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. [1] 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. [1]
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. [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 be siphoned 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 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. [4] Many substances have historically been used as fining agents, including dried blood powder. [5] There are two general types of fining agents — organic compounds and solid/mineral materials. [4]
Organic compounds used as fining agents are generally animal based, a possible cause of concern to vegans. [6] 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. [4] 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. [7] Silica and kaolin are also sometimes used. [4]
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. [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 and aroma from the phenols before they are removed.[ citation needed ]
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. [4] 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.
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. [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 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. [10] 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. [1]
A wine that has not been sterilized by filtration might well still contain live yeast cells and bacteria. If both alcoholic and malolactic fermentation have run to completion, and neither excessive oxygen nor Brettanomyces yeast are present, this ought to cause no problems; modern hygiene has largely eliminated spoilage by bacteria such as acetobacter, which turns wine into vinegar. If there is residual sugar, however, it may undergo secondary 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 complete malolactic fermentation may undergo it in bottle, reducing its acidity, generating carbon dioxide, and adding a diacetyl butterscotch aroma. Brettanomyces yeasts add 4-ethylphenol, 4-ethylguaiacol and isovaleric acid horse-sweat aromas. These phenomena may be prevented by sterile filtration, by the addition of relatively large quantities of sulfur dioxide and sometimes sorbic acid, [1] by mixing in alcoholic spirit to give a fortified wine of sufficient strength to kill all yeast and bacteria, or by pasteurization.
Pasteurization gives a kosher wine of the type called mevushal, literally "cooked" or "boiled", that can be handled by non-Jews and non-observant Jews without losing its kosher 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 as flash 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 barrel aging 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 as Tom 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 red Bordeaux and Port, may expect to see tartrates and sediment after aging in bottle. [1]
Winemaking or vinification is the production of wine, starting with the selection of the fruit, its fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over millennia. There is evidence that suggests that the earliest wine production took place in Georgia and Iran around 6000 to 5000 B.C. The science of wine and winemaking is known as oenology. A winemaker may also be called a vintner. The growing of grapes is viticulture and there are many varieties of grapes.
Oak is used in winemaking to vary the color, flavor, tannin profile and texture of wine. It can be introduced in the form of a barrel during the fermentation or aging periods, or as free-floating chips or staves added to wine fermented in a vessel like stainless steel. Oak barrels can impart other qualities to wine through evaporation and low level exposure to oxygen.
Carbonic maceration is a winemaking technique, often associated with the French wine region of Beaujolais, in which whole grapes are fermented in a carbon dioxide rich environment before crushing. Conventional alcoholic fermentation involves crushing the grapes to free the juice and pulp from the skin with yeast serving to convert sugar into ethanol. Carbonic maceration ferments most of the juice while it is still inside the grape, although grapes at the bottom of the vessel are crushed by gravity and undergo conventional fermentation. The resulting wine is fruity with very low tannins. It is ready to drink quickly but lacks the structure for long-term aging. In extreme cases such as Beaujolais nouveau, the period between picking and bottling can be less than six weeks.
Red wine is a type of wine made from dark-colored grape varieties. The color of the wine can range from intense violet, typical of young wines, through to brick red for mature wines and brown for older red wines. The juice from most purple grapes is greenish-white, the red color coming from anthocyan pigments present in the skin of the grape. Much of the red wine production process involves extraction of color and flavor components from the grape skin.
White wine is a wine that is fermented without skin contact. The colour can be straw-yellow, yellow-green, or yellow-gold. It is produced by the alcoholic fermentation of the non-coloured pulp of grapes, which may have a skin of any colour. White wine has existed for at least 4,000 years.
Malolactic conversion is a process in winemaking in which tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation is most often performed as a secondary fermentation shortly after the end of the primary fermentation, but can sometimes run concurrently with it. The process is standard for most red wine production and common for some white grape varieties such as Chardonnay, where it can impart a "buttery" flavor from diacetyl, a byproduct of the reaction.
Maceration is the winemaking process where the phenolic materials of the grape—tannins, coloring agents (anthocyanins) and flavor compounds—are leached from the grape skins, seeds and stems into the must. To macerate is to soften by soaking, and maceration is the process by which the red wine receives its red color, since raw grape juice is clear-grayish in color. In the production of white wines, maceration is either avoided or allowed only in very limited manner in the form of a short amount of skin contact with the juice prior to pressing. This is more common in the production of varietals with less natural flavor and body structure like Sauvignon blanc and Sémillon. For Rosé, red wine grapes are allowed some maceration between the skins and must, but not to the extent of red wine production.
Finings are substances that are usually added at or near the completion of the processing of making wine, beer, and various nonalcoholic juice beverages. They are used to remove organic compounds, either to improve clarity or adjust flavor or aroma. The removed compounds may be sulfides, proteins, polyphenols, benzenoids, or copper ions. Unless they form a stable sediment in the final container, the spent finings are usually discarded from the beverage along with the target compounds that they capture.
A tartrate is a salt or ester of the organic compound tartaric acid, a dicarboxylic acid. The formula of the tartrate dianion is O−OC-CH(OH)-CH(OH)-COO− or C4H4O62−.
A wine fault is a sensory-associated (organoleptic) characteristic of a wine that is unpleasant, and may include elements of taste, smell, or appearance, elements that may arise from a "chemical or a microbial origin", where particular sensory experiences might arise from more than one wine fault. Wine faults may result from poor winemaking practices or storage conditions that lead to wine spoilage.
