Explain the process of chemical reaction that takes place in souring of milk and also tell the chemical equation of this chemical reaction.

Dear student,
Milk is made up of lactose, sugar, bacteria among other things. So, when ambient temperature is provided (by keeping milk in room temperature) the bacteria feeds on the sugar present in the milk. Thus, milk becomes sour. This process of souring of milk is fermentation process, which converts the lactose present in milk to lactic acid, thus making milk acidic (lower pH value).
The following reaction will take place:
C₁₂H₂₂O₁₁ +H₂O--> 4C₃H₆O₃
Regards!