Clarification

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

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 fining agent.

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

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

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

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)