Wine Science: Principles and Applications, Third Edition - Vinum Vine

426 8. Postfermentation Treatments and Related Topics
misinterpreted by consumers. As a consequence, considerable
effort is expended in avoiding the formation of
crystalline deposits in bottled wine. Stabilization is normally
achieved by enhancing crystallization, followed by
removal. Less frequently, it may be achieved by delaying
or inhibiting crystallization.
Potassium Bitartrate Instability Juice is typically
supersaturated with potassium bitartrate at crushing.
As the alcohol content rises during fermentation, the
solubility of bitartrate decreases. This induces the slow
precipitation of potassium bitartrate (cream of tartar).
Given suffi cient time, the salt crystals precipitate spontaneously.
In northern regions, low cellar temperatures
may induce adequately rapid precipitation. Spontaneous
precipitation is seldom satisfactory in warmer areas.
Early bottling aggravates the problem. Where spontaneous
precipitation is inadequate, refrigeration often
achieves rapid and satisfactory bitartrate stability.
Because the rate of bitartrate crystallization is directly
dependent on the degree of supersaturation, wines that
are only mildly unstable may be insuffi ciently stabilized
by cold treatment. In addition, protective colloids may
retard crystallization (Lubbers et al., 1993). Typically,
protective colloids have been viewed negatively, as their
precipitation after bottling releases tartrates that could
subsequently crystallize. Although this may be true for
most protective colloids, some mannoproteins appear
suffi ciently stable to donate tartrate stability in bottled
wine (Dubourdieu and Moine, 1998b). Mannoproteins
are released during the latter stages of fermentation,
and especially during maturation, as yeast cells in the
lees autolyze. The addition of yeast-cell-wall enzymic
digest (yeast hulls) can promote tartrate stability, without
cold or other stabilization treatments.
In the absence of protective colloids, potassium
bitartrate exists in a dynamic equilibrium between ionized
and salt states.
crystallization
Under supersaturated conditions, crystals form and
eventually reach a critical mass that provokes precipitation.
Crystallization continues until an equilibrium
develops. If suffi cient crystallization and removal occur
before bottling, bitartrate stability is achieved. Because
chilling decreases solubility, it speeds crystallization.
In red wines, crystal formation is often associated
with yeast cells (about 20% by weight) (Vernhet et al.,
1999b). This compares with about 2% in white wines
(Vernhet et al., 1999a). Potassium hydrogen tartrate
crystals are also associated with smaller amounts of
phenolic compounds (notably anthocyanins and tannins
in red wines), as well as with polysaccharides, such
as rhamnogalacturonans and mannoproteins.
Theoretically, chilling should establish bitartrate stability.
However, charged particles in the wine can interfere
with crystal initiation and growth. For example,
positively charged bitartrate crystals are attracted to negatively
charged colloids, blocking growth. The charge on
the crystals is created by the tendency of more potassium
than bitartrate ions to associate with the crystals early
in growth (Rodriguez-Clemente and Correa-Gorospe,
1988). Crystal growth also may be delayed by the binding
of bitartrate ions to positively charged proteins. This
reduces the amount of free bitartrate and, thereby, the
rate of crystallization. Because both bitartrate and potassium
ions may bind with tannins, crystallization tends to
be delayed more in red than in white wines. The binding
of potassium with sulfi tes is another source of delayed
bitartrate stabilization.
For cold stabilization, table wines are routinely chilled
to near the wine’s freezing point. Five days is usually
suffi cient at 5.5 ºC, but 2 weeks may be necessary
at 3.9 ºC. Fortifi ed wines are customarily chilled to
between 7.2 and 9.4 ºC, depending on the alcoholic
strength. The stabilization temperature can be estimated
using the formula empirically established by Perin (1977).
Direct seeding with potassium bitartrate crystals is
occasionally employed to stimulate crystal growth and
deposition. Another technique involves fi lters incorporating
seed crystals. The chilled wine is agitated and
then passed through the fi lter. Crystal growth is encouraged
by the dense concentration of “seed” nuclei in the
fi lter. The fi lter acts as a support medium for the crystal
nuclei.
At the end of conventional chilling, the wine is fi ltered
or centrifuged to remove the crystals. Crystal
removal is performed before the wine warms to ambient
temperatures.
Because of the expense of refrigeration, various procedures
have been developed to determine the need for
cold stabilization. None of the techniques appears to be
suffi ciently adequate. Potassium conductivity, although
valuable, is too complex for regular use in most wineries.