Freeze Extra Dry Cat Food

I have two kitties on prescription food. It is rather expensive, but there is a noticeable break if you buy the larger bag, 17 lbs. However, I have noticed in the past that my discriminating cats don't eat stale food as reliably as when it is fresh. I was mentioning this to the lady behind the counter at my vet's office and she recommended that I bag up the excess and freeze it for future use. She has done this herself in the past. I tried it and this works really well. I use gallon size Ziploc freezer bags and keep them in the deep freezer.