To make wine, the juice of grapes is fermented. The process of fermentation involves yeast converting sugars in the juice into alcohol and carbon dioxide gas.
The process begins with berries being crushed to release their juices, which are called must. If you think about what happens when you crush a grape, this makes sense—the skin and flesh of a berry contain different concentrations of sugar than its juice does. As it turns out, there are two main types of fermentation: alcoholic and malolactic (or ML).