Prevent the idly flour from absorbing moisture as well as odors and flavors,.... add any chemicals?

After grinding the batter keep it overnight in air tight container for fermentation and next morning beat the batter to mix the fluffiness and store it in fridge. If you are keeping them for long days outside then it starts adoring and give the souring taste due to change in room temperature. There is no need of any chemicals to prevent it. Just follow the simple steps. Hope it helps!

Check out the Idli preparation in detail:
http://desimejwani.blogspot.com/2012/03/idli.html