Mature ales are stronger versions of regular ales. These drinks provide even bigger skill boosts and drawbacks than the regular versions. They can only be obtained by brewing, where there is a chance that a normal batch of ale will become mature. This happens during the brewing process, not after. Leaving a normal ale in the vat after it has fully set will not make it turn mature, nor will it go bad. The chance of brewing a mature ale can be increased to 64% by adding a package of "the stuff" to the brewing vat. Without "the stuff", the chance of maturity is only 5%. Chance of maturity may also be affected by a player's Cooking level, but not by all that much; higher Cooking level mostly results in a lower chance of the ale going bad.