Alistair Cooper MW replies: During the early stages of fermentation, open-top fermenters allow for increased oxygen contact which can help the yeast build a strong population.This can help prevent what is known as ‘stuck ferments’ – when the yeasts become dormant before fermentation has finished.Open-top fermenters also allow easy access to the cap (the grape skins that rise to the top) and allows them to be punched down into the fermenting must easily.The heat generated during fermentation can easily escape the vessel, and an open-top can help better manage the fermentation temperatures.