Stefano Lubiana Pinot Noir ferment vintage 2010

The role of yeast in winemaking is the most important element that distinguishes

wine Wine is an alcoholic drink made from Fermentation in winemaking, fermented fruit. Yeast in winemaking, Yeast consumes the sugar in the fruit and converts it to ethanol and carbon dioxide, releasing heat in the process. Wine is most often made f ...

from

fruit In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

juice Juice is a drink made from the extraction or Cold-pressed juice, pressing of the natural liquid contained in fruit and vegetables. It can also refer to liquids that are flavored with concentrate or other biological food sources, such as meat ...

. In the absence of oxygen,

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...

converts the

sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides ...

of the fruit into

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

through the process of

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

.Jeff Cox ''"From Vines to Wines: The Complete Guide to Growing Grapes and Making Your Own Wine"'' pp. 133–36 Storey Publishing 1999 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.D. Bird ''"Understanding Wine Technology"'' pp. 67–73 DBQA Publishing 2005 Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with

dessert wine Dessert wines, sometimes called pudding wines in the United Kingdom, 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 mea ...

s. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile

filtering Filtration is a physical process that separates solid matter and fluid from a mixture. Filter, filtering, filters or filtration may also refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Fil ...

the wine to remove the yeast or

fortification A fortification (also called a fort, fortress, fastness, or stronghold) is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime. The term is derived from Lati ...

with

brandy Brandy is a liquor produced by distilling wine. Brandy generally contains 35–60% alcohol by volume (70–120 US proof) and is typically consumed as an after-dinner digestif. Some brandies are aged in wooden casks. Others are coloured ...

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 A stuck fermentation is the term used in brewing beer or winemaking when the yeast becomes dormant before the fermentation has completed. Unlike an "arrested fermentation", where the winemaker intentionally stops fermentation (such as in the pro ...

.J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pp. 778–80

Oxford University Press Oxford University Press (OUP) is the publishing house of the University of Oxford. It is the largest university press in the world. Its first book was printed in Oxford in 1478, with the Press officially granted the legal right to print books ...

2006 The most common yeast associated with

winemaking Winemaking, wine-making, or vinification is the production of wine, starting with the selection of the fruit, its Ethanol fermentation, fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over ...

is ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have be ...

'' which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

as well as its ability to thrive in normal wine pH between 2.8 and 4. Despite its widespread use which often includes deliberate

inoculation Inoculation is the act of implanting a pathogen or other microbe or virus into a person or other organism. It is a method of artificially inducing immunity against various infectious diseases. The term "inoculation" is also used more generally ...

from cultured stock, ''S. cerevisiae'' is rarely the only yeast species involved in a fermentation. Grapes brought in from

harvest Harvesting is the process of collecting plants, animals, or fish (as well as fungi) as food, especially the process of gathering mature crops, and "the harvest" also refers to the collected crops. Reaping is the cutting of grain or pulses fo ...

are usually teeming with a variety of "wild yeast" from the ''

Kloeckera ''Hanseniaspora'' is a genus of yeasts. The name ''Kloeckera'' is applied to its anamorph form. They are typically apiculate (lemon-shaped) in shape and often found in grape musts pre-fermentation. The genus name ''Hanseniaspora'' honours Emi ...

'' and ''

Candida Candida, or Cándida (Spanish), may refer to: Biology and medicine * ''Candida'' (fungus), a genus of yeasts ** Candidiasis, an infection by ''Candida'' organisms * Malvasia Candida, a variety of grape Places * Candida, Campania, a ''comu ...

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

. These yeasts often begin the fermentation process almost as soon as the grapes are picked when the weight of the clusters in the harvest bins begin to crush the grapes, releasing the sugar-rich

must Must is freshly crushed Juice, fruit juice (usually grape juice) that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace and typically makes up 7–23% of the total weight of the must. Making must ...

.K. Fugelsang, C. Edwards ''Wine Microbiology'' Second Edition pp. 3–28 Springer Science and Business Media, New York (2010) While additions of sulfur dioxide (often added at the crusher) may limit some of the wild yeast activities, these yeasts will usually die out once the alcohol level reaches about 15% due to the

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...

of alcohol on the yeast cells

physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...

while the more alcohol tolerant ''

Saccharomyces ''Saccharomyces'' is a genus of fungi that includes many species of yeasts. ''Saccharomyces'' is from Greek σάκχαρον (sugar) and μύκης (fungus) and means ''sugar fungus''. Many members of this genus are considered very important in f ...

'' species take over. In addition to ''S. cerevisiae'', ''

Saccharomyces bayanus ''Saccharomyces bayanus'' is a yeast of the genus ''Saccharomyces'', and is used in winemaking and cider fermentation, and to make distilled beverages. ''Saccharomyces bayanus'', like ''Saccharomyces pastorianus'', is now accepted to be the resul ...

'' is a species of yeast that can tolerate alcohol levels of 17–20% and is often used in fortified wine production such as

ports Ports collections (or ports trees, or just ports) are the sets of makefiles and Patch (Unix), patches provided by the BSD-based operating systems, FreeBSD, NetBSD, and OpenBSD, as a simple method of installing software or creating binary packages. T ...

and varieties such as

Zinfandel Zinfandel (also known as Primitivo) is a variety of black-skinned wine grape. The variety is grown in over 10 percent of California vineyards. DNA analysis has revealed that it is genetically equivalent to the Croatian grapes Crljenak Kašt ...

and

Syrah Syrah (), also known as Shiraz, is a dark-skinned grape variety grown throughout the world and used primarily to produce red wine. In 1999, Syrah was found to be the offspring of two obscure grapes from southeastern France, Dureza and Mondeuse ...

harvested at high

Brix Degrees Brix (symbol °Bx) is a measure of the dissolved solids in a liquid, based on its specific gravity, and is commonly used to measure dissolved sugar content of a solution. One degree Brix is 1 gram of sucrose solute dissolved in 100 grams ...

sugar levels. Another common yeast involved in wine production is ''

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 teleomor ...

'' whose presence in a wine may be viewed by different winemakers as either a

wine fault 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 expe ...

or in limited quantities as an added note of complexity.B. Zoecklein, K. Fugelsang, B. Gump, F. Nury ''Wine Analysis and Production'' pp. 281–90 Kluwer Academic Publishers, New York (1999)

History

For most of the

history of wine The oldest evidence of ancient wine production has been found in Georgia from BC (the earliest known traces of grape wine), Greece from BC, Armenia from BC (large-scale production), and Sicily from BC. The earliest evidence of fermente ...

, winemakers did not know the mechanism that somehow converted sugary grape juice into alcoholic wine. They could observe the fermentation process which was often described as "boiling", "seething" or the wine being "troubled" due to release of carbon dioxide that gave the wine a frothy, bubbling appearance. This history is preserved in the

etymology Etymology ( ) is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century a subfield within linguistics, etymology has become a more rigorously scientific study. ...

of the word "yeast" itself which essentially means "to boil". In the mid-19th century, the French scientist

Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist, pharmacist, and microbiologist renowned for his discoveries of the principles of vaccination, Fermentation, microbial fermentation, and pasteurization, the la ...

was tasked by the French government to study what made some wines spoil. His work, which would later lead to Pasteur being considered one of the "Fathers of

Microbiology Microbiology () is the branches of science, scientific study of microorganisms, those being of unicellular organism, unicellular (single-celled), multicellular organism, multicellular (consisting of complex cells), or non-cellular life, acellula ...

