We get a little over a 2 year rotation on cheapie dog food from walmart. We pour bags into 60 gallon drums. Usually after 2 years the top inch or so is WHITE (fat going rancid is a big cause of this IMO). We scoop that part off and throw in the pond for the fish. Everything below that is given to the dogs. Actually they have eaten the white part before also with no problems.
Mylar and o2s aren't going to help in this one situation.