Middle did not cook - Apples too wet?

I've made this cake multiple times and always had it turn out wonderfully. The problem you had could be caused by extra moisture from the apples or the ricotta, or even problems with the oven temperature variations and where in the oven it baked. It should be moist, but not raw in the middle. Is it possible that your oven temperature is not holding? If it gets hot, and then cycles off to cool, your outside cake would bake or even overbake before the middle could bake and set. It would look baked, of course, right up until you cut into it. Extra moisture could also make the cake wet in the center, and it would sink as a rule. While you don't want the apples as dry as say, grated carrot, you don't necessarily want a lot of extra juice either. Same for the ricotta. Some brands are wetter than others, and for this one you would want to drain extra whey that may have accumulated in the carton. If you think your apples are extra juicy, you can squeeze dry them by hand and get just enough extra out without removing too much. You can also drain ricotta in a sieve for a few minutes to do the same thing. It's also a good idea to test the center of a cake before your remove it from the oven, rather than risk baking by clock time alone. If your cake seems to be browning up too quickly, and the center is lagging behind, you can always cover it with a sheet of foil for protection while you add baking time. I'm sorry yours didn't turn out so well this time, but give it another go. Check the oven temp, and then consider the other fixes to remove extra moisture. It is really a nice cake with a cup of coffee or tea.