Fermentation (wine)

Fermentation (wine)

Overview
The process of fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

in wine
Wine
Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...

turns grape juice
Grape juice
Grape juice is obtained from crushing and blending grapes into a liquid. The juice is often sold in stores or fermented and made into wine, brandy, or vinegar. In the wine industry, grape juice that contains 7-23 percent of pulp, skins, stems and seeds is often referred to as "must"...

 into an alcoholic beverage
Alcoholic beverage
An alcoholic beverage is a drink containing ethanol, commonly known as alcohol. Alcoholic beverages are divided into three general classes: beers, wines, and spirits. They are legally consumed in most countries, and over 100 countries have laws regulating their production, sale, and consumption...

. During fermentation, yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 interact with sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s in the juice to create ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, commonly known as ethyl alcohol, and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 (as a by-product
By-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....

). In winemaking
Winemaking
Winemaking, or vinification, is the production of wine, starting with selection of the grapes or other produce and ending with bottling the finished wine. Although most wine is made from grapes, it may also be made from other fruit or non-toxic plant material...

, the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and speed of fermentation are important considerations as well as the levels of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 present in the must
Must
Must is freshly pressed fruit juice that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace; it typically makes up 7%–23% of the total weight of the must. Making must is the first step in winemaking...

 at the start of the fermentation. The risk of stuck fermentation
Stuck fermentation
A stuck fermentation occurs in brewing beer or winemaking when the yeast become dormant before the fermentation has completed. Unlike an "arrested fermentation" where the winemaker intentionally stops fermentation , a stuck fermentation is an unintentional and unwanted occurrence that can lead to...

 and the development of several wine fault
Wine fault
A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine but at insufficient concentrations to adversely...

s can also occur during this stage, which can last anywhere from 5 to 14 days for primary fermentation and potentially another 5 to 10 days for a secondary fermentation
Secondary fermentation
Secondary fermentation is a process commonly associated with winemaking, which entails a second period of fermentation in a different vessel than what was used when the fermentation process first started. An example of this would be starting fermentation in a carboy or stainless steel tank and then...

.
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Encyclopedia
The process of fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

in wine
Wine
Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...

turns grape juice
Grape juice
Grape juice is obtained from crushing and blending grapes into a liquid. The juice is often sold in stores or fermented and made into wine, brandy, or vinegar. In the wine industry, grape juice that contains 7-23 percent of pulp, skins, stems and seeds is often referred to as "must"...

 into an alcoholic beverage
Alcoholic beverage
An alcoholic beverage is a drink containing ethanol, commonly known as alcohol. Alcoholic beverages are divided into three general classes: beers, wines, and spirits. They are legally consumed in most countries, and over 100 countries have laws regulating their production, sale, and consumption...

. During fermentation, yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 interact with sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s in the juice to create ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, commonly known as ethyl alcohol, and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 (as a by-product
By-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....

). In winemaking
Winemaking
Winemaking, or vinification, is the production of wine, starting with selection of the grapes or other produce and ending with bottling the finished wine. Although most wine is made from grapes, it may also be made from other fruit or non-toxic plant material...

, the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and speed of fermentation are important considerations as well as the levels of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 present in the must
Must
Must is freshly pressed fruit juice that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace; it typically makes up 7%–23% of the total weight of the must. Making must is the first step in winemaking...

 at the start of the fermentation. The risk of stuck fermentation
Stuck fermentation
A stuck fermentation occurs in brewing beer or winemaking when the yeast become dormant before the fermentation has completed. Unlike an "arrested fermentation" where the winemaker intentionally stops fermentation , a stuck fermentation is an unintentional and unwanted occurrence that can lead to...

 and the development of several wine fault
Wine fault
A wine fault or defect is an unpleasant characteristic of a wine often resulting from poor winemaking practices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine but at insufficient concentrations to adversely...

s can also occur during this stage, which can last anywhere from 5 to 14 days for primary fermentation and potentially another 5 to 10 days for a secondary fermentation
Secondary fermentation
Secondary fermentation is a process commonly associated with winemaking, which entails a second period of fermentation in a different vessel than what was used when the fermentation process first started. An example of this would be starting fermentation in a carboy or stainless steel tank and then...

