Yeast in winemaking

The process of fermentation at work on Pinot noir. As yeast consume the sugar in themustit releases alcohol and carbon dioxide (seen here as the foaming bubbles) as byproducts.

The role ofyeast in winemakingis the most important element that distinguisheswinefromfruitjuice.In theabsence of oxygen,yeastconverts thesugarsof the fruit intoalcoholandcarbon dioxidethrough the process offermentation.The moresugars in the grapes,the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation todryness.Sometimes winemakers will stop fermentation early in order to leave someresidual sugarsand sweetness in the wine such as withdessert wines.This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterilefilteringthe wine to remove the yeast orfortificationwithbrandyor 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 astuck fermentation.

The most common yeast associated withwinemakingisSaccharomyces cerevisiaewhich has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol andsulfur dioxideas well as its ability to thrive in normal winepHbetween 2.8 and 4. Despite its widespread use which often includes deliberateinoculationfrom cultured stock,S. cerevisiaeis rarely the only yeast species involved in a fermentation. Grapes brought in fromharvestare usually teeming with a variety of "wild yeast" from theKloeckeraandCandidagenera.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-richmust.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 thetoxicityof alcohol on the yeast cellsphysiologywhile the more alcohol tolerantSaccharomycesspecies take over. In addition toS. cerevisiae,Saccharomyces bayanusis a species of yeast that can tolerate alcohol levels of 17–20% and is often used in fortified wine production such asportsand varieties such asZinfandelandSyrahharvested at highBrixsugar levels. Another common yeast involved in wine production isBrettanomyceswhose presence in a wine may be viewed by different winemakers as either awine faultor in limited quantities as an added note of complexity.

History

French scientist Louis Pasteur discovered the connection between microscopic yeast and the process of fermentation.

For most of thehistory of wine,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 theetymologyof the word "yeast" itself which essentially means "to boil".

In the mid-19th century, the French scientistLouis Pasteurwas 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 ofMicrobiology",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 theEmbden–Meyerhof–Parnas pathway.

The yeast species commonly known asSaccharomyces cerevisiaewas first identified in late 19th century enology text asSaccharomyces ellipsoideusdue to theelliptical(as opposed to circular) shape of the cells. Throughout the 20th century, more than 700 differentstrainsofSaccharomyces cerevisiaewere 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 particulargrape varieties.Some of these differences include the "vigor" or speed of fermentation, temperature tolerance, the production of volatilesulfurcompounds (such ashydrogen sulfide) and other compounds that may influence thearomaof 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

In the absence of oxygen, yeast cells will take the pyruvate produced by glycolysis and reduce it into acetaldehyde which is further reduced into ethanol "recharging" the NAD+ co-enzymes that is needed for various metabolic processes of the yeast.

See also:Fermentation (biochemistry)andFermentation (wine)

The primary role of yeast is to convert the sugars present (namelyglucose) 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 oxygen), the cell will continue some metabolic functions (such asglycolysis) but will rely on other pathways such as reduction ofacetaldehydeinto ethanol (fermentation) to "recharge" theco-enzymesneeded 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 unfermentablepentoseleaving behind a negligible amount of residual sugar.

Other compounds in wine produced by yeast

If a Chardonnay has too much "buttery" diacetyl notes, winemakers may add fresh yeast to the wine to consume the diacetyl and reduce it to the more neutral-smelling fusel oil 2,3-Butanediol.

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 includesglycerolwhich is produced when an intermediate of the glycolysis cycle (dihydroxyacetone) is reduced to "recharge" theNADH enzymeneeded 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 maintainingredoxbalance. As glycerol contributes increasedbodyand 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 likeKloeckeraandMetschnikowiato 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 withbisulfitemolecules in the wine, but it would take a substantial amount of sulfur dioxide addition (far beyondlegal limits) to prolong glycerol production beyond just these very nascent stages of fermentation.

