Some wines deposit their suspended material (yeast cells,
particles of skins, etc.) very quickly, and the supernatant wine
remains nearly brilliant. This is particularly true when
50-gallon wooden barrels, which have greater surface-to-volume
ratio than larger containers, are employed. The rough interior of
wooden cooperage facilitates deposition of suspended material.
Other wines, particularly in warm regions or when large tanks are
used, may remain somewhat cloudy for long periods. Removal of the
suspended material during aging is called clarification. The
major procedures involved are fining, filtration, centrifugation,
refrigeration, ion exchange, and heating.
Fining is an ancient practice in which a material that aids
clarification is added to the wine. The main processes involved
are adsorption, chemical reaction and adsorption, and possibly
physical movement. Proteins and yeast cells are adsorbed on
fining agents such as bentonite (a type of clay formed mainly of
montmorillonite) or gelatin. Chemical reactions occurring with
tannins and gelatin may be followed by adsorption of suspended
compounds. If an inert material, such as silica, is added to a
cloudy wine, some clarification will occur simply by the movement
of the particles of inert silica through the wine. This action
probably occurs to a certain extent with the addition of any
Bentonite has largely replaced all other fining agents. Such
fining agents as gelatin, casein, isinglass, albumin, egg white,
nylon, and PVPP (polyvinyl pyrrolidone) may be used for special
purposes, including removal of excess tannin or color.
Excessive amounts of metals, particularly iron and copper, may
be present in the wine, usually from contact with iron or metal
surfaces. These result in persistent cloudiness and require
removal by such special fining materials as potassium
ferrocyanide (blue fining), long recommended in Germany. Cufex, a
proprietary product containing potassium ferrocyanide, may be
used in the United States under strict control. Phytates have
been used for removing iron. In modern winery operations
excessive metal content is rare, mainly owing to the use of
stainless steel equipment.
Filtration is another ancient practice, and early filters
consisted of rough cloth-covered screens through which the wine
was poured. Modern filter pads are made of cellulose fiber of
various porosities or consist of membrane filters, also in a
range of porosity. The pore size of some filters is sufficiently
small to remove yeast cells and most bacterial cells, but filters
operate not only because of pore size but also by a certain
amount of adsorption.
Centrifugation, or high-speed spinning, used to clarify musts,
is also applied to wines that are difficult to clarify by other
means. This operation requires careful control to avoid undue
oxidation and loss of alcohol during the process.
Refrigeration aids wine clarification in several ways.
Temperature reduction often prevents both yeast growth and the
evolution of carbon dioxide, which tends to keep the yeast cells
suspended. Carbon dioxide is more soluble at lower temperatures.
A major cause of cloudiness is the slow precipitation of
potassium acid tartrate (cream of tartar)
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