When I was testing the horse gourmet food, I made some of the other pet foods and found what you found that it can't be put in the fridge (the horse granola bars are the only gourmet pet food that can actually be placed in the fridge), but it can stay in your inventory without going off liked food cooked for sims. I found this out because I didn't have a cat or dog, so it didn't get used and just sat there for quite some time. Not sure if it will eventually go off, but it was there for about 2 weeks and didn't go off.
I find it funny that the pet food you make for a horse using fruit can be put in the fridge, yet the pet food you make using meat cannot.