Other by-products of yeast include:

• Methanol– Caused by thedemethylationofpectinsin 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– Formed by the decomposition of amino acids by the yeast. This includes2,3-Butanediolwhich is formed by yeast that are consumingdiacetyl,the compound that givesChardonnayand other wines a "buttery"aroma, reducing it first toacetoinand 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– 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 overallacidity of wine.
• Acetic acid– Considered a main component ofvolatile aciditythat can make a wine taste unbalanced and overly acidic. While acetic acid is the main volatile acid produced by yeast, trace amounts ofbutyric,formicandpropionicacids 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 faultethyl acetatewhich 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.

The distinctive "aldehydic" notes ofSherry winesare caused by special yeast native to theJerez wine region.

• Acetaldehyde– 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. Thefloryeast strains that produce theSpanish wineSherrywill 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 creatingoxidizedaromas.
• Hydrogen sulfide– Often produced by yeast during fermentation because of a nitrogen deficiency in the must. This can be done by a reduction ofsulfatesorsulfitesavailable 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 betweenrackings.In the presence of alcohol, hydrogen sulfide can react with ethanol to formethyl mercaptansanddisulfidesthat contribute to off aromas and wine faults. Some commercial yeast strains, such asMontrachet 522are known to produce higher levels of hydrogen sulfides than other strains, particularly if the must has some nutrient deficiencies.
• Pyruvic acid– Along with acetaldehyde, this compound can react withanthocyaninsextracted fromcontact with grape skinsto create a more stable color pigment (pyranoanthocyanin) that can enhance the color of some red wines.
• Variousesters,ketones,lactones,phenolsandacetals.

Lees

The lees left over from the secondary fermentation of sparkling wine can be seen on the bottom side of this bottle being inspected. Eventually this wine will go through riddling to collect the lees in the neck, where it will be removed prior to corking.

When yeast cells die, they sink to the bottom of the fermentation vessel where they combine with insolubletartrates,grape seeds, skin and pulp fragments to form thelees.During fermentation, the first significant racking which removes the bulk of dead yeast cells is often referred to as thegross leesas opposed to the less coarsefine leesthat come as the wine continues tosettleand age. During the time that the wine spends in contact with the lees, a number of changes can impact the wine due to both theautolysis(or self-metabolize) of the dead yeast cells as well as thereductiveconditions that can develop if the lees are not aerated or stirred (a process that the French callbâtonnage). The length of time that a wine spends on its lees (calledsur lie) will depend on the winemaking style and type of wine.

The process of leaving the wine to spend some contact with the lees has a long history in winemaking, being known to theAncient Romansand described byCato the Elderin the 2nd century BC. Today the practice is widely associated with any red wines that arebarrel fermented,Muscadet,sparkling wineChampagneas well asChardonnayproduced 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 themannoproteins,polysaccharidesand other compounds that were present in the yeast cell walls and membranes. This stirring also helps avoid the development of reductive sulfur compounds likemercaptansand hydrogen sulfide that can appear if the lees layer is more than 10 cm (3.9 in) 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 ofmannoseand proteins, with some glucose, mannoproteins are often bound in the cell wall of yeast withhydrophobicaroma compounds that becomevolatilizedas the cell wall breaks down. Not only does the release of mannoproteins impart sensory changes in the wine but they can contribute totartrateandprotein stability,help enhance thebodyandmouthfeelof the wine as well as decrease the perception of bitterness andastringencyoftannins.

Secondary fermentation

Main article:Sparkling wine production

The production ofChampagneand manysparkling winesrequires a second fermentation to occur in the bottle in order to produce thecarbonationnecessary 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 andcarbon dioxide.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

Film yeast on the surface of wine in a barrel ofVin jaunefrom theJura wineregion of France

Yeasttaxonomyincludes classification of yeast species depending onthe presence or absence of a sexual phase.Therefore, some winemaking yeasts are classified by their asexualanamorph(or "imperfect" form) while others may be classified by their sexualteleomorph(or "perfect" form). A common example of this isBrettanomyces(or "Brett" ) that is usually referenced in wine andviticulturetext under its asexual classification though some scientific and winemaking texts may describe specific species (such asDekkera bruxellensis) under itssporulatingsexual classification ofDekkera.Unless otherwise noted, this article will commonly refer to the asexual form of wine yeast.