", would uncover the connection between microscopic yeast cells and the process of the fermentation. It was Pasteur who discovered that yeast converted sugars in the must into alcohol and carbon dioxide, though the exact mechanisms of how the yeast would accomplish this task was not discovered till the 20th century with the Embden–Meyerhof–Parnas pathway.J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pgs 267 & 508

2006 The yeast species commonly known as ''Saccharomyces cerevisiae'' was first identified in late 19th century enology text as ''Saccharomyces ellipsoideus'' due to the elliptical (as opposed to circular) shape of the cells. Throughout the 20th century, more than 700 different strains of ''Saccharomyces cerevisiae'' were identified. The differences between the vast majority of these strains are mostly minor, though individual winemakers will develop a preference for particular strains when making certain wines or working with particular

grape varieties This list of grape varieties includes cultivated grapes, whether used for wine, or eating as a table grape, fresh or dried (raisin, currant, sultana). For a complete list of all grape species, including those unimportant to agriculture, see ''V ...

. Some of these differences include the "vigor" or speed of fermentation, temperature tolerance, the production of volatile

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

compounds (such as

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

) and other compounds that may influence the

aroma An odor (American English) or odour (Commonwealth English; see spelling differences) is a smell or a scent caused by one or more volatilized chemical compounds generally found in low concentrations that humans and many animals can perceive v ...

of the wine. In modern winemaking, winemakers have the option to select from a diverse range of yeast strains, each offering distinct characteristics that influence the wine's sensory profile. These strains are readily available for purchase from specialized suppliers. Winemakers can now easily access yeast strains that accentuate desirable features in wine, such as aromatic compounds, mouthfeel, and fermentation kinetics. This commercial availability of yeast strains has revolutionized the art of winemaking by allowing for more precise control over the fermentation process and the resultant wine's character.

Role in winemaking

The primary role of yeast is to convert the sugars present (namely

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

) in the grape must into alcohol. The yeast accomplishes this by utilizing glucose through a series of metabolic pathways that, in the presence of oxygen, produces not only large amounts of energy for the cell but also many different intermediates that the cell needs to function. In the absence of oxygen ( and sometimes even in the presence of oxygenB. Zoecklein, K. Fugelsang, B. Gump, F. Nury ''Wine Analysis and Production'' pp. 97–114 Kluwer Academic Publishers, New York (1999) ), the cell will continue some metabolic functions (such as

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

) but will rely on other pathways such as reduction of

acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...

into ethanol (fermentation) to "recharge" the co-enzymes needed to keep metabolism going. It is through this process of fermentation that ethanol is released by the yeast cells as a waste product. Eventually, if the yeast cells are healthy and fermentation is allowed to run to the completion, all fermentable sugars will be used up by the yeast with only the unfermentable

pentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.

Other compounds in wine produced by yeast

While the production of alcohol is the most noteworthy by-product of yeast metabolism from a winemaking perspective, there are a number of other products that yeast produce that can be also influence the resulting wine. This includes

glycerol Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...

which is produced when an intermediate of the glycolysis cycle (

dihydroxyacetone Dihydroxyacetone (; DHA), also known as glycerone, is a simple saccharide (a triose) with formula . DHA is primarily used as an ingredient in sunless tanning products. It is often derived from plant sources such as sugar beets and sugar cane, ...

) is reduced to "recharge" the NADH enzyme needed to continue other metabolic activities. This is usually produced early in the fermentation process before the mechanisms to reduce acetaldehyde into ethanol to recharge NADH becomes the cell's primary means of maintaining

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

balance. As glycerol contributes increased

body Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anim ...

and a slightly sweet taste without increasing the alcohol level of the wine, some winemakers try to intentionally favor conditions that would promote glycerol production in wine. This includes selecting yeast strains that favor glycerol production (or allowing some wild yeast like ''Kloeckera'' and ''Metschnikowia'' to ferment), increased oxygen exposure and aeration as well as fermenting at higher temperatures. Glycerol production is also encouraged if most available acetaldehyde is made unavailable by binding with

bisulfite The bisulfite ion (IUPAC-recommended nomenclature: hydrogensulfite) is the ion . Salts containing the ion are also known as "sulfite lyes". Sodium bisulfite is used interchangeably with sodium metabisulfite (Na2S2O5). Sodium metabisulfite diss ...

molecules in the wine, but it would take a substantial amount of sulfur dioxide addition (far beyond legal limits) to prolong glycerol production beyond just these very nascent stages of fermentation.Dr. Yair Margalit, ''Winery Technology & Operations A Handbook for Small Wineries'' pp. 67–74 The Wine Appreciation Guild (1996) Other by-products of yeast include: *

Methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

– Caused by the

demethylation Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen at ...

pectins Pectin ( ': "congealed" and "curdled") is a heteropolysaccharide, a structural polymer contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal chemical component of pectin is galactu ...

in the must by enzymes of the yeast. More commonly found in red wines than white but only in very small amounts between 20 and 200 mg/L. *

Fusel oils Fusel alcohols or fuselol, also sometimes called fusel oils in Europe, are mixtures of several higher alcohols (those with more than two carbons, chiefly amyl alcohol) produced as a by-product of alcoholic fermentation. The word ''Fusel'' is Ger ...

– Formed by the decomposition of amino acids by the yeast. This includes

2,3-Butanediol 2,3-Butanediol is the organic compound with the formula (CH3CHOH)2. It is classified as a ''vic''-diol (glycol). It exists as three stereoisomers, a chiral pair and the meso isomer. All are colorless liquids. Applications include precursors to ...

which is formed by yeast that are consuming

diacetyl Diacetyl ( ; IUPAC systematic name: butanedione or butane-2,3-dione) is an organic compound with the chemical formula (CH3CO)2. It is a yellow liquid with an intensely buttery flavor. It is a vicinal diketone (two C=O groups, side-by-side). Di ...

, the compound that gives

Chardonnay Chardonnay (, ; ) is a green-skinned grape variety used in the production of white wine. The variety originated in the Burgundy wine region of eastern France, but is now grown wherever wine is produced, from England to New Zealand. For new a ...

and other wines a "

butter Butter is a dairy product made from the fat and protein components of Churning (butter), churned cream. It is a semi-solid emulsion at room temperature, consisting of approximately 81% butterfat. It is used at room temperature as a spread (food ...

y" aroma, reducing it first to

acetoin Acetoin, also known as 3-hydroxybutanone or acetyl methyl carbinol, is an organic compound with the formula CH3CH(OH)C(O)CH3. It is a colorless liquid with a pleasant, buttery odor. It is chiral. The form produced by bacteria is (''R'')-acetoin. ...

and then to the more neutral-smelling 2,3-Butanediol. Many beer and winemakers who have a wine with too much "butteriness" will often "pitch" fresh yeast cultures into the no longer fermenting tank so that the yeast will consume the diacetyl and reduce the aroma. *

Succinic acid Succinic acid () is a dicarboxylic acid with the chemical formula (CH2)2(CO2H)2. In living organisms, succinic acid takes the form of an anion, succinate, which has multiple biological roles as a metabolic intermediate being converted into fum ...

