Is it necessary to use a cleanser to wash the bottle and nipple after usage?

Yes.

Milk contains fat and protein that cannot be washed away using just plain water.  Before sterilizing the bottles, be sure to wash them clean with a cleanser and rinse them thoroughly. 

We recommend using Pigeon Liquid Cleanser to wash the bottles and nipples as it is made with 100% food grade ingredients, and therefore a safer option as compared with general dishwashing detergent.