The most common yeast generally associated with winemaking isSaccharomyces cerevisiaewhich is also used inbread makingandbrewing.Other genera of yeast that can be involved in winemaking (either beneficially or as the cause of potentialwine faults) include:

• Brettanomyces(TeleomorphDekkera)
• Candida(Teleomorphs for different species from several genera includingPichia,Metschnikowia,Issatchenkia,TorulasporaandKluyveromyces)
• Kloeckera(TeleomorphHanseniaspora), usually the most common "wild yeast" found in the vineyard. Some species are known as "killer yeast"that produce inhibitory levels ofethyl acetateandacetic acidthat can kill off sensitive strains ofSaccharomyces cerevisiae
• Saccharomycodes
• Schizosaccharomyces,the only wine yeast that reproduced byfissionwhereas most wine yeast reproduce bybudding.
• Zygosaccharomyces,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 60Brix) and is very resistant to sulfur dioxide.
• Aureobasidium,particularly the "black yeast" species ofAureobasidium pullulansfound in moist cellars that can contaminateaging winein barrels.

Saccharomyces

Saccharomyces cerevisiaeas seen under a Differential Interference Contrast (DIC) microscope

The yeast genusSaccharomyces(sugar mold) is favored for winemaking (for both grapes as well as otherfruit winesin 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 readilyfermentglucose,sucroseandraffinoseand metabolize glucose, sucrose, raffinose,maltoseandethanol.However,Saccharomycescannot ferment or utilizepentoses(such asarabinose) which is usually present in small amount in wines as residual sugars.

In addition toSaccharomyces cerevisiae,other species within the genusSaccharomycesthat are involved with winemaking include:

• Saccharomyces bayanus
• Saccharomyces beticus
• Saccharomyces fermentati
• Saccharomyces paradoxus
• Saccharomyces pastorianus
• Saccharomyces uvarum

Influences of different strains on fermentation

While some strains of yeast may influence the sensory characteristics and aromas of young wine, these differences seem to fade as the wine ages.

In 1996,Saccharomyces cerevisiaewas the first single-celled,eukaryoticorganism to have its entiregenomesequenced.This sequencing helped confirm the nearly century of work bymycologistsandenologistsin identifying differentstrainsofSaccharomyces cerevisiaethat are used inbeer,breadandwinemaking.Today there are several hundred different strains ofS. cerevisiaeidentified.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 thewines 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 likehydrogen sulfide) 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 thevarietalflavors of certainly grape varieties such asSauvignon blancandSémillon.It is believed that these wines can be influenced bythiolsproduced by thehydrolysisof certaincysteine-linked compounds byenzymesthat are more prevalent in particular strains. Other aromatic varieties such asGewürztraminer,RieslingandMuscatmay also be influenced by yeast strains containing high levels ofglycosidasesenzymes that can modifymonoterpenes.Similarly, though potentially to a much smaller extent, other varieties could be influenced by hydrolytic enzymes working onaliphatics,norisoprenoids,andbenzenederivatives such aspolyphenolsin themust.

Insparkling wine productionsome winemakers select strains (such as one known asÉpernaynamed after the town in theChampagne wine regionofFranceandCalifornia Champagne,also known asUC-Davisstrain 505) that are known toflocculatewell, allowing the dead yeast cells to be removed easily byriddlinganddisgorgement.InSherryproduction, the surface film of yeast known asflorused to make the distinctive style offinoandmanzanillasherries comes from different strains ofSaccharomyces cerevisiae,though the commercial flor yeast available for inoculation is often from different species ofSaccharomyces,Saccharomyces beticus,Saccharomyces fermentatiandSaccharomyces bayanus.

Wild yeasts and natural fermentation

Fruit flies are a common vector that transports ambient or "wild" yeast strains within wineries.

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 naturalfloraof a winery. Very often these are strains ofSaccharomyces cerevisiaethat 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 asambient,indigenousornaturalyeast as opposed toinoculated,selectedorcultured yeast.Wineries that often solely rely on these "in-house" strains will sometimes market their wines as being the product of wild ornatural fermentations.The (c. 304)Nanfang Caomu Zhuanghas the earliest description of winemaking using "herb ferment"(cǎoqūThảo khúc ) wild yeast with rice and various herbs, including the poisonousGelsemium elegans(yěgéDã cát ).

