Clarification and stabilization of wine

In wine tasting, a wine is considered "clear" when there are no visible particles suspended in the liquid and, especially in the case of white wines, when there is some degree of transparency. A wine with too much suspended matter will appear cloudy and dull, even if its aroma and flavor are unaffected; wines therefore generally undergo some kind of clarification. After fermentation, the force of gravity may eventually cause the wine to "fall bright" or clarify naturally, as the larger suspended particles gradually settle to the bottom of the storage vessel. The wine can then be siphoned or "racked" off the compact solids into a new container. But this process may take many months, or even years, as well as several rackings, in order to produce a perfectly clear wine. Producers can accelerate the process by using fining agents, filtration and/or flotation. Many substances have historically been used as fining agents, including dried blood powder. There are two general types of fining agents — organic compounds and solid/mineral materials. The most common organic compounds used include egg whites, casein derived from milk, gelatin and isinglass obtained from the bladders of fish. Pulverized minerals and solid materials can also be used, with bentonite clay being one of the most common, thanks to its effectiveness in absorbing proteins and some bacteria. Activated carbon from charcoal is used to remove some phenols that contribute to browning as well as some particles that produce "off-odors" in the wine. Silica and kaolin are also sometimes used. Some producers of premium wine avoid fining, or delay it in order to leach more flavor and aroma from the phenols before they are removed. Filtration While fining clarifies wine by binding to suspended particles and precipitating out as larger particles, filtration works by passing the wine through a filter medium that captures particles larger than the medium's holes. Complete filtration may require a series of filtering through progressively finer filters. Many white wines require the removal of all potentially active yeast and/or lactic acid bacteria if they are to remain reliably stable in bottle, and this is usually achieved by fine filtration. Most filtration in a winery can be classified as either the coarser depth filtration or the finer surface filtration. In depth filtration, often done after fermentation, the wine is pushed through a thick layer of pads made from cellulose fibers, diatomaceous earth, or perlite. In surface filtration, the wine passes through a thin membrane. Running the wine parallel to the filter surface, known as cross-flow filtration, will minimize the filter clogging. The finest surface filtration, microfiltration, can sterilize the wine by trapping all yeast and, optionally, bacteria, and so is often done immediately prior to bottling. An absolute rated filter of 0.45 μm is generally considered to result in a microbially stable wine and is accomplished by the use of membrane cartridges, most commonly polyvinylidene fluoride (PVDF). Certain red wines may be filtered to 0.65 μm, to remove yeast, or to 1.0 μm to remove viable brettanomyces only. Flotation The winemaking technique of flotation was adapted from the froth flotation process used in the mining industry for ore refining. In this process, small bubbles of air (or compressed nitrogen) are injected into the bottom of a tank. As the bubbles rise through the must, grape solids, including phenolic compounds prone to oxidation and browning, will tend to cling to the bubbles, creating a froth that can be removed from the wine. This must be done prior to fermentation, since yeast will inhibit the flocculation involved. ==Stabilization==

As a complex chemical mixture dependent on the activity of microorganisms, wine can be unstable and reactive to changes in its environment. Once bottled, a wine may be exposed to extremes of temperature and humidity, as well as violent movement during transportation and storage. These may cause cloudiness, sedimentation and/or the formation of tartrate crystals; more seriously, they may also cause spoilage or the production of carbonic gas. Temperature instability Tartaric acid is the most prominent acid in wine with the majority of the concentration present as potassium bitartrate. During fermentation, these tartrates bind with the lees, pulp debris and precipitated tannins and pigments. While there is some variation according to grape variety and climate, usually about half of the deposits are soluble in the wine, but on exposure to low temperature they may crystallize out unpredictably. The crystals, though harmless, may be mistaken for broken glass, or simply reckoned unattractive by consumers. To prevent this the wine may undergo "cold stabilization", in which it is cooled to near its freezing point to provoke crystallization before bottling. In some white wines there are significant quantities of proteins that, being "heat-unstable", will coagulate if exposed to excessively fluctuating heat; the use of fining agents such as bentonite can prevent the haze this causes. Other methods of stabilization Clarification tends to stabilize wine, since it removes some of the same particles that promote instability. The gradual oxidation that occurs during barrel aging also has a naturally stabilizing effect. ==Premium wine production==

Some producers prefer not to thoroughly clarify and stabilize their wines, believing that the processes involved may diminish a wine's aroma, flavor, texture, color or aging potential. Wine experts such as Tom Stevenson note that they may improve wine quality when used with moderation and care, or diminish it when used to excess. Winemakers deliberately leave more tartrates and phenolics in wines designed for long aging in bottle so that they are able to develop the aromatic compounds that constitute bouquet. The consumers of some wines, such as red Bordeaux and Port, may expect to see tartrates and sediment after aging in bottle. ==References==