The role ofyeast in winemaking is the most important element that distinguisheswine fromfruitjuice. In theabsence of oxygen,yeast converts thesugars of the fruit intoalcohol andcarbon dioxide through the process offermentation.[1] The moresugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation todryness.[2] Sometimes winemakers will stop fermentation early in order to leave someresidual sugars and sweetness in the wine such as withdessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterilefiltering the wine to remove the yeast orfortification withbrandy 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 astuck fermentation.[3]
The most common yeast associated withwinemaking isSaccharomyces cerevisiae which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol andsulfur dioxide as well as its ability to thrive in normal winepH between 2.8 and 4. Despite its widespread use which often includes deliberateinoculation from cultured stock,S. cerevisiae is rarely the only yeast species involved in a fermentation. Grapes brought in fromharvest are usually teeming with a variety of "wild yeast" from theKloeckera andCandidagenera. 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.[4] 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 thetoxicity of alcohol on the yeast cellsphysiology while the more alcohol tolerantSaccharomyces species take over. In addition toS. cerevisiae,Saccharomyces bayanus is a species of yeast that can tolerate alcohol levels of 17–20% and is often used in fortified wine production such asports and varieties such asZinfandel andSyrah harvested at highBrix sugar levels. Another common yeast involved in wine production isBrettanomyces whose presence in a wine may be viewed by different winemakers as either awine fault or in limited quantities as an added note of complexity.[5]
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 theetymology of the word "yeast" itself which essentially means "to boil".[3][6]
In the mid-19th century, the French scientistLouis Pasteur 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 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.[7]
The yeast species commonly known asSaccharomyces cerevisiae was first identified in late 19th century enology text asSaccharomyces ellipsoideus due to theelliptical (as opposed to circular) shape of the cells. Throughout the 20th century, more than 700 differentstrains ofSaccharomyces 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 particulargrape varieties. Some of these differences include the "vigor" or speed of fermentation, temperature tolerance, the production of volatilesulfur compounds (such ashydrogen sulfide) and other compounds that may influence thearoma of the wine.[3]
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.[8] 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.
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[9]), the cell will continue some metabolic functions (such asglycolysis) but will rely on other pathways such as reduction ofacetaldehyde into ethanol (fermentation) to "recharge" theco-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 unfermentablepentose leaving behind a negligible amount of residual sugar.[4]
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 includesglycerol which is produced when an intermediate of the glycolysis cycle (dihydroxyacetone) is reduced to "recharge" theNADH enzyme needed to continue other metabolic activities.[4] 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 maintainingredox balance. As glycerol contributes increasedbody 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 likeKloeckera andMetschnikowia to ferment), increased oxygen exposure and aeration as well as fermenting at higher temperatures.[9] Glycerol production is also encouraged if most available acetaldehyde is made unavailable by binding withbisulfite molecules 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.[10]
Other by-products of yeast include:[3][10]
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 lees as opposed to the less coarsefine lees that come as the wine continues tosettle 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 theautolysis (or self-metabolize) of the dead yeast cells as well as thereductive conditions 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.[12]
The process of leaving the wine to spend some contact with the lees has a long history in winemaking, being known to theAncient Romans and described byCato the Elder in the 2nd century BC. Today the practice is widely associated with any red wines that arebarrel fermented,Muscadet, sparkling wineChampagne as well asChardonnay 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 themannoproteins,polysaccharides and other compounds that were present in the yeast cell walls and membranes. This stirring also helps avoid the development of reductive sulfur compounds likemercaptans and 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.[12]
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 ofmannose and proteins, with some glucose, mannoproteins are often bound in the cell wall of yeast withhydrophobic aroma compounds that becomevolatilized as the cell wall breaks down. Not only does the release of mannoproteins impart sensory changes in the wine but they can contribute totartrate andprotein stability, help enhance thebody andmouthfeel of the wine as well as decrease the perception of bitterness andastringency oftannins.[4]
The production ofChampagne and manysparkling wines requires a second fermentation to occur in the bottle in order to produce thecarbonation 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 andcarbon dioxide. The lees are then ricked into the neck of the bottle, frozen, and expelled via pressure of the carbonated wine.
Yeasttaxonomy includes 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 andviticulture text under its asexual classification though some scientific and winemaking texts may describe specific species (such asDekkera bruxellensis) under itssporulating sexual classification ofDekkera.[4] Unless otherwise noted, this article will commonly refer to the asexual form of wine yeast.
