Bottles starts smelling and gets dirty too when kept unused for long time. Initially in the beginning of summers people wash their bottles properly before using them. In spite of cleaning them with washing powder it smells a lot .To get rid of this problem fill bottle half with water and add 1/2tsp of baking soda in it. Shake the bottle well and let the solution stay in the bottle for an hour. Wash the bottle thoroughly after an hour.
Babies milk bottle can also be washed in similar way.