How to clean containers

[ickmagnet]

I just bought two food grade 55g barrels to store water when I do my tank change. They are food grade and smell like they had juice in them. They are pretty clean but I want them perfectly safe so that there are minimal contaminants. How would I clean them? Just water? Hot water? Laundry soap? Dish soap? Don't want to see my fish and coral dead.

Thanks