Vacuum sealing is important and of course it creates a window of time in which the contents can not be smelled. You wash with alcohol or similar to clean any traces off the layers of the vacuum bag. A vacuum seal holds molecules in, so it keeps them away from the dogs nose. Over time scent can permeate the vacuum seal bag. Some vacuum seal bags are made with less porous materials than others and thus hold the scent away from the dog longer. I would be hesitant about using metal foil in packages, but would look into less porous plastic bags.