ビール can be placed inside a Cask to age from normal quality to silver, gold, and eventually iridium quality. Iridium quality doubles the base sell price of ビール.
Normal, silver, and gold quality ビール can be prematurely removed from a cask at any time by striking the cask with an Axe, Hoe, or Pickaxe. As of PC v1.11, striking a cask that is ready to harvest (with iridium quality ビール) with an Axe or Pickaxe will crash the game.