Inwinemaking,clarification and stabilization are the processes by whichinsoluble mattersuspended in thewine is removed before bottling. This matter may include deadyeast cells (lees),bacteria,tartrates,proteins,pectins, varioustannins and otherphenolic compounds, as well as pieces of grape skin,pulp, stems andgums.[1] Clarification and stabilization may involvefining,filtration,centrifugation,flotation,refrigeration,pasteurization, and/orbarrel maturation andracking.
Inwine 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 oftransparency. 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.[1]
Beforefermentation,pectin-splittingenzymes and, for white wine, fining agents such asbentonite may be added to themust in order to promote the eventualagglomeration andsettling ofcolloids.[1] Pectins are structural molecules in the cell walls of fruits which have the important function of 'gumming' plant cells together. The pectin content of grapes increases steadily throughout ripening, reaching levels of about 1 g/L, although it varies by varietal and pre-fermentation handling processes. Large pectin molecules can affect the amount of juice yielded at pressing, ease of filtration and clarification, and extraction of tannins. Grapes contain natural pectolytic enzymes responsible for softening the grape berries during ripening, but these are not active under wine-making conditions (due to pH level, SO2, and alcohol.) Therefore, fungal pectolytic enzymes are often added to white must to break up pectins, decrease the viscosity of the juice, and speed up settling. In red musts, this increases color and tannin extraction.[2]
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 besiphoned or "racked" off the compact solids into a new container.[3] 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.[1]
In winemaking, fining is the process by which a substance (fining agent) is added to the wine to create anadsorbent,enzymatic orionic bond with the suspended particles, producing larger molecules and larger particles that willprecipitate out of the wine more readily and rapidly. Unlike filtration, which can only removeparticulates (such as dead yeast cells and grape fragments), fining can remove soluble substances such aspolymerized tannins, coloringphenols andproteins; some of these proteins can cause haziness in wines exposed to high temperatures after bottling. The reduction of tannin can reduce astringency in red wines.[4] Many substances have historically been used as fining agents, including driedblood powder.[5] There are two general types of fining agents — organic compounds and solid/mineral materials.[4]
Organic compounds used as fining agents are generallyanimal based, a possible cause of concern tovegans.[6] The most common organic compounds used includeegg whites,casein derived frommilk,gelatin andisinglass obtained from thebladders 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.[4] In a process known as blue fining,potassium ferrocyanide is sometimes used to remove anycopper andiron particles that have entered the wine from bentonite, metal winery and vineyard equipment, or vineyard sprays such asBordeaux mixture. Because potassium ferrocyanide may formhydrogen cyanide its use is highly regulated and, in manywine producing countries,[which?] illegal.[7]Silica andkaolin are also sometimes used.[4]
Some countries, such as Australia and New Zealand, havewine labeling laws that require the use of fining agents that may be anallergenic substance to appear on the wine label. A study conducted by theUniversity of California, Davis Department of Viticulture and Enology, however, found that no detectable amount of inorganic fining agents, and only trace quantities of proteinaceous agents, are left in the wine.[4]
There is the risk of valuable aromatic molecules being precipitated out along with the less desirable matter.[8] Some producers of premium wine avoid fining, or delay it in order to leach more flavor andaroma from the phenols before they are removed.[citation needed]
While fining clarifies wine bybinding 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/orlactic 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 coarserdepth filtration or the finersurface filtration.[4] In depth filtration, often done after fermentation, the wine is pushed through a thick layer of pads made fromcellulose fibers,diatomaceous earth, orperlite. In surface filtration, the wine passes through a thin membrane. Running the wine parallel to the filter surface, known ascross-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 commonlypolyvinylidene 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.
The winemaking technique of flotation was adapted from thefroth 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 theflocculation involved.[1]
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.
Tartaric acid[9] is the most prominentacid in wine with the majority of the concentration present aspotassium bitartrate. During fermentation, thesetartrates 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.[10] In some white wines there are significant quantities of proteins that, being "heat-unstable", willcoagulate if exposed to excessively fluctuating heat; the use of fining agents such as bentonite can prevent the haze this causes.[1]
A wine that has not been sterilized by filtration might well still contain live yeast cells and bacteria. If bothalcoholic andmalolactic fermentation have run to completion, and neither excessive oxygen norBrettanomyces yeast are present, this ought to cause no problems; modern hygiene has largely eliminated spoilage by bacteria such asacetobacter, which turns wine intovinegar. If there is residual sugar, however, it may undergosecondary fermentation, creating dissolved carbon dioxide as a by-product. When the wine is opened, it will be spritzy or "sparkling". In a wine intended to be still this is regarded as a serious fault; it can even cause the bottle to explode. Similarly, a wine that has not been put through completemalolactic fermentation may undergo it in bottle, reducing its acidity, generating carbon dioxide, and adding adiacetyl butterscotch aroma.Brettanomyces yeasts add4-ethylphenol,4-ethylguaiacol andisovaleric acid horse-sweat aromas. These phenomena may be prevented by sterile filtration, by the addition of relatively large quantities of sulfur dioxide and sometimessorbic acid,[1] by mixing inalcoholic spirit to give afortified wine of sufficient strength to kill all yeast and bacteria, or bypasteurization.
Pasteurization gives akosher wine of the type calledmevushal, literally "cooked" or "boiled", that can be handled by non-Jews and non-observant Jews without losing itskosher status.[citation needed] Typically, the wine is heated to 185 °F (85 °C) for a minute, then cooled to 122 °F (50 °C), at which temperature it remains for up to three days, killing all yeast and bacteria. It may then be allowed to cool, or be bottled "hot" and cooled by water sprays. Since pasteurization affects a wine's flavor and aging potential it is not used for premium wines. A gentler procedure known asflash pasteurization involves heating to 205 °F (96 °C) for a few seconds, followed by rapid cooling.[11]
Clarification tends to stabilize wine, since it removes some of the same particles that promote instability. The gradual oxidation that occurs during barrelaging also has a naturally stabilizing effect.[1]
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 asTom Stevenson note that they may improve wine quality when used with moderation and care, or diminish it when used to excess.[3] 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.[2] The consumers of some wines, such as redBordeaux andPort, may expect to see tartrates and sediment after aging in bottle.[1]