I have experienced the same rancid food for some time now.  I also have
used TF for years but ever since Performance Foods admitted they had some
bad batches many months ago, I have yet to get any good food.  I have
purchased food directly from Performance Foods as well as my vet.  Both
Regular formula plus Older Ferret formula smells rancid continually but
my 11 ferrets have not experienced any diarrhea from it.  I did throw away
several bags and had them replaced by PFoods but I received more rancid
food in return.  I reluctantly feed TF but do mix it with other foods.
PFoods says their foods are fine and they are not experiencing problems,
but personally I don't agree.  My nose knows!
[Posted in FML issue 3511]