. Fermentation may be done in stainless steel tanks, which is common with many white wines like Riesling
Riesling
Riesling is a white grape variety which originated in the Rhine region of Germany. Riesling is an aromatic grape variety displaying flowery, almost perfumed, aromas as well as high acidity. It is used to make dry, semi-sweet, sweet and sparkling white wines. Riesling wines are usually varietally...

, in an open wooden vat, inside a wine barrel
Wine barrel
The use of oak plays a significant role in winemaking and can have a profound effect on the resulting wine, affecting the color, flavor, tannin profile and texture of the wine. Oak can come into contact with wine in the form of a barrel during the fermentation or aging periods...

 and inside the wine bottle
Wine bottle
A wine bottle is a bottle used for holding wine, generally made of glass. Some wines are fermented in the bottle, others are bottled only after fermentation. They come in a large variety of sizes, several named for Biblical kings and other figures. The standard bottle contains 750 ml,...

 itself as in the production
Sparkling wine production
There are four main methods of sparkling wine production. The first is simple injection of carbon dioxide , the process used in soft drinks, but this produces big bubbles that dissipate quickly in the glass. The second is the Metodo Italiano – Charmat process, in which the wine undergoes a...

 of many sparkling wine
Sparkling wine
Sparkling wine is a wine with significant levels of carbon dioxide in it making it fizzy. The carbon dioxide may result from natural fermentation, either in a bottle, as with the méthode champenoise, in a large tank designed to withstand the pressures involved , or as a result of carbon dioxide...

s.

History



The natural occurrence of fermentation means it was probably first observed long ago by humans. The earliest uses of the word "Fermentation" in relation to winemaking was in reference to the apparent "boiling" within the must that came from the anaerobic reaction
Anaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...

 of the yeast to the sugars
Sugars in wine
The sugars in wine grapes are what make winemaking possible. During the process of fermentation, sugars are broken down and converted by yeasts into alcohol and carbon dioxide. Grapes accumulate sugars as they grow on the grapevine through the translocation of sucrose molecules that are produced...

 in the grape juice and the release of carbon dioxide. The Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 fervere means, literally, to boil. In the mid-19th century, Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 noted the connection between yeast and the process of the fermentation in which the yeast act as catalyst and mediator through a series of a reaction that convert sugar into alcohol. The discovery of the Embden–Meyerhof–Parnas
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...

 pathway by Gustav Embden
Gustav Embden
Gustav Georg Embden was a German chemist who conducted studies on carbohydrate metabolism and muscle contraction, and was the first to discover and link together all the steps involved in the conversion of glycogen to lactic acid...

, Otto Fritz Meyerhof
Otto Fritz Meyerhof
-External links:* *...

 and Jakub Karol Parnas
Jakub Karol Parnas
Jakub Karol Parnas, also known as Yakov Oskarovich Parnas was a prominent Jewish-Polish–Soviet biochemist who contributed to the discovery of the Embden–Meyerhof–Parnas pathway, together with Otto Fritz Meyerhof and Gustav Georg Embden...

 in the early 20th century contributed more to the understanding of the complex chemical processes involved in the conversion of sugar to alcohol.

Process



In winemaking, there are distinctions made between ambient yeasts which are naturally present in wine cellars, vineyards and on the grapes themselves (sometimes known as a grape's "bloom" or "blush") and cultured yeast which are specifically isolated and inoculated for use in winemaking. The most common genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of wild yeasts found in winemaking include Candida
Candida (genus)
Candida is a genus of yeasts. Many species are harmless commensals or endosymbionts of animal hosts including humans, but other species, or harmless species in the wrong location, can cause disease. Candida albicans can cause infections in humans and other animals, especially in immunocompromised...

, Klöckera/Hanseniaspora, Metschnikowiaceae
Metschnikowiaceae
The Metschnikowiaceae are a family of yeasts in the order Saccharomycetales that reproduce by budding. It contains the genera Clavispora and Metschnikowia. Species in the family have a widespread distribution, especially in tropical areas....