– Like glycerol, this is often formed early in fermentation. Usually found in concentrations of 500–1200 mg/L, it is a minor acid in the overall acidity of wine. *

Acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

– Considered a main component of

volatile acidity Volatility or volatile may refer to: Chemistry * Volatility (chemistry), a measuring tendency of a substance or liquid to vaporize easily ** Volatile organic compounds, organic or carbon compounds that can evaporate at normal temperature and p ...

that can make a wine taste unbalanced and overly acidic. While acetic acid is the main volatile acid produced by yeast, trace amounts of butyric, formic and propionic acids can also be formed depending on the yeast strain. Most countries have wine laws setting the legal limit of volatile acidity, usually expressed as acetic acid, to 1200–2000 mg/L. Acetic acid can also lead to the development of the wine fault

ethyl acetate Ethyl acetate commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This flammable, colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues, nail polish removers, ...

which is characterized by a "nail polish remover" smell. However, small amounts of acetic acid are actually beneficial for the yeast as they use them to synthesis lipids in the cell membrane. CatavinoEnMano

Acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...

– While most of the acetaldehyde produce gets reduced to ethanol or is bound by sulfur dioxide, concentrations between 50 and 100 mg/L can remain in the wine. The

flor Flor (Spanish and Portuguese for ''flower'') in winemaking, is a film of yeast on the surface of wine, important in the manufacture of some styles of sherry. The flor is formed naturally under certain winemaking conditions, from indigenous yeast ...

yeast strains that produce the

Spanish wine Spanish wine ( or ) includes red wine, red, white wine, white, and sparkling wine, sparkling wines produced throughout the country. Located on the Iberian Peninsula, Spain has over planted in wine grapes, making it the most widely planted wine- ...

Sherry Sherry ( ) is a fortified wine produced from white grapes grown around the city of Jerez de la Frontera in Andalusia, Spain. Sherry is a drink produced in a variety of styles made primarily from the Palomino grape, ranging from light versio ...

will produce higher amounts that contributes to the characterized "aldehydic" aromas of Sherries. In the presence of oxygen, yeast can convert some of the ethanol presence in the wine back into acetaldehyde creating

oxidized Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

aromas. *

Hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

– Often produced by yeast during fermentation because of a nitrogen deficiency in the must. This can be done by a reduction of

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

s or

sulfite Sulfites or sulphites are compounds that contain the sulfite ion (systematic name: sulfate(IV) ion), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widely used. Sulfites are ...

s available in the must or by the decomposition of dead yeast cells by other yeast that releases sulfur-containing amino acids that are further broken down by the yeast. The latter often happens with wines that sit in contact with their lees for long periods of time between

racking Racking, often referred to as Soutirage or Soutirage traditionnel (meaning racking in French), also filtering or fining, is the process of moving wine or beer from one container to another using gravity rather than a pump, which can be disrupti ...

s. In the presence of alcohol, hydrogen sulfide can react with ethanol to form

ethyl mercaptan Ethanethiol, commonly known as ethyl mercaptan, is an organosulfur compound with the formula CH3CH2SH. It is a colorless liquid with a distinct odor. Abbreviated EtSH, it consists of an ethyl group (Et), CH3CH2, attached to a thiol group, SH. It ...

s and

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

s that contribute to off aromas and wine faults. Some commercial yeast strains, such as ''Montrachet 522'' are known to produce higher levels of hydrogen sulfides than other strains, particularly if the must has some nutrient deficiencies. *

Pyruvic acid Pyruvic acid (CH3COCOOH) is the simplest of the keto acids, alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is an metabolic intermediate, intermediate in several m ...

– Along with acetaldehyde, this compound can react with

anthocyanins Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart named a chemical compound that gives flow ...

extracted from contact with grape skins to create a more stable color pigment (

pyranoanthocyanin The pyranoanthocyanins are a type of pyranoflavonoids. They are chemical compounds formed in red wines by yeast during fermentation processes or during controlled oxygenation processes during the aging of wine. The different classes of pyranoantho ...

) that can enhance the color of some red wines. *Various

esters In chemistry, an ester is a chemical compound, compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds c ...

ketones In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

lactones Lactones are cyclic carboxylic esters. They are derived from the corresponding hydroxycarboxylic acids by esterification. They can be saturated or unsaturated. Lactones are formed by lactonization, the intramolecular esterification of the corre ...

phenols In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (− O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds ar ...

and

acetals In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments no ...

Lees

When yeast cells die, they sink to the bottom of the fermentation vessel where they combine with insoluble

tartrates A tartrate is a salt (chemistry), 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−. The main forms of tartrates used commercially are p ...

, grape seeds, skin and pulp fragments to form the lees. During fermentation, the first significant racking which removes the bulk of dead yeast cells is often referred to as the ''gross lees'' as opposed to the less coarse ''fine lees'' that come as the wine continues to

settle Settle or SETTLE may refer to: __NOTOC__ Places * Settle, Kentucky, United States, an unincorporated community * Settle, North Yorkshire, a town in England ** Settle Rural District, a historical administrative district Music * Settle (band), an in ...

and age. During the time that the wine spends in contact with the lees, a number of changes can impact the wine due to both the autolysis (or self-metabolize) of the dead yeast cells as well as the reductive conditions that can develop if the lees are not aerated or stirred (a process that the French call ''bâtonnage''). The length of time that a wine spends on its lees (called ''sur lie'') will depend on the winemaking style and type of wine.J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pp. 398–99

2006 The process of leaving the wine to spend some contact with the lees has a long history in winemaking, being known to the

Ancient Romans The Roman people was the ethnicity and the body of Roman citizenship, Roman citizens (; ) during the Roman Kingdom, the Roman Republic, and the Roman Empire. This concept underwent considerable changes throughout the long history of the Roman ...

and described by

Cato the Elder Marcus Porcius Cato (, ; 234–149 BC), also known as Cato the Censor (), the Elder and the Wise, was a Roman soldier, Roman Senate, senator, and Roman historiography, historian known for his conservatism and opposition to Hellenization. He wa ...

in the 2nd century BC. Today the practice is widely associated with any red wines that are barrel fermented,

Muscadet Muscadet ( , , ) is a French white wine. It is made at the western end of the Loire Valley, near the city of Nantes in the Pays de la Loire region. It is made from the Melon de Bourgogne grape, often referred to simply as ''melon''. While m ...

, sparkling wine

Champagne Champagne (; ) is a sparkling wine originated and produced in the Champagne wine region of France under the rules of the appellation, which demand specific vineyard practices, sourcing of grapes exclusively from designated places within it, spe ...

as well as

produced in many wine regions across the globe. Typically when wines are left in contact with their lees, they are regularly stirred in order to release the mannoproteins,

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

and other compounds that were present in the yeast cell walls and membranes. This stirring also helps avoid the development of reductive sulfur compounds like

mercaptans In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

and hydrogen sulfide that can appear if the lees layer is more than thick and undisturbed for more than a week. Most of the benefits associated with lees contact deals with the influence on the wine of the mannoproteins released during the autolysis of the yeast cells. Composed primarily of

mannose Mannose is a sugar with the formula , which sometimes is abbreviated Man. It is one of the monomers of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylatio ...

and proteins, with some glucose, mannoproteins are often bound in the cell wall of yeast with

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

aroma compounds that become volatilized as the cell wall breaks down. Not only does the release of mannoproteins impart sensory changes in the wine but they can contribute to

tartrate 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−. The main forms of tartrates used commercially are pure crystalline ...

and protein stability, help enhance the

and

mouthfeel Mouthfeel refers to the physical sensations in the mouth caused by food or drink, making it distinct from taste. It is a fundamental sensory attribute which, along with taste and smell, determines the overall flavor of a food item. Mouthfeel i ...

of the wine as well as decrease the perception of bitterness and

astringency An astringent (sometimes called adstringent) is a chemical that shrinks or constricts body tissues. The word derives from the Latin '' adstringere'', which means "to bind fast". Astringency, the dry, puckering or numbing mouthfeel caused by th ...

tannins Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' is widely applied to any large po ...

