We have probably 10 pounds of dry food and will rarely use it (occasional puzzle feeders, long weekends, etc). It is stored in an airtight plastic container. How long should we keep it before it is no longer good, or how do we tell if it is no longer good?
So, dry foods do have a shelf life. Dry cat food does contain moisture, usually less than 10%, but enough to run the risk of molds developing. In addition, the vitamins and other nutrients in the food are not everlasting, they do weaken, and that poses some risk.
In any event, the exact answer to your question is that it varies. Not all foods have the same shelf life, even within the same brand. If the food is in the original bag, the expiry date should be on it somewhere, if it is not, and you're not sure how long you've had it and can compare to the same in the store, then I would toss it. It's not so expensive to replace in comparison to a trip to the vets. It's probably a good idea to write the expiration date of any food on the outside of the container using masking tape and avoid mixing them.
In terms of determining if it's gone bad, there's no one answer to that. Some obvious possibilities:
- Mold on the food.
- It becomes very crumbly and powdery (almost all moisture gone).
- Doesn't smell right (can be hard for us to tell) or doesn't really smell at all.
The problem is, I'm not aware of anything especially obvious. Which, in some ways, is part of the concern I would have with dry foods because the flavor enhancers on the food may convince a cat to eat it anyways.