, Pichia
Pichia
Pichia is a genus of yeasts in the family Saccharomycetaceae with spherical, elliptical or oblong acuminate cells. Pichia is a teleomorph, and forms during sexual reproduction hat-shaped, hemispherical or round ascospores. The anamorphs of some Pichia species are Candida species...

and Zygosaccharomyces
Zygosaccharomyces
Zygosaccharomyces is a genus of yeast in the family Saccharomycetaceae. It was first described under the Saccharomyces genus but in 1983 it was reclassified to its current name in the work by Barnett et al. The yeast has a long history as a spoilage yeast within the food industry. This is mainly...

. Wild yeasts can produce high-quality, unique-flavored wines; however, they are often unpredictable and may introduce less desirable traits to the wine, and can even contribute to spoilage. Traditional wine makers, particularly in Europe, advocate use of ambient yeast as a characteristic of the region's terroir
Terroir
Terroir comes from the word terre "land". It was originally a French term in wine, coffee and tea used to denote the special characteristics that the geography, geology and climate of a certain place bestowed upon particular varieties...

; nevertheless, many winemakers prefer to control fermentation with predictable cultured yeast. The cultured yeasts most commonly used in winemaking belong to the Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...

(also known as "sugar yeast") species. Within this species are several hundred different strain
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...

s of yeast that can be used during fermentation to affect the heat or vigor of the process and enhance or suppress certain flavor characteristics of the varietal
Varietal
"Varietal" describes wines made primarily from a single named grape variety, and which typically displays the name of that variety on the wine label. Examples of grape varieties commonly used in varietal wines are Cabernet Sauvignon, Chardonnay and Merlot...

. The use of different strains of yeasts are a major contributor to the diversity of wine, even among the same grape variety.

The addition of cultured yeast normally occurs with the yeast first in a dried or "inactive" state and is reactivated in warm water or diluted grape juice prior to being added to the must
Must
Must is freshly pressed fruit juice that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace; it typically makes up 7%–23% of the total weight of the must. Making must is the first step in winemaking...

. To thrive and be active in fermentation, the yeast needs access to a continuous supply of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 as well as access to various vitamins and minerals
Dietary mineral
Dietary minerals are the chemical elements required by living organisms, other than the four elements carbon, hydrogen, nitrogen, and oxygen present in common organic molecules. Examples of mineral elements include calcium, magnesium, potassium, sodium, zinc, and iodine...

. These components are naturally present in the grape must
Must
Must is freshly pressed fruit juice that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace; it typically makes up 7%–23% of the total weight of the must. Making must is the first step in winemaking...

 but their amount may be corrected by adding nutrient packets to the wine, in order to foster a more encouraging environment for the yeast. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 is needed as well, but in wine making, the risk of oxidation and the lack of alcohol production from oxygenated yeast requires the exposure of oxygen to be kept at a minimum.


Upon the introduction of active yeasts to the grape must, phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

s are attached to the sugar and the six-carbon sugar molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s begin to be split into three-carbon pieces and go through a series of rearrangement reaction
Rearrangement reaction
A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule...

s. During this process, the carboxylic carbon atom is released in the form of carbon dioxide with the remaining components becoming acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...

. The absence of oxygen in this anaerobic process allows the acetaldehyde to be eventually converted, by reduction, to ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

. During the conversion of acetaldehyde, a small amount is converted, by oxidation, to acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

 which, in excess, can contribute to the wine fault known as volatile acidity (vinegar taint). After the yeast has exhausted its life cycle, they fall to the bottom of the fermentation tank as sediment known as lees
Lees (fermentation)
Lees refers to deposits of dead yeast or residual yeast and other particles that precipitate, or are carried by the action of "fining", to the bottom of a vat of wine after fermentation and ageing. The yeast deposits in beer brewing are known as trub...

.

Other compounds involved



The metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 of amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s and breakdown of sugars by yeasts has the effect of creating other biochemical compounds that can contribute to the flavor and aroma of wine
Aroma of wine
It is through the aromas of wine that wine is actually tasted. The human tongue is limited to the primary tastes perceived by taste receptors on the tongue-acidity, bitterness, saltiness, sweetness and umami. The wide array of fruit, earthy, floral, herbal, mineral and woodsy flavor perceived in...