Another use of the term "wild yeast" refers to the non-Saccharomycesgenera of yeasts that are present in the vineyard, on the surface ofgrapevinesand of the grapes themselves. Anywhere from 160 to 100,000colony forming unitsof 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 byfruit flies). The most common wild yeasts found in the vineyard are from the generaKloeckera,CandidaandPichiawith the speciesKloeckera apiculatabeing 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 asleesandpomace) into the vineyard.

Unlike the "ambient"Saccharomyceswild 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 arepressedor allowed tomaceratewith 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"Saccharomycesstrains finish the fermentation.

Wineries that wish to cultivate an "in-house" ambient yeast strain will often recycle the leftover pomace of previous vintages as compost in the vineyard.

The use of both "ambient" and non-Saccharomyceswild yeasts carries both potential benefits and risk. Some winemakers feel that the use of resident/indigenous yeast helps contribute to the unique expression ofterroirin the wine. In wine regions such asBordeaux,classifiedand highly regarded estates will often tout the quality of their resident "chateau" strains. To this extent, wineries will often take the leftoverpomaceand lees from winemaking and return them to the vineyard to be used ascompostin 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 highervolatile aciditybut 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-Saccharomyceswild 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-Saccharomycesferment 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 asphenylethanol,which can impart arose-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

Some winemakers favor the use of freeze-dried cultured yeast (left) and yeast nutrients (right) because of their relative predictability in beginning and completing a fermentation.

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 ordimethyl sulfide
• Produce a minimum amount of residualpyruvate,acetic acidandacetaldehyde
• Produce minimum foaming during fermentation which may create difficulties for cap management duringmacerationor cause bungs to pop out duringbarrelfermentation.
• Have high levels offlocculationandleescompaction that makesracking,fining and filteringof the wine easier.

Inoculated (orpure cultured) yeasts are strains ofSaccharomyces cerevisiaethat have been identified and plated from wineries across the world (including notable producers from well-known wine regions such asBordeaux,Burgundy,Napa Valleyand theBarossa Valley). 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 sweetlate-harvest wines), development of surface film on the wine (positive forsome Sherry stylesbut 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") and tolerance for nutritional deficiencies in a must that may lead to a stuck fermentation.

Re-hydrating freeze dried yeast cultures

Preparing a yeast starter culture and gradually cooling the culture down to the must temperature by adding some wine

Pure culture yeasts that are grown in a lab are oftenfreeze driedand 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 bycold shock.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 40 °C (104 °F) prior to the introduction of the yeast (though some yeast strains may need temperatures below 38 °C (100 °F)) 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 solublecytoplasmiccomponents 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 15 °C (59 °F). 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 5–10 °C (41–50 °F) 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

Diammonium phosphate (or DAP) is a common additive that provides two necessary nutrients for yeast to have a healthy and sustained fermentation – nitrogen and phosphate.

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) andyeast assimilable nitrogen(ammoniaandamino acidsor YAN) but alsominerals(such asmagnesium) andvitamins(such asthiaminandriboflavin) that serve as important growth and survival factors. Among the other nutritional needs of wine yeast:

• Phosphate– used for the production ofnucleic acids,phospholipids(an important component of the cell membrane) and ATP (Adenosine triphosphatewhich the cell uses for transferring energy for metabolism).
• Potassium– important for the uptake and utilization of phosphate
• Biotin– involved in the synthesis ofproteins,fatty acidsand nucleic acids.
• Pantothenic acid– involved in the metabolism of sugars andlipids.A deficiency of this vitamin could lead into increase hydrogen sulfide production with off-aromas in the resulting wine.
• Nicotinic acid– involved in the synthesis ofNicotinamide adenine dinucleotide(NAD+), a co-enzyme that is important in maintaining theredoxbalance of the cell as well as in the process ofethanol fermentationitself.
• Inositol– involved with thesecondary messengermolecules that facilitatecell division.
• Trace amounts ofcalcium,chlorine,copper,iron,manganeseandzincfor healthy cell function.