Secondary fermentation

The production of

and many

sparkling wine Sparkling wine is a wine with significant levels of carbon dioxide in it, making it fizzy. While it is common to refer to this as champagne, European Union countries legally reserve that word for products exclusively produced in the Champagne ( ...

s requires a second fermentation to occur in the bottle in order to produce the

carbonation Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids. In inorganic che ...

necessary for the style. A small amount of sugared liquid is added to individual bottles, and the yeast is allowed to convert this to more alcohol and

. The lees are then ricked into the neck of the bottle, frozen, and expelled via pressure of the carbonated wine.

Types of yeasts used in winemaking

Yeast

taxonomy image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...

includes classification of yeast species depending on the presence or absence of a sexual phase. Therefore, some winemaking yeasts are classified by their asexual

anamorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...

(or "imperfect" form) while others may be classified by their sexual

teleomorph In mycology, the terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota: *Teleomorph: the sexual reproductive stage (morph), typically a fruiting body. *Anamorph: an ase ...

(or "perfect" form). A common example of this is ''

'' (or "Brett") that is usually referenced in wine and

viticulture Viticulture (, "vine-growing"), viniculture (, "wine-growing"), or winegrowing is the cultivation and harvesting of grapes. It is a branch of the science of horticulture. While the native territory of ''Vitis vinifera'', the common grape vine ...

text under its asexual classification though some scientific and winemaking texts may describe specific species (such as ''

Dekkera bruxellensis ''Brettanomyces bruxellensis'' (the anamorph of ''Dekkera bruxellensis'') is a yeast associated with the Senne valley near Brussels, Belgium. Despite its Latin species name, ''B. bruxellensis'' is found all over the globe. In the wild, it is oft ...

'') under its

sporulating In biology, a spore is a unit of sexual (in fungi) or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plant ...

sexual classification of '' Dekkera''. Unless otherwise noted, this article will commonly refer to the asexual form of wine yeast. The most common yeast generally associated with winemaking is ''Saccharomyces cerevisiae'' which is also used in bread making and

brewing Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and #Fermenting, fermenting the resulting sweet liquid with Yeast#Beer, yeast. It may be done in a brewery ...

. Other genera of yeast that can be involved in winemaking (either beneficially or as the cause of potential

wine faults 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 expe ...

) include: *''

'' (Teleomorph ''Dekkera'') *''

'' (Teleomorphs for different species from several genera including ''

Pichia ''Pichia'' (''Hansenula'' and '' Hyphopichia'' are obsolete synonyms) is a genus of yeasts in the family Pichiaceae with spherical, elliptical, or oblong acuminate cells. ''Pichia'' is a teleomorph, and forms hat-shaped, hemispherical, or roun ...

'', '' Metschnikowia'', '' Issatchenkia'', ''

Torulaspora ''Torulaspora'' is a genus of ascomycetous yeasts in the family Saccharomycetaceae. See also *Yeast in winemaking The role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. In the absence of oxyge ...

'' and ''

Kluyveromyces ''Kluyveromyces'' is a genus of ascomycetous yeasts in the family Saccharomycetaceae. Some of the species, such as ''Kluyveromyces marxianus, K. marxianus'', are the teleomorphs of ''Candida (genus), Candida species''. The genus name of ''Kluyve ...

'') *''

'' (Teleomorph ''Hanseniaspora''), usually the most common "wild yeast" found in the vineyard. Some species are known as "

killer yeast A killer yeast is a yeast, such as ''Saccharomyces cerevisiae'', which is able to secrete one of a number of toxic proteins which are lethal to susceptible cells. These "killer toxins" are polypeptides that kill sensitive cells of the same or relat ...

" that produce inhibitory levels of

and

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

that can kill off sensitive strains of ''

'' *'' Saccharomycodes'' *''

Schizosaccharomyces ''Schizosaccharomyces'' is a genus of fission yeasts. The most well-studied species is '' S. pombe''. At present five Schizosaccharomyces species have been described ('' S. pombe'', ''S. japonicus'', ''S. octosporus'', ''S. cryophilus'' and ''S ...

'', the only wine yeast that reproduced by fission whereas most wine yeast reproduce by

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

. *''

Zygosaccharomyces ''Zygosaccharomyces'' is a genus of yeasts in the family Saccharomycetaceae. It was first described under the genus ''Saccharomyces'', but in 1983, it was reclassified to its current name in the work by Barnett ''et al''. The yeast has a long hi ...

'', very alcohol-tolerant and can grow in wines up to 18% v/v. Additionally this yeast can survive in extremely high sugar levels (as much as 60% w/w or 60

) and is very resistant to sulfur dioxide. *'' Aureobasidium'', particularly the "black yeast" species of ''

Aureobasidium pullulans ''Aureobasidium pullulans'' is a ubiquitous and generalistic black, yeast-like fungus that can be found in different environments (e.g. soil, water, air and limestone). It is well known as a naturally occurring epiphyte or endophyte of a wide ra ...

'' found in moist cellars that can contaminate

aging wine 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 ...

in barrels.

Saccharomyces

The yeast genus ''Saccharomyces'' (sugar mold) is favored for winemaking (for both grapes as well as other

fruit wines Fruit wines are Ethanol fermentation, fermented alcoholic beverages made from a variety of base ingredients (other than grapes); they may also have additional flavors taken from fruits, flowers, and herbs. This definition is sometimes broadened t ...

in addition to being used in brewing and breadmaking) because of the generally reliable and positive attributes it can bring to the wine. These yeasts will usually readily

ferment Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic compound, Organic molecules, such as glucose or other sugars, are Catabo ...

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

and

raffinose Raffinose is a trisaccharide composed of galactose, glucose, and fructose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains. Raffinose can be hydrolyzed to D-galactose and sucrose by th ...

and metabolize glucose, sucrose, raffinose,

maltose } Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the tw ...

and

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

. However, ''Saccharomyces'' cannot ferment or utilize

pentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.arabinose Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. Properties For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, o ...

) which is usually present in small amount in wines as residual sugars. In addition to ''Saccharomyces cerevisiae'', other species within the genus ''Saccharomyces'' that are involved with winemaking include: *''

'' *'' Saccharomyces beticus'' *'' Saccharomyces fermentati'' *''

Saccharomyces paradoxus ''Saccharomyces paradoxus'' is a wild yeast and the closest known species to the baker's yeast ''Saccharomyces cerevisiae''. It is used in population genomics and phylogenetic studies to compare its wild characteristics to laboratory yeasts. Eco ...

