Two things you can do:
Globalise drink0_ingred
:
with open(file="drink0_ingred.txt") as ingred0:
global drink0_ingred
drink0_ingred = ingred0.read()
Call the function instead of calling the variable:
self.l0 = ttk.Label(self, text=callbackkb0()) # Notice how I called the function
# because it returned drink0_ingred