Carrying a ferment forward
Mosto is not an ingredient. It is a microbial lineage the estate keeps alive from one batch to the next.
The first time a tank is opened after a mosto ferment, the smell is closer to a winery than a coffee mill. That is not a coincidence — the technique comes directly from winemaking, where the leachate from one pressing seeds the next.
What makes it worth the trouble is continuity. A successful ferment produces a liquid dense with the specific yeasts and bacteria that made it successful. Drawing that liquid off and using it to inoculate the following batch means the estate is not starting from zero each time. It is carrying a population forward.
The risk
Concentrated microbial activity moves fast. The same density that produces depth in twenty-four hours produces vinegar in thirty-six. There is no recovering a tank that has gone over.
Which is why mosto lots are always small, and why they are always the last thing cupped before a decision is made about the season.