'' *''

Saccharomyces pastorianus ''Saccharomyces pastorianus'' is a yeast used industrially for the production of lager beer, and was named in honour of Louis Pasteur by the German Max Reess in 1870. This yeast's complicated genome appears to be the result of Hybridisation (bio ...

'' *''

Saccharomyces uvarum ''Saccharomyces uvarum'' is a species of yeast that is commonly found in fermented beverages, particularly those fermented at colder temperatures. It was originally described by Martinus Willem Beijerinck in 1898, but was long considered identica ...

Influences of different strains on fermentation

In 1996, ''Saccharomyces cerevisiae'' was the first single-celled,

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

organism to have its entire

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

sequenced In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which succi ...

. This sequencing helped confirm the nearly century of work by

mycologist Mycology is the branch of biology concerned with the study of fungi, including their taxonomy, genetics, biochemical properties, and use by humans. Fungi can be a source of tinder, food, traditional medicine, as well as entheogens, poison, and ...

s and

enologist Oenology (also enology; ) is the science and study of wine and winemaking. Oenology is distinct from viticulture, which is the science of the growing, cultivation, and harvesting of grapes. The English word oenology derives from the Greek word ' ...

s in identifying different strains of ''Saccharomyces cerevisiae'' that are used in

beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...

bread Bread is a baked food product made from water, flour, and often yeast. It is a staple food across the world, particularly in Europe and the Middle East. Throughout recorded history and around the world, it has been an important part of many cu ...

and

. Today there are several hundred different strains of ''S. cerevisiae'' identified. Not all of the strains are suitable for winemaking and even among the strains that are, there is debate among winemakers and scientists about the actual magnitude of differences between the various strains and their potential impact on the wine. Even among strains that have demonstrated distinctive difference when compared among young wines, these differences seem to fade and become less distinctive as the wines age. Some distinct difference among various strains include the production of certain "off-flavor" and aromas that may be temporary (but producing a "stinky fermentation") or could stay with the wine and either have to be dealt with through other winemaking means (such as the presence of volatile sulfur compounds like

) or leave a faulty wine. Another difference includes the "vigor" or speed of fermentation (which can also be influenced by other factors beyond yeast selection) with some yeast strains having the tendency to do "fast ferments" while others may take longer to get going. Another less measurable difference that are subject to more debate and questions of winemakers preference is the influence of strain selection on the

varietal A varietal wine is a wine made primarily from a single named grape variety, and which typically displays the name of that variety on the wine label.The American Heritage Dictionary of the English Language, Fourth Edition, 2000.winepros.com.au. ...

flavors of certainly grape varieties such as

Sauvignon blanc Sauvignon blanc () is a green-skinned grape variety that originates from the city of Bordeaux in France. The grape most likely gets its name from the French words ''sauvage'' ("wild") and ''blanc'' ("white") due to its early origins as an ind ...

and

Sémillon Sémillon () is a golden-skinned grape used to make dry and sweet white wines, mostly in French wine, France and Australian wine, Australia. Its thin skin and susceptibility to Botrytis cinerea, botrytis make it dominate the sweet wine region S ...

. It is believed that these wines can be influenced by

thiols In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

produced by the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of certain

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

-linked compounds by

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

that are more prevalent in particular strains. Other aromatic varieties such as

Gewürztraminer Gewürztraminer () is an aromatic wine grape variety, used in white wines, and which performs best in cooler climates. In English, it is sometimes referred to colloquially as Gewürz ( ; although this is never the case in German, because mean ...

Riesling Riesling ( , ) is a white grape variety that originated in the Rhine region. 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 ...

and

Muscat Muscat (, ) is the capital and most populous city in Oman. It is the seat of the Governorate of Muscat. According to the National Centre for Statistics and Information (NCSI), the population of the Muscat Governorate in 2022 was 1.72 million. ...

may also be influenced by yeast strains containing high levels of

glycosidases In biochemistry, glycoside hydrolases (also called glycosidases or glycosyl hydrolases) are a class of enzymes which catalyze the hydrolysis of glycosidic bonds in complex sugars. They are extremely common enzymes, with roles in nature inclu ...

enzymes that can modify

monoterpenes Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen funct ...

. Similarly, though potentially to a much smaller extent, other varieties could be influenced by hydrolytic enzymes working on aliphatics, norisoprenoids, and

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

derivatives such as

polyphenols Polyphenols () are a large family of naturally occurring phenols. They are abundant in plants and structurally diverse. Polyphenols include phenolic acids, flavonoids, tannic acid, and ellagitannin, some of which have been used historically as ...

in the

. In

sparkling wine production 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''. Pressure and terminology In popular parlance and also in the ...

some winemakers select strains (such as one known as ''Épernay'' named after the town in the

Champagne wine region The wine region within the historical province of Champagne in the northeast of France is best known for the production of champagne, the sparkling white wine that bears the region's name. EU law and the laws of most countries reserve the term ...

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

and ''California Champagne'', also known as

UC-Davis The University of California, Davis (UC Davis, UCD, or Davis) is a public land-grant research university in Davis, California, United States. It is the northernmost of the ten campuses of the University of California system. The institution was ...

strain 505) that are known to

flocculate In colloidal chemistry, flocculation is a process by which colloidal particles come out of suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipi ...

well, allowing the dead yeast cells to be removed easily by

riddling The traditional method for producing sparkling wine is the process used in the Champagne wine region, Champagne region of France to produce Champagne. It is also the method used in various French regions to produce sparkling wines (not called " ...

and

disgorgement Disgorgement is the act of giving up something on demand or by legal compulsion, for example giving up profits that were obtained illegally. In United States regulatory law, disgorgement is often a civil remedy imposed by some regulatory agenci ...

. In

production, the surface film of yeast known as

used to make the distinctive style of

fino In computer science, FINO is a humorous scheduling algorithm. It is an acronym for ''first in, never out'' as opposed to traditional ''first in, first out'' (FIFO) and ''last in, first out'' (LIFO) algorithms. A similar acronym is "FISH", for ...

and manzanilla sherries comes from different strains of ''Saccharomyces cerevisiae'', though the commercial flor yeast available for inoculation is often from different species of ''Saccharomyces'', '' Saccharomyces beticus'', '' Saccharomyces fermentati'' and ''

''.

Wild yeasts and natural fermentation

In winemaking, the term "wild yeast" has multiple meanings. In its most basic context, it refers to yeast that has not been introduced to the must by intentional inoculation of a cultured strain. Instead, these "wild yeasts" often come into contact with the must through their presence on harvest equipment, transport bins, the surface winemaking equipment and as part of the natural

flora Flora (: floras or florae) is all the plant life present in a particular region or time, generally the naturally occurring (indigenous (ecology), indigenous) native plant, native plants. The corresponding term for animals is ''fauna'', and for f ...

of a winery. Very often these are strains of ''Saccharomyces cerevisiae'' that have taken residence in these places over the years, sometimes being previously introduced by inoculation of prior vintages. In this context, these wild yeasts are often referred to as ambient, indigenous or natural yeast as opposed to inoculated, selected or cultured yeast. Wineries that often solely rely on these "in-house" strains will sometimes market their wines as being the product of wild or ''natural fermentations''. The (c. 304) ''

Nanfang Caomu Zhuang The (c. 304 CE) ''Nanfang caomu zhuang'' (南方草木狀 ''Plants of the Southern Regions''), attributed to the Jin dynasty (265-420), Western Jin dynasty scholar and botanist Ji Han (嵇含, 263-307), is a Flora (publication), Flora describi ...