. These compounds can be considered "volatile
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...

" like aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s, ethyl acetate
Ethyl acetate
Ethyl acetate is the organic compound with the formula CH3COOCH2CH3. This colorless liquid has a characteristic sweet smell and is used in glues, nail polish removers, and cigarettes...

, ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

, fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s, fusel oils, hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

s and mercaptans) or "non-volatile" like glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

, acetic acid and succinic acid
Succinic acid
Succinic acid is a dicarboxylic acid. Succinate plays a biochemical role in the citric acid cycle. The name derives from Latin succinum, meaning amber, from which the acid may be obtained....

. Yeast also has the effect during fermentation of releasing glycoside hydrolase
Glycoside hydrolase
Glycoside hydrolases catalyze the hydrolysis of the glycosidic linkage to release smaller sugars...

 which can hydrolyse the flavor precursors of aliphatics (a flavor component that reacts with oak), benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 derivatives, monoterpene
Monoterpene
Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear or contain rings...

s (responsible for floral aromas from grapes like Muscat
Muscat (grape and wine)
The Muscat variety of grapes of the species Vitis vinifera is widely grown for wine, raisins and table grapes. Their color ranges from white to near black. Muscat almost always has a pronounced sweet floral aroma. Muscat grapes are grown around the world...

 and Traminer), norisoprenoids (responsible for some of the spice notes in Chardonnay
Chardonnay
Chardonnay is a green-skinned grape variety used to make white wine. It is originated from the Burgundy wine region of eastern France but is now grown wherever wine is produced, from England to New Zealand...

), and phenols
Natural phenol
Natural phenols, bioavailable phenols, plant phenolics, low molecular weight phenols or phenoloids are a class of natural products. They are small molecules containing one or more phenolic group. These molecules are smaller in size than polyphenols, containing less than 12 phenolic groups...

.

Some strains of yeasts can generate volatile thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

s which contribute to the fruity aromas in many wines such as the gooseberry
Gooseberry
The gooseberry or ; Ribes uva-crispa, syn. R. grossularia) is a species of Ribes, native to Europe, northwestern Africa and southwestern Asia...

 scent commonly associated with Sauvignon blanc
Sauvignon blanc
Sauvignon Blanc is a green-skinned grape variety which originates from the Bordeaux region of France. The grape most likely gets its name from the French word sauvage and blanc due to its early origins as an indigenous grape in South West France., a possible descendant of savagnin...

.
Brettanomyces
Brettanomyces
Brettanomyces is a non-spore forming genus of yeast in the family Saccharomycetaceae, and is often colloquially referred to as "Brett". The genus name Dekkera is used interchangeably with Brettanomyces, as it describes the teleomorph or spore forming form of the yeast. The cellular morphology of...

yeasts are responsible for the "barnyard aroma" characteristic in some red wines like Burgundy
Burgundy wine
Burgundy wine is wine made in the Burgundy region in eastern France, in the valleys and slopes west of the Saône River, a tributary of the Rhône. The most famous wines produced here - those commonly referred to as "Burgundies" - are red wines made from Pinot Noir grapes or white wines made from...

 and Pinot noir
Pinot Noir
Pinot noir is a black wine grape variety of the species Vitis vinifera. The name may also refer to wines created predominantly from Pinot noir grapes...

.

Winemaking considerations


During fermentation, there are several factors that winemakers take into consideration. The most notable is that of the internal temperature of the must. The biochemical process of fermentation itself creates a lot of residual heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 which can take the must out of the ideal temperature range for the wine. Typically, white wine is fermented between 64-68 °F (18-20 °C) though a wine maker may choose to use a higher temperature to bring out some of the complexity of the wine. Red wine is typically fermented at higher temperatures up to 85 °F (29 °C). Fermentation at higher temperatures may have adverse effect on the wine in stunning the yeast to inactivity and even "boiling off" some of the flavors of the wines. Some winemakers may ferment their red wines at cooler temperatures, more typical of white wines, in order to bring out more fruit flavors.

