaction between the carboxylic acid bacteria and casein. During fermentation, the bacteria use enzymes to supply energy (ATP) from lactose. The by-product of ATP production is carboxylic acid. Lactic acid acts on the globular proteins present within the milk and denatures them. This denaturation destroys the tertiary and quaternary structures of proteins and the globular proteins are converted into fibrous proteins thus giving a thick texture to the milk due to coagulation of the proteins.
This bacterium develops and reproduces several times to make curd in the warm milk. Sometimes if the milk is extremely hot or cool, the bacteria won't have the capacity to work up to its full potential and the curd won't be formed.
The ideal temperature for Lactobacillus acidophilus microorganisms which change the milk into curd is 40 degrees Celsius. Below this temperature, the microscopic organisms do not work up to their ultimate potential.

Note:
Even if we add milk to the curd, the result will not be altered; one can add milk to the curd instead of curd to the milk and get the desired result i.e., the production of yoghurt or curd. This occurs as long as there is presence of casein proteins and Lactobacillus bacteria in milk and curd respectively.