Cut, peel, and soak 5-6 Granny Smith apples inwater and 1 Tbsp lemon juice.
Mix 1 cup sugar, 2 Tbsp flour, 1/2 tsp cinnamon, 1/4 salt, and 1 Tbsp cider vinegar.
Drain and mix apples and sugar mixture.
Dump into unbaked pie crust in deep dish pie pan, Dot with 4 pats of butter.
Cover with unbaked pie crust. Pinch edges and cover edges with foil. Cut 4 slits in pie crust.
Bake 50-60 minutes at 400 degrees F. Remove foil. Brush with egg whites. Bake 10 more minutes.