What is a 'must' in the context of winemaking?

In winemaking, 'must' refers to the crushed grape mixture that contains not only the juice but also the grape skins, seeds, and sometimes stems. This mixture is a crucial step in the winemaking process as it is during fermentation that the sugars in the grape juice convert into alcohol. The presence of skins and seeds in the must is important because they contribute to the flavor, color, and tannin structure of the wine. The fermentation process occurs in the must, allowing the extraction of flavors and other compounds that shape the final product.

The other options represent different components or stages in the winemaking process: fermented grape juice refers to the product after fermentation has occurred, the final wine product is the completed wine after aging, and the aging vessel is the container where the wine is matured after fermentation. Each of these aspects plays an important role in winemaking but specifically, 'must' denotes the initial combination of grape material before fermentation begins.