Fermentation is the part of rum-making where you have the most opportunity to create flavor, in the form of esters and other congeners. In the previous posts I explained how this works, but there are a few ways to supercharge this process.
Today we will shortly cover 4 of those methods. For more in-depth meditations on this subject, I refer to the @cocktailwonk blog, where you can find the most comprehensive breakdown of these techniques to date.
One way to boost ester-levels in the rum wash is to seriously prolong the fermentation and continuously feed it more molasses for weeks, even months. The other ways involve adding some highly acidic ingredients to the wash. Firstly sugar cane vinegar is rich in acetic acid and made by fermenting fresh cane juice. Similar to Sour Mash in Whiskey- or Vinasse in Brandy-making, Dunder is the acidic sludge left in the still after the first distillation.
The third, and perhaps most mysterious, ingredient is called Muck. The make-up of Muck is an intuitive recipe mastered by very few people, including but not limited to Dunder, spent lees from the retorts, ferment-bottoms packed with dead yeast, cane trash, and even rotting fruits. The idea is to add large amounts of e.g. acetic, carboxylic & butyric acids to the wash, in order to maximize the amount of esters created during fermentation. The resulting levels of hi-octane flavor is what many refer to as Funk or Hogo.