When the lid for a vat of wine is slightly open, the yeast inside can continue to perform alcoholic fermentation because the carbon dioxide produced by the yeast during fermentation will act as a blanket over the wine. Nevertheless, the air around the fermenter must be still and enough carbon dioxide should be produced to continue alcoholic fermentation. However, at some point, carbon dioxide will fall and can no longer protect the wine. This is the right time to get your wine covered before it will be vulnerable to undesirable microorganisms.
Therefore, open fermentation is good in the early stages of fermentation because yeast need oxygen to synthesize sterols and assimilate fatty acids to ensure the nutrients it needs to multiply and ferment the 70% of the sugar from the fruit. Furthermore, the yeast is responsible in decomposing sugar from the grapes into alcohol and carbon dioxide.