To control the heat generated during fermentation, the winemaker must choose a suitable vessel size or else use a cooling device. Various kinds of cooling devices are available, ranging from the ancient Bordeaux
Bordeaux wine
A Bordeaux wine is any wine produced in the Bordeaux region of France. Average vintages produce over 700 million bottles of Bordeaux wine, ranging from large quantities of everyday table wine, to some of the most expensive and prestigious wines in the world...

 practice of placing the fermentation vat atop blocks of ice to sophisticated fermentation tanks that have built-in cooling rings.

A risk factor involved with fermentation is the development of chemical residue and spoilage which can be corrected with the addition of sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 (SO2), although excess SO2 can lead to a wine fault. A winemaker who wishes to make a wine with high levels of residual sugar (like a dessert wine
Dessert wine
Dessert wines are sweet wines typically served with dessert.There is no simple definition of a dessert wine. In the UK, a dessert wine is considered to be any sweet wine drunk with a meal, as opposed to the white fortified wines drunk before the meal, and the red fortified wines drunk after it...

) may stop fermentation early either by dropping the temperature of the must to stun the yeast or by adding a high level of alcohol (like brandy
Brandy
Brandy is a spirit produced by distilling wine. Brandy generally contains 35%–60% alcohol by volume and is typically taken as an after-dinner drink...

) to the must to kill off the yeast and create a fortified wine
Fortified wine
Fortified wine is wine to which a distilled beverage has been added. Fortified wine is distinguished from spirits made from wine in that spirits are produced by means of distillation, while fortified wine is simply wine that has had a spirit added to it...

.

Other types of fermentation


In winemaking, there are different processes that fall under the title of "Fermentation" but might not follow the same procedure commonly associated with wine fermentation.

Bottle fermentation


Bottle fermentation is a method of sparkling wine production
Sparkling wine production
There are four main methods of sparkling wine production. The first is simple injection of carbon dioxide , the process used in soft drinks, but this produces big bubbles that dissipate quickly in the glass. The second is the Metodo Italiano – Charmat process, in which the wine undergoes a...

, originating in the Champagne region where after the cuvee
Cuvee
Cuvée is a French wine term derived from cuve, meaning vat or tank. The term cuvée is used with several different meanings, more or less based on the concept of a tank of wine put to some purpose:...

 has gone through a primary yeast fermentation the wine is then bottled and goes through a secondary fermentation where sugar and additional yeast known as liqueur de tirage is added to the wine. This secondary fermentation is what creates the carbon dioxide bubbles that sparkling wine is known for.

Carbonic maceration


The process of carbonic maceration
Carbonic maceration
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 prior to crushing. Conventional alcoholic fermentation involves crushing the grapes to free the juice and pulp from the...

 is also known as whole grape fermentation where instead of yeast being added, the grapes fermentation is encouraged to take place inside the individual grape berries. This method is common in the creation of Beaujolais
Beaujolais
Beaujolais is a French Appellation d'Origine Contrôlée wine generally made of the Gamay grape which has a thin skin and is low in tannins. Like most AOC wines they are not labeled varietally. Whites from the region, which make up only 1% of its production, are made mostly with Chardonnay grapes...

 wine and involves whole clusters of grapes being stored in a closed container with the oxygen in the container being replaced with carbon dioxide. Unlike normal fermentation where yeast converts sugar into alcohol, carbonic maceration works by enzymes within the grape breaking down the cellular matter to form ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 and other chemical properties. The resulting wines are typically soft and fruity.

Malolactic fermentation


Instead of yeast, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 play a fundamental role in malolactic fermentation
Malolactic fermentation
Malolactic fermentation is a process in winemaking where tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation tends to create a rounder, fuller mouthfeel. It has been said that malic acid tastes of green apples...

 which is essentially the conversion of malic acid
Malic acid
Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists...

 into lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

. This has the benefit of reducing some of the tartness and making the resulting wine taste softer. Depending on the style of wine that the winemaker is trying to produce, malolactic fermentation may take place at the very same time as the yeast fermentation.