Bottle Cap is a Legends of Idleon item: dropped by 1 monster, upgrades 1 bonus. With the main prey of the Soda Can being the Soda Bottle, one can conclude that bottle caps are indigestible, and will remain untarnished after being consumed.

Bottle Cap | Idleon Toolbox

With the main prey of the Soda Can being the Soda Bottle, one can conclude that bottle caps are indigestible, and will remain untarnished after being consumed.

Bottle_Cap

Have_a_stack_of_exactly_800,000_Bottle_Caps_in_your_Storage_Chest.