Thanks Melev...
I would fill them to the brim with water, let it soak 24 hours, rinse it out several times, let it air out for a day, then it is ready to go. You may need to tilt the barrel. If you have the plugs, you can screw it shut and roll it around to get the underside of the lid area soaked as well. That was the hardest spot for me t to clean with my Dr Pepper syrup barrels. Eventually I just used them and quit worrying about the sweet smell.