Fermentation is the name of process by which milk changes into curd. Fermentation is a chemical process that usually is initiated in the presence of bacteria. These bacteria produce certain chemicals that act as catalysts for fermentation processes.

The bacteria that convert milk into curd, are called lactobacillus. These bacteria convert lactose (present in the milk) into lactic acid.

The formation of curd from milk is a chemical reaction and it is irreversible, that is, once the milk has been converted into curd, it cannot be changed back into milk.

Processes happening around us-

There are two types of processes that generally occur in nature. Chemical processes & Physical processes.

Chemical Reactions/ processes- These are the process in which-

Molecular or ionic structure of a single compound changes (only 1 substance, as conversion of milk into curd)    OR

Reactions between two or more compounds in which the molecular or ionic structures of substances are changed and new compounds are formed post reaction.

As a result chemical processes, properties of matter formed are different from the initial substances. For example, the formation of curd by milk, burning of substances etc.

Physical processes– There are one other type of processes that occur in nature- physical processes. Under such processes, physical or molecular properties of materials do not change. Examples of physical processes are inter-conversions of water into ice or steam. Thermal expansions of metals etc.