We been just going thru this...

Here is a link that might be useful: Low temp. for apples..

One thing not in the linked discussion is that apples are divided into three types scientifically, and have different damage properties.

Type I correspond to Summer apples, and these will damage easily, maybe taking 28F for a short time. They also have the property that if the apple is damaged anywhere, the entire apple will usually rot.

Type II corresponds to Winter apples, and these are very robust, taking down to 20F without damage. They also have the property that if any part of the apple is damaged, it will heal itself, even if off the tree. In the native apple forests, there are many apple trees that will go bare and hold on to fruit for quite a while longer without the fruit having any damage.

Type III corresponds to all the hybrids in between the two, these behave specific to the variety.

I forget the reference, I can dig it up if anyone cares.