Cap

[Vlabruz]

Does the cap serve a purpose during fermentation or is it something undesirable that occurs?

 

[FlamingoEmporium]

I think it’s more a result than a purpose. Fermention gasses keep pushing the must upward. It’s necessary to reintroduce the top part to the active fermentation by stirring or punching it down

 

[Jim Welch]

I think the only thing that could be undesirable is if it is allowed to dry out. Then molds can grow on it, or so I've read.

 

[heatherd]

It does have a purpose, here's an article that tells more about the cap and management of it: Red Wine Fermentation: Cap Management