One traditional way of providing nutrients for the yeast is theripassomethod where the leftover grape skins and pomace(pictured)from a previous fermentation is added to a newly fermenting wine.

Many of these nutrients are available in the must and skins of the grapes themselves but sometimes are supplemented by winemakers with additions such asdiammonium phosphate(DAP), freeze-dried micro-nutrients (such asGo-FermandFerm-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 someItalian wineregions is theripassomethod of adding the leftoverpomacefrom thepressingof other wines into a newly fermenting batch of wine as an additional food source for the yeast.

Saccharomyces cerevisiaecan assimilate nitrogen from both inorganic (ammonia andammonium) and organic forms (amino acids, particularlyarginine). As yeast cells die, enzymes within the cells beginautolyzingby 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 acidcysteine) which can combine with other molecules and react with alcohol to create volatilethiolsthat can contribute to a "stinky fermentation" or later development into various wine faults.

The role of oxygen

Yeasts arefacultative anaerobesmeaning that they can exist in both the presence and absence of oxygen. While fermentation is traditionally thought of as ananaerobicprocess 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 asergosterolandlanosterol.Thesesterolsare important in maintaining theselective permeabilityof the yeast cell membrane which becomes critical as the yeast becomes exposed to increasingosmotic pressureand 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-Saccharomycesyeasts 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

Film yeast likeCandida(pictured) andPichiacan cover the surface of a wine with a film layer that not only consumed most of the free sulfur dioxide available to protect the wine but also produces high levels of acetic acid that will contribute to volatile acidity in a wine.

Either directly or indirectly, wine yeast can be a culprit behind a wide variety ofwine faults.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 ofKloeckeraandCandida.Even the common wine yeastSaccharomyces cerevisiaecan be behind some wine faults with some strains of the yeast known to produce higher than ideal levels ofacetic acid,acetaldehydeand volatile sulfur compounds such asthiols.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 astuck fermentation.

In the presence ofoxygenseveral species ofCandidaandPichiacan create afilm surfaceon 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 othermicrobialattack. The presence of these yeasts is often identified by elevated levels ofvolatile acidity,particularly acetic acid. Some strains ofPichiawill metabolize acetic acid (as well asethyl acetateandisoamyl acetatethat may also be produced) with the side-effect of substantially decreasing thetitratable acidityand 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 theflorsherry 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 favorableSaccharomycesyeast kick in, will often chill their must, such as the practice of "cold soaking" the must during a pre-fermentationmacerationat temperatures between 4–15 °C (39–59 °F). Though some species, such asBrettanomyces,will not be inhibited and may even thrive during an extended period of cold soaking.

Brettanomyces

While some wine regions view the influence ofBrettanomyceson the wine, in limited amounts, as added complexity, many winemakers view the presence ofBrettanomycesspecies such asBrettanomyces bruxellensis(pictured) in their wineries as a negative influence that needs to be controlled.

The wine yeastBrettanomyces(or "Brett" ) produces very distinctive aroma compounds,4-Ethylphenol(4-EP) and4-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 asPinot noirfromBurgundy), 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 asRieslingfrom theMosel), the presence of any Brett will be considered a fault.Fruit fliesare commonvectorin the transfer ofBrettanomycesbetween tanks and even nearby wineries.

As a fermentation yeast,Brettanomycescan usually ferment a wine up to 10–11% alcohol levels before they die out. SometimesBrettanomycesalready present in a wine that has been inoculated withSaccharomyces cerevisiaewill out compete theSaccharomycesstrain for nutrients and even inhibit it due to the high levels of acetic acid,decanoic acidandoctanoic acidthat many strains ofBrettanomycescan 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 ofBrettanomycescan use a wide variety of carbon sources in wine and grape must, includingethanol,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.

External links

• P. Romano, C. Fiore, M. Paraggio, M. Caruso, A. Capece"Function of yeast species and strains in wine flavour".Archived2015-06-06 at theWayback Machine.International Journal of Food Microbiology 86 (2003). pp. 169–180.