'' has the earliest description of winemaking using " herb ferment" (''cǎoqū'' 草麴) wild yeast with rice and various herbs, including the poisonous '' Gelsemium elegans'' (''yěgé'' 冶葛).Li Hui-Lin (1979), ''Nan-fang ts'ao-mu chuang: a fourth century flora of Southeast Asia'', The Chinese University Press, p. 59. Another use of the term "wild yeast" refers to the non-''Saccharomyces'' genera of yeasts that are present in the vineyard, on the surface of

grapevines ''Vitis'' (grapevine) is a genus of 81 accepted species of vining plants in the flowering plant family Vitaceae. The genus consists of species predominantly from the Northern Hemisphere. It is economically important as the source of grapes, bot ...

and of the grapes themselves. Anywhere from 160 to 100,000

colony forming units In microbiology, a colony-forming unit (CFU, cfu or Cfu) is a unit which estimates the number of microbial cells (bacteria, fungi, viruses etc.) in a sample that are viable, able to multiply via binary fission under the controlled conditions. Coun ...

of wild yeasts per berry could exist in a typical vineyard. These yeasts can be carried by air currents, birds and insects through the vineyard and even into the winery (such as by fruit flies). The most common wild yeasts found in the vineyard are from the genera ''

'', ''

'' and ''

'' with the species '' Kloeckera apiculata'' being the most dominant species by far. ''Saccharomyces cerevisiae'', itself, is actually quite rarely found in the vineyard or on the surface freshly harvested wine grapes unless the winery frequently reintroduced winery waste (such as lees and

pomace Pomace ( ), or marc (; from French ''marc'' ), is the solid remains of grapes, olives, or other fruit after pressing (wine), pressing for juice or Vegetable oil, oil. It contains the skins, pulp, seeds, and stems of the fruit. Grape pomace has ...

) into the vineyard. Unlike the "ambient" ''Saccharomyces'' wild yeast, these genera of wild yeasts have very low tolerance to both alcohol and sulfur dioxide. They are capable of starting a fermentation and often begin this process as early as the harvest bin when clusters of grapes get slightly crushed under their own weight. Some winemakers will try to "knock out" these yeasts with doses of sulfur dioxide, most often at the crusher before the grapes are pressed or allowed to macerate with skin contact. Other winemakers may allow the wild yeasts to continue fermenting until they succumb to the toxicity of the alcohol they produce which is often between 3–5% alcohol by volume and then letting either inoculated or "ambient" ''Saccharomyces'' strains finish the fermentation. Pomace in the vineyard after pressing

The use of both "ambient" and non-''Saccharomyces'' wild yeasts carries both potential benefits and risk. Some winemakers feel that the use of resident/indigenous yeast helps contribute to the unique expression of ''

terroir (; ; from ''terre'', ) is a French language, French term used to describe the environmental factors that affect a crop's phenotype, including unique environment contexts, farming practices and a crop's specific growth habitat. Collectively, th ...

'' in the wine. In wine regions such as

Bordeaux Bordeaux ( ; ; Gascon language, Gascon ; ) is a city on the river Garonne in the Gironde Departments of France, department, southwestern France. A port city, it is the capital of the Nouvelle-Aquitaine region, as well as the Prefectures in F ...

classified Classified may refer to: General *Classified information, material that a government body deems to be sensitive *Classified advertising or "classifieds" Music *Classified (rapper) (born 1977), Canadian rapper * The Classified, a 1980s American ro ...

and highly regarded estates will often tout the quality of their resident "chateau" strains. To this extent, wineries will often take the leftover

and lees from winemaking and return them to the vineyard to be used as

compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is commonly prepared by Decomposition, decomposing plant and food waste, recycling organic materials, and man ...

in order to encourage the sustained presence of favorable strains. But compared to inoculated yeast, these ambient yeasts hold the risk of having a more unpredictable fermentation. Not only could this unpredictability include the presence of off-flavors/aromas and higher

but also the potential for a stuck fermentation if the indigenous yeast strains are not vigorous enough to fully convert all the sugars. It is virtually inevitable that non-''Saccharomyces'' wild yeast will have a role in beginning the fermentation of virtually every wine but for the wineries that choose to allow these yeasts to continue fermenting versus minimizing their influence do so with the intent of enhancing complexity through bio-diversity. While these non-''Saccharomyces'' ferment glucose and fructose into alcohol, they also have the potential to create other intermediates that could influence the aroma and flavor profile of the wine. Some of these intermediates could be positive, such as phenylethanol, which can impart a

rose A rose is either a woody perennial plant, perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred Rose species, species and Garden roses, tens of thousands of cultivar ...

-like aroma. However, as with ambient yeasts, the products of these yeasts can be very unpredictable – especially in terms of the types of flavors and aromas that these yeasts can produce.

Inoculated yeast

When winemakers select a cultured yeast strain, it is largely done because the winemaker wants a predictable fermentation taken to completion by a strain that has a track record of dependability. Among the particular considerations that are often important to winemakers is a yeast's tendency to: *Quickly begin fermentation, out-competing other "wild yeasts" for nutrients in the must *Completely utilize all fermentable sugars with a predictable sugar-to-alcohol conversion rate *Have an alcohol-tolerance up to 15% or even higher depending on the winemaking style *Have a high sulfur dioxide tolerance but low production of sulfur compounds such as hydrogen sulfide or

dimethyl sulfide Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula . It is the simplest thioether and has a characteristic disagreeable odor. It is a flammable liquid that boils at . It is a component of the smell produc ...

*Produce a minimum amount of residual

pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ...

and

*Produce minimum foaming during fermentation which may create difficulties for cap management during maceration or cause bungs to pop out during

barrel A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden stave (wood), staves and bound by wooden or metal hoops. The word vat is often used for large containers ...

fermentation. *Have high levels of

flocculation In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...

and lees compaction that makes

fining and filtering Fining may refer to: * Finings, additives to beer or wine, see also fining * Fining, metallurgy process done in a finery forge * Fining, glass production process, also called refining Refining is the process of purification of a (1) substanc ...

of the wine easier. Inoculated (or ''pure cultured'') yeasts are strains of ''Saccharomyces cerevisiae'' that have been identified and plated from wineries across the world (including notable producers from well-known wine regions such as

Burgundy Burgundy ( ; ; Burgundian: ''Bregogne'') is a historical territory and former administrative region and province of east-central France. The province was once home to the Dukes of Burgundy from the early 11th until the late 15th century. ...

Napa Valley Napa Valley is an American Viticultural Area (AVA) in Napa County, California. The area was established by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) on February 27, 1981, after a 1978 petition submitted by the Napa Valley Vin ...

and the

Barossa Valley The Barossa Valley (Barossa German: ''Barossa Tal'') is a valley in South Australia located northeast of Adelaide city centre. The valley is formed by the North Para River. It is notable as a major list of wine-producing regions, wine-producin ...

). These strains are tested in laboratories to determine a strain's vigor, sulfur dioxide and alcohol tolerance, production levels of acetic acid and sulfur compounds, ability to re-ferment (positive for sparkling wine but a negative attribute for sweet

late-harvest wines Late harvest wine is wine made from grapes left on the vine longer than usual. ''Late harvest'' is usually an indication of a sweet dessert wine, such as late harvest Riesling. Late harvest grapes are often more similar to raisins, but have been ...

