Clarification and stabilization of wine

In winemaking, clarification and stabilization are the processes by which insoluble matter suspended in the wine is removed before bottling. This matter may include dead yeast cells (lees), bacteria, tartrates, proteins, pectins, various tannins and other phenolic compounds, as well as pieces of grape skin, pulp, stems and gums. Clarification and stabilization may involve fining, filtration, centrifugation, flotation, refrigeration, pasteurization, and/or barrel maturation and racking.

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.

Before fermentation, pectin-splitting enzymes and, for white wine, fining agents such as bentonite may be added to the must in order to promote the eventual agglomeration and settling of colloids. 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.

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.

In winemaking, fining is the process by which a substance (fining agent) is added to the wine to create an adsorbent, enzymatic or ionic bond with the suspended particles, producing larger molecules and larger particles that will precipitate out of the wine more readily and rapidly. Unlike filtration, which can only remove particulates (such as dead yeast cells and grape fragments), fining can remove soluble substances such as polymerized tannins, coloring phenols and proteins; 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. 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.

Organic compounds used as fining agents are generally animal based, a possible cause of concern to vegans. 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. In a process known as blue fining, potassium ferrocyanide is sometimes used to remove any copper and iron particles that have entered the wine from bentonite, metal winery and vineyard equipment, or vineyard sprays such as Bordeaux mixture. Because potassium ferrocyanide may form hydrogen cyanide its use is highly regulated and, in many wine producing countries,^[which?] illegal. Silica and kaolin are also sometimes used.

Some countries, such as Australia and New Zealand, have wine labeling laws that require the use of fining agents that may be an allergenic substance to appear on the wine label. A study conducted by the University 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.

There is the risk of valuable aromatic molecules being precipitated out along with the less desirable matter. 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.^{[citation needed]}