Crushing is the process of removing the grapes from the rachis
and breaking the skins of the grapes. In smaller-scale wine
making, the harvested grapes are sometimes crushed by trampling
them bare-footed. However, in larger wineries, a mechanical
crusher/destemmer is used. Because the stems of the grapes have
a relatively high tannin content, they are usually removed
beforehand; otherwise they can give a vegetal aroma to the wine
(due to extraction of 2-methoxy-3-isopropylpyrazine which has an
aroma reminiscent of green bell peppers.) However the winemaker
can decide to leave them in if the grapes themselves contain
less tannin than desired. If increased skin extraction is
desired, a winemaker might choose to crush the grapes after
destemming. In these cases the grapes pass between two rollers
which squeeze the grapes enough to separate the skin and pulp,
but not so much as to cause excessive shearing or tearing of the
skin tissues. In some cases, notably with "delicate" red
varietals such as Pinot noir or Syrah, all or part of the grapes
might be left uncrushed (called "whole berry") to encourage the
retention of fruity aromas through partial carbonic maceration.
