Definition - What does Crushing mean?
Crushing is the step in the winemaking process that introduces the pulp of the berry to the skins through, as the name implies, gentle "crushing." The pressure applied allows the juices free access to the yeast, tannins and characteristics of the skin. Crushing is combined with the de-stemming process. This is the point where the stems are removed from the berries, or left attached, depending on the preference of the winemaker. Crussing and de-stemming is the catalyst for the process whereby wine attains it color and characteristics.
There are two ways to complete this stage in the winemaking process: manual crushing and de-stemming; or mechanical crushing and de-stemming. Smaller estates and those that wish to hold true to traditional winemaking techniques usually opt for the manual crushing - the berries are de-stemmed by hand, placed in a barrel and stomped with bare feet. Large estates and those wishing to use modern technology use crushing/de-stemming machines to complete this step.
WineFrog explains Crushing
The crushing process must be gentle to prevent crushing the seeds. Seeds add unnecessary tannins and astringency to the wine; for the same reason are stems usually removed - the stems and seeds are high in tannin and create a vegetal "green plant" taste in a wine.
Because the grape's juice is naturally colorless, white, rosé and red wines are treated differently at this stage. White wines usually aren’t crushed because the wine does not need to be exposed to the skins - winemakers don't want the color or tannins to effect the wine. White wines are picked, sorted and thrown into the pressing machine. The wine press separates the juices from the skins, seeds and stems. This process is how it is possible to get a white wine from a red wine grape. The only time a white wine is crushed is to create a deeper color and heavier wine, perhaps meant for cellaring. In which case, the stems are left on the grapes to facilitate the juice extraction, and the mixture is pressed immediately after crushing.
Rosés are crushed and left to absorb enough of the color and skin characteristics as will allow the final wine to take on the pretty, pink color and fruity flavours. If the winemaker chooses to leave the stems, it is with the intention of allowing the juices to flow freely and/or add a bit more tannic structure.
Reds are crushed, de-stemmed and left to stew in their juices before pressing. This allows the juice to absorb characteristics from the skin. When the winemaker is satisfied that the must has reached the desired color, flavor and tannic structure, the must is sent into the wine press.