Crushing is the process when gently squeezing the berries and breaking the skins to start to liberate the contents of the berries. De-stemming is the process of removing the grapes from the rachis (the stem which holds the grapes). In traditional and smaller-scale wine making, the harvested grapes are sometimes crushed by trampling them barefoot or by the use of inexpensive small scale crushers. These can also de-stem at the same time. However, in larger wineries, a mechanical crusher/de-stemmer is used.
Yeast is normally already present on the grapes, often visible as a powdery appearance of the grapes. The primary, or alcoholic, fermentation can be done with this natural yeast, but since this can give unpredictable results depending on the exact types of yeast that are present, cultured yeast is often added to the must. During the primary fermentation, the yeast cells feed on the sugars in the must and multiply, producing carbon dioxide gas and alcohol.