), development of surface film on the wine (positive for some Sherry styles but a negative attribute for many other wines), enhancement of a wine's color or certain varietal characteristics by enzymes in the yeast cells and other metabolic products produced by the yeast, foaming and flocculation tendencies, yeasticidal properties (a trait known as "

Killer yeast A killer yeast is a yeast, such as ''Saccharomyces cerevisiae'', which is able to secrete one of a number of toxic proteins which are lethal to susceptible cells. These "killer toxins" are polypeptides that kill sensitive cells of the same or relat ...

") and tolerance for nutritional deficiencies in a must that may lead to a stuck fermentation.

Re-hydrating freeze dried yeast cultures

Adding Agria wine to rehydrated yeast to prepare culture inoculum

Pure culture yeasts that are grown in a lab are often

freeze dried Freeze drying, also known as lyophilization or cryodesiccation, is a low temperature dehydration process that involves freezing the product and lowering pressure, thereby removing the ice by sublimation. This is in contrast to dehydration by m ...

and packaged for commercial use. Prior to their addition into must, these yeasts need to be re-hydrated in "starter cultures" that must be carefully monitored (particularly in regards to temperature) to ensure that the yeast cells are not killed off by

cold shock Cold shock response is a series of neurogenic cardio-respiratory responses caused by sudden immersion in cold water. In cold water immersions, such as by falling through thin ice, cold shock response is perhaps the most common cause of death. Al ...

. Ideally winemakers want to add enough inoculum to have a viable cell population density of 5 million cells per milliliter. The exact amount of freeze-dried culture varies by manufacturer and strain of yeast but it is often around 1 gram per gallon (or 25 grams per 100 liters). Wines that could have potentially problematic fermentation (such as high sugar level late harvest or botryized wines) may have more yeast added. Similarly, re-hydration procedures will also vary depending on the manufacturer and winery. Yeast is often inoculated in a volume of water or grape must that is 5–10 times the weight of the dry yeast. This liquid is often brought to temperature of prior to the introduction of the yeast (though some yeast strains may need temperatures below ) to allow the cells to disperse easily rather than clump and sink to the bottom of the container. The heat activation also allows the cells to quickly reestablish their membrane barrier before soluble

cytoplasmic The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell and ...

components escape the cell. Re-hydration at lower temperatures can greatly reduce the viability of the yeast with up to 60% cell death if the yeast is re-hydrated at . The culture is then stirred and aerated to incorporate oxygen into the culture which the yeast uses in the synthesis of needed survival factors. The temperature of the starter culture is then slowly reduced, often by the graduated addition of must to get within of the must that the culture will be added to. This is done to avoid the sudden cold shock that the yeast cells may experience if the starter culture was added directly to the must itself which can kill up to 60% of the culture. Additionally, surviving cells exposed to cold shock tend to see an increase in hydrogen sulfide production.

Nutritional needs of wine yeast

In order to successfully complete a fermentation with minimum to no negative attributes being added to the wine, yeast needs to have the full assortment of its nutritional needs met. These include not only an available energy source (carbon in the form of sugars such as glucose) and

yeast assimilable nitrogen Yeast assimilable nitrogen or YAN is the combination of free amino nitrogen (FAN), ammonia (NH3) and ammonium (NH4+) that is available for a yeast, e.g. the wine yeast ''Saccharomyces cerevisiae'', to use during fermentation (wine), fermentation. ...

(

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

and

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

or YAN) but also

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s (such as

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

) and

vitamin Vitamins are Organic compound, organic molecules (or a set of closely related molecules called vitamer, vitamers) that are essential to an organism in small quantities for proper metabolism, metabolic function. Nutrient#Essential nutrients, ...

s (such as

thiamin Thiamine, also known as thiamin and vitamin B1, is a vitamin – an essential micronutrient for humans and animals. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosphorylated forms of thiam ...

and

riboflavin Riboflavin, also known as vitamin B2, is a vitamin found in food and sold as a dietary supplement. It is essential to the formation of two major coenzymes, flavin mononucleotide and flavin adenine dinucleotide. These coenzymes are involved in ...

) that serve as important growth and survival factors. Among the other nutritional needs of wine yeast: *

Phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

– used for the production of

nucleic acids Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic a ...

phospholipids Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typi ...

(an important component of the cell membrane) and ATP (

Adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...

which the cell uses for transferring energy for metabolism). *

Potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

– important for the uptake and utilization of phosphate *

Biotin Biotin (also known as vitamin B7 or vitamin H) is one of the B vitamins. It is involved in a wide range of metabolic processes, both in humans and in other organisms, primarily related to the utilization of fats, carbohydrates, and amino acids. ...

– involved in the synthesis of

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

and nucleic acids. *

Pantothenic acid Pantothenic acid (vitamin B5) is a B vitamin and an essential nutrient. All animals need pantothenic acid in order to synthesize coenzyme A (CoA), which is essential for cellular energy production and for the synthesis and degradation of prote ...

– involved in the metabolism of sugars and

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

s. A deficiency of this vitamin could lead into increase hydrogen sulfide production with off-aromas in the resulting wine. *

Nicotinic acid Nicotinic acid, or niacin, is an organic compound and a vitamer of vitamin B3, an essential human nutrient. It is produced by plants and animals from the amino acid tryptophan. Nicotinic acid is also a prescription medication. Amounts f ...

– involved in the synthesis of

Nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphat ...

(NAD+), a co-enzyme that is important in maintaining the

balance of the cell as well as in the process of

ethanol fermentation Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this ...

itself. *

Inositol In biochemistry, medicine, and related sciences, inositol generally refers to ''myo''-inositol (formerly ''meso''-inositol), the most important stereoisomer of the chemical compound cyclohexane-1,2,3,4,5,6-hexol. Its elemental formula, formula is ...

– involved with the

secondary messenger Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. (Intercellular signals, a non-local form of cell signaling, encompassing both first m ...

molecules that facilitate

cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...

. *Trace amounts of

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

and

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

for healthy cell function. Red wine grape pomace

Many of these nutrients are available in the must and skins of the grapes themselves but sometimes are supplemented by winemakers with additions such as

diammonium phosphate Diammonium phosphate (DAP; IUPAC name diammonium hydrogen phosphate; chemical formula (NH4)2(HPO4)) is one of a series of water- soluble ammonium phosphate salts that can be produced when ammonia reacts with phosphoric acid. Solid diammonium ph ...

(DAP), freeze-dried micro-nutrients (such as ''Go-Ferm'' and ''Ferm-K'') and even the remnant of dead or extracted yeast cells such that the fermenting yeast can break down to mine for available nitrogen and nutrients. One historical winemaking tradition that is still practiced in some

Italian wine Italian wine () is produced in every region of Italy. Italy is the country with the widest variety of indigenous grapevine in the world, with an area of under vineyard cultivation, as well as the List of wine-producing regions#Countries, world's ...

regions is the ''

ripasso Valpolicella (, , ) is a viticultural zone of the province of Verona, Italy, east of Lake Garda. The hilly agricultural and marble-quarrying region of small holdings north of the Adige is famous for wine production. Valpolicella ranks just after ...

