The filled vats are left for several days while fermentation of plums starts. During this time the level of fruit in the vat is lowered from squashing the plums (the mash thickens) and a top cover is formed. During fermentation foam is formed and it comes to the surface around the internal periphery of the vat, or if the vat is filled to the brim, the foam comes out.

As during the plum mash fermentation the formed alcohol evaporates, many people put wooden lid on the barrel hole as they seal the edge to stop the emission of alcohol.

When the crust of the mash falls down, then it is overrisen for distilling. The fermentation of the plum mash usually lasts 10-14 days. To speed up the process, some farmers put 1-2 copper cauldrons of hot water into the mash.