When macerating apples to use in a pie, what should you do with the liquid?
On Serious Eats, Kenji Alt recommends precooking your apples (by a quick microwave or hot water bath) to set the pectin and prevent shrinking:

http://sweets.seriouseats.com/2011/10/the-food-labs-apple-pie-part-2-how-to-make-perfect-apple-pie-filling.html

Kenji is my favorite active practical food scientist, and a former recipe developer for Cooks Illustrated.