'' method of adding the leftover

from the pressing of other wines into a newly fermenting batch of wine as an additional food source for the yeast. ''Saccharomyces cerevisiae'' can assimilate nitrogen from both inorganic (ammonia and

ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

) and organic forms (amino acids, particularly

arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...

). As yeast cells die, enzymes within the cells begin autolyzing by breaking down the cell, including the amino acids. This autolysis of the cell provides an available nitrogen source for the still-fermenting and viable yeast cells. However, this autolysis can also release sulfur-link compounds (such as the breakdown of amino acid

) which can combine with other molecules and react with alcohol to create volatile

that can contribute to a "stinky fermentation" or later development into various wine faults.

The role of oxygen

Yeasts are

facultative anaerobes A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...

meaning that they can exist in both the presence and absence of oxygen. While fermentation is traditionally thought of as an

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...

process done in the absence of oxygen, early exposure of the yeast to oxygen can be a vital component in the successful completion of that fermentation. This is because oxygen is important in the synthesis of cell "survival factors" such as

ergosterol Ergosterol (ergosta-5,7,22-trien-3β-ol) is a mycosterol found in cell membranes of fungi and protozoa, serving many of the same functions that cholesterol serves in animal cells. Because many fungi and protozoa cannot survive without ergostero ...

and

lanosterol Lanosterol is a tetracyclic triterpenoid and is the compound from which all animal and fungal steroids are derived. By contrast, plant steroids are produced via cycloartenol. In the eyes of vertebrates, lanosterol is a natural constituent, havin ...

. These

sterols A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...

are important in maintaining the

selective permeability Semipermeable membrane is a type of synthetic or biologic, polymeric membrane that allows certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecules o ...

of the yeast cell membrane which becomes critical as the yeast becomes exposed to increasing

osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a Solution (chemistry), solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a soluti ...

and levels of alcohol in the wine. As a waste product of its own metabolism, alcohol is actually very toxic to yeast cells. Yeast with weak survival factors and lacking sterols may succumb to these conditions before fermenting a wine to complete dryness, leaving a stuck fermentation. Cultured yeasts that are freeze-dried and available for inoculation of wine must are deliberately grown in commercial labs in high oxygen/low sugar conditions that favor the development of these survival factors. One of the reasons that some winemakers prefer using inoculated yeast is the predictability of fermentation due to the high level of survival factors that cultured yeast are assured of having without necessarily needing to expose the wine to additional levels of oxygen. Winemakers using "ambient" yeasts that are resident in their winery may not have this same assurance of survival factors and may need to compensate with other winemaking techniques. Wild non-''Saccharomyces'' yeasts often need a much greater exposure to oxygen in order to build up survival factors which is why many of these yeasts are often found living oxidatively as "film yeast" on the surface of wines in tanks or barrels.

Wine faults related to yeast

Either directly or indirectly, wine yeast can be a culprit behind a wide variety of

. These can include the presence of " off flavors" and aromas that can be the by-product of some "wild yeast" fermentation such as those by species within the genera of ''Kloeckera'' and ''Candida''. Even the common wine yeast ''Saccharomyces cerevisiae'' can be behind some wine faults with some strains of the yeast known to produce higher than ideal levels of

and volatile sulfur compounds such as

. Also any yeast can have a low tolerance to nutritional deficiencies, temperature fluctuation or extremes and excessive or low sugar levels that may lead to a

. In the presence of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

several species of ''Candida'' and ''Pichia'' can create a film surface on top of the wine in the tank of barrel. Allowed to go unchecked, these yeasts can rapidly deplete the available free sulfur compounds that keeps a wine protected from oxidation and other

microbial A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

attack. The presence of these yeasts is often identified by elevated levels of

, particularly acetic acid. Some strains of ''Pichia'' will metabolize acetic acid (as well as

and

isoamyl acetate Isoamyl acetate, also known as isopentyl acetate, is an ester formed from isoamyl alcohol and acetic acid, with the molecular formula . It is a colorless liquid that is only slightly soluble in water, but very soluble in most organic solvents. ...

that may also be produced) with the side-effect of substantially decreasing the titratable acidity and shifting the pH of wine upwards to levels that make the wine prone to attack by other spoilage microbes. Commonly called "film yeast", these yeasts are distinguished from the

sherry yeast that are usually welcomed by winemakers in producing the delicate fino-style wines. Growth of many unfavorable wild yeasts is generally slowed at lower cellar temperatures, so many winemakers who wish to inhibit the activities of these yeasts before the more favorable ''Saccharomyces'' yeast kick in, will often chill their must, such as the practice of "cold soaking" the must during a pre-fermentation maceration at temperatures between . Though some species, such as ''Brettanomyces'', will not be inhibited and may even thrive during an extended period of cold soaking.

Brettanomyces

The wine yeast ''Brettanomyces'' (or "Brett") produces very distinctive aroma compounds, 4-Ethylphenol (4-EP) and 4-Ethylguaiacol (4-EG), that can have a wine being described as smelling like a "barnyard", "wet saddle" or "band-aid". To some winemakers and with some wine styles (such as

Pinot noir Pinot noir (), also known as Pinot nero, is a red-wine grape variety of the species ''Vitis vinifera''. The name also refers to wines created predominantly from Pinot noir grapes. The name is derived from the French language, French words fo ...

from

), a limited amount of these compounds could be considered a positive attribute that adds to the complexity of wine. To other winemakers and with other wine styles (such as

from the Mosel), the presence of any Brett will be considered a fault.M. Baldy ''The University Wine Course'' p. 80 The Wine Appreciation Guild Third Edition 2009 Fruit flies are common

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

in the transfer of ''Brettanomyces'' between tanks and even nearby wineries. As a fermentation yeast, ''Brettanomyces'' can usually ferment a wine up to 10–11% alcohol levels before they die out. Sometimes ''Brettanomyces'' already present in a wine that has been inoculated with ''Saccharomyces cerevisiae'' will out compete the ''Saccharomyces'' strain for nutrients and even inhibit it due to the high levels of acetic acid,

decanoic acid Capric acid, also known as decanoic acid or decylic acid, is a saturated fatty acid, medium-chain fatty acid (MCFA), and carboxylic acid. Its formula is . Salts and esters of decanoic acid are called caprates or decanoates. The term capric aci ...

and

octanoic acid Caprylic acid (), also known under the systematic name octanoic acid or C8 Acid, is a saturated fatty acid, medium-chain fatty acid (MCFA). It has the structural formula , and is a colorless oily liquid that is minimally soluble in water with a ...

that many strains of ''Brettanomyces'' can produce. Once Brett is in a winery, it is very difficult to control even with strict hygiene and the discarding of barrels and equipment that has previously come into contact with "Bretty" wine. This is because many species of ''Brettanomyces'' can use a wide variety of carbon sources in wine and grape must, including

, for metabolism. Additionally, Brett can produce a wide range of by-products that could influence the wine beyond just the 4-EP and 4-EG compounds previously discussed. Many of these compounds, such as the "footprints" of the 4-EP and 4-EG, will still remain in the wine even after yeast cells die and are removed by racking and sterile filtration.

References

External links

*P. Romano, C. Fiore, M. Paraggio, M. Caruso, A. Capece '
Function of yeast species and strains in wine flavour
''. . International Journal of Food Microbiology 86 (2003). pp. 169–180. {{Authority control Food microbiology Saccharomycetes Winemaking