Micro-oxygenation is a process used in winemaking to introduce oxygen into wine in a controlled manner. Developed in 1991 by Patrick DuCournau, working with the exceptionally tannic grape Tannat in Madiran, the process gained usage in modern winemaking following the 1996 authorization by the European Commission. Today, the technique is widely employed in Bordeaux, as well as at least 11 different countries, including the United States and Chile.
Sparkling wine production is the method of winemaking used to produce sparkling wine. The oldest known production of sparkling wine took place in 1531 with the ancestral method.
Sugars in wine are at the heart of what makes winemaking possible. During the process of fermentation, sugars from wine grapes are broken down and converted by yeast into alcohol (ethanol) and carbon dioxide. Grapes accumulate sugars as they grow on the grapevine through the translocation of sucrose molecules that are produced by photosynthesis from the leaves. During ripening the sucrose molecules are hydrolyzed (separated) by the enzyme invertase into glucose and fructose. By the time of harvest, between 15 and 25% of the grape will be composed of simple sugars. Both glucose and fructose are six-carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape. Not all sugars are fermentable, with sugars like the five-carbon arabinose, rhamnose and xylose still being present in the wine after fermentation. Very high sugar content will effectively kill the yeast once a certain (high) alcohol content is reached. For these reasons, no wine is ever fermented completely "dry". Sugar's role in dictating the final alcohol content of the wine sometimes encourages winemakers to add sugar during winemaking in a process known as chaptalization solely in order to boost the alcohol content – chaptalization does not increase the sweetness of a wine.
The acids in wine are an important component in both winemaking and the finished product of wine. They are present in both grapes and wine, having direct influences on the color, balance and taste of the wine as well as the growth and vitality of yeast during fermentation and protecting the wine from bacteria. The measure of the amount of acidity in wine is known as the “titratable acidity” or “total acidity”, which refers to the test that yields the total of all acids present, while strength of acidity is measured according to pH, with most wines having a pH between 2.9 and 3.9. Generally, the lower the pH, the higher the acidity in the wine. There is no direct connection between total acidity and pH. In wine tasting, the term “acidity” refers to the fresh, tart and sour attributes of the wine which are evaluated in relation to how well the acidity balances out the sweetness and bitter components of the wine such as tannins. Three primary acids are found in wine grapes: tartaric, malic, and citric acids. During the course of winemaking and in the finished wines, acetic, butyric, lactic, and succinic acids can play significant roles. Most of the acids involved with wine are fixed acids with the notable exception of acetic acid, mostly found in vinegar, which is volatile and can contribute to the wine fault known as volatile acidity. Sometimes, additional acids, such as ascorbic, sorbic and sulfurous acids, are used in winemaking.
The aging of wine is potentially able to improve the quality of wine. This distinguishes wine from most other consumable goods. While wine is perishable and capable of deteriorating, complex chemical reactions involving a wine's sugars, acids and phenolic compounds can alter the aroma, color, mouthfeel and taste of the wine in a way that may be more pleasing to the taster. The ability of a wine to age is influenced by many factors including grape variety, vintage, viticultural practices, wine region and winemaking style. The condition that the wine is kept in after bottling can also influence how well a wine ages and may require significant time and financial investment. The quality of an aged wine varies significantly bottle-by-bottle, depending on the conditions under which it was stored, and the condition of the bottle and cork, and thus it is said that rather than good old vintages, there are good old bottles. There is a significant mystique around the aging of wine, as its chemistry was not understood for a long time, and old wines are often sold for extraordinary prices. However, the vast majority of wine is not aged, and even wine that is aged is rarely aged for long; it is estimated that 90% of wine is meant to be consumed within a year of production, and 99% of wine within 5 years.
The aromas of wine are more diverse than its flavours. The human tongue is limited to the primary tastes perceived by taste receptors on the tongue – sourness, bitterness, saltiness, sweetness and savouriness. The wide array of fruit, earthy, leathery, floral, herbal, mineral, and woodsy flavour present in wine are derived from aroma notes sensed by the olfactory bulb. In wine tasting, wine is sometimes smelled before taking a sip in order to identify some components of the wine that may be present. Different terms are used to describe what is being smelled. The most basic term is aroma which generally refers to a "pleasant" smell as opposed to odour which refers to an unpleasant smell or possible wine fault. The term aroma may be further distinguished from bouquet which generally refers to the smells that arise from the chemical reactions of fermentation and aging of the wine.
This glossary of winemaking terms lists some of terms and definitions involved in making wine, fruit wine, and mead.
Autolysis in winemaking relates to the complex chemical reactions that take place when a wine spends time in contact with the lees, or dead yeast cells, after fermentation. While for some wines - and all beers - autolysis is undesirable, it is a vital component in shaping the flavors and mouth feel associated with premium Champagne production. The practice of leaving a wine to age on its lees has a long history in winemaking dating back to Roman winemaking. The chemical process and details of autolysis were not originally understood scientifically, but the positive effects such as a creamy mouthfeel, breadlike and floral aromas, and reduced astringency were noticed early in the history of wine.
In winemaking, pressing is the process where juice is extracted from the grapes with the aid of a wine-press, by hand, or even by the weight of the grape berries and clusters. Historically, intact grape clusters were trodden by feet but in most wineries today the grapes are sent through a crusher/destemmer, which removes the individual grape berries from the stems and breaks the skins, releasing some juice, prior to being pressed. There are exceptions, such as the case of sparkling wine production in regions such as Champagne where grapes are traditionally whole-cluster pressed with stems included to produce a lighter must that is low in phenolics.
The role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. In the absence of oxygen, yeast converts the sugars of the fruit into alcohol and carbon dioxide through the process of fermentation. The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness. Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy or neutral spirits to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients and the wine has not yet reached dryness, this is considered a stuck fermentation.