The liquid left in a still after a run of rum is completed, also know as stillage. The dunder is used in subsequent mashes to aid in fermentation and add flavor to the distillate. Dunder is put into a open container (the famed Dunder Pit) to encourage bacterial infection. These bacteria create Carboxylic acid that is then converted into an Ester. Dunder pits must be maintain by the removal and addition of fresh dunder or some materials in the dunder pit will decompose into ammonia.
Typically for Rum 10-25% of the wash liquid is Dunder. Some papers show usage up to 50%. Per Arroyo, "Infected" Dunder should be around 4% of total volume, and included after most primary fermentation has been completed.
For "Infected" Dunder one must understand the safety of its use: Dunder Pit Safety
- Dunder pit infections
- Does a dunder pit create esters or a feedstock for them?
- When to add aged dunder?