If the zoo is only selling the food for the purpose of giving it to the animals, is that a schirus or michirah or michirah al minas lihachzir or just buying the right to feed the animals? Especially the machines that give food out for free. Except that shielah of chametz is also included in bal yeraeh but schirus is a machlokes R’M and R’Y if it is like a shomer chinam or shomer sachar which is related to a machlokes rishonim what type of shomer is chayav in bal yiraeh.(Rashi,Tos,Rambam)
But lichoreh they would also sell it to someone who wanted to take it home so maybe irrelevant shaylah.