The most common yeast generally associated with winemaking isSaccharomyces cerevisiae which is also used inbread making andbrewing. Other genera of yeast that can be involved in winemaking (either beneficially or as the cause of potentialwine faults) include:[3][4]
The yeast genusSaccharomyces (sugar mold) is favored for winemaking (for both grapes as well as otherfruit wines 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 readilyfermentglucose,sucrose andraffinose and metabolize glucose, sucrose, raffinose,maltose andethanol. However,Saccharomyces cannot ferment or utilizepentoses (such asarabinose) which is usually present in small amount in wines as residual sugars.[4]
In addition toSaccharomyces cerevisiae, other species within the genusSaccharomyces that are involved with winemaking include:[1][3][4]
In 1996,Saccharomyces cerevisiae was the first single-celled,eukaryotic organism to have its entiregenomesequenced. This sequencing helped confirm the nearly century of work bymycologists andenologists in identifying differentstrains ofSaccharomyces cerevisiae that are used inbeer,bread andwinemaking. Today there are several hundred different strains ofS. cerevisiae identified.[3] 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.[5] 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.[2]
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.[3]
Another less measurable difference that are subject to more debate and questions of winemakers preference is the influence of strain selection on thevarietal flavors of certainly grape varieties such asSauvignon blanc andSémillon. It is believed that these wines can be influenced bythiols produced by thehydrolysis of certaincysteine-linked compounds byenzymes that are more prevalent in particular strains. Other aromatic varieties such asGewürztraminer,Riesling andMuscat may also be influenced by yeast strains containing high levels ofglycosidases enzymes that can modifymonoterpenes. Similarly, though potentially to a much smaller extent, other varieties could be influenced by hydrolytic enzymes working onaliphatics,norisoprenoids, andbenzene derivatives such aspolyphenols in themust.[3]
Insparkling wine production some winemakers select strains (such as one known asÉpernay named after the town in theChampagne wine region ofFrance andCalifornia Champagne, also known asUC-Davis strain 505) that are known toflocculate well, allowing the dead yeast cells to be removed easily byriddling anddisgorgement. InSherry production, the surface film of yeast known asflor used to make the distinctive style offino andmanzanilla sherries comes from different strains ofSaccharomyces cerevisiae,[3] though the commercial flor yeast available for inoculation is often from different species ofSaccharomyces,Saccharomyces beticus,Saccharomyces fermentati andSaccharomyces bayanus.[1][2][5]
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 naturalflora of a winery. Very often these are strains ofSaccharomyces 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 asambient,indigenous ornatural yeast as opposed toinoculated,selected orcultured yeast. Wineries that often solely rely on these "in-house" strains will sometimes market their wines as being the product of wild ornatural fermentations.[3] The (c. 304)Nanfang Caomu Zhuang has the earliest description of winemaking using "herb ferment" (cǎoqū 草麴) wild yeast with rice and various herbs, including the poisonousGelsemium elegans (yěgé 冶葛).[13][14]
Another use of the term "wild yeast" refers to the non-Saccharomyces genera of yeasts that are present in the vineyard, on the surface ofgrapevines and of the grapes themselves. Anywhere from 160 to 100,000colony forming units 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 byfruit flies). The most common wild yeasts found in the vineyard are from the generaKloeckera,Candida andPichia with the speciesKloeckera apiculata being the most dominant species by far.[5]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 aslees andpomace) into the vineyard.[3]
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 arepressed or allowed tomacerate 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.[3]
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 ofterroir in the wine. In wine regions such asBordeaux,classified and highly regarded estates will often tout the quality of their resident "chateau" strains. To this extent, wineries will often take the leftoverpomace and lees from winemaking and return them to the vineyard to be used ascompost 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 highervolatile acidity but also the potential for a stuck fermentation if the indigenous yeast strains are not vigorous enough to fully convert all the sugars.[3]
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 asphenylethanol, which can impart arose-like aroma.[5] 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.[3]
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:[5]
Inoculated (orpure cultured) yeasts are strains ofSaccharomyces cerevisiae that have been identified and plated from wineries across the world (including notable producers from well-known wine regions such asBordeaux,Burgundy,Napa Valley and 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 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") and tolerance for nutritional deficiencies in a must that may lead to a stuck fermentation.[3]
Pure culture yeasts that are grown in a lab are oftenfreeze dried 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 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.[5]
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)[1]) 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 solublecytoplasmic 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 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.[5]
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.[5]
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 (ammonia andamino acids or YAN) but alsominerals (such asmagnesium) andvitamins (such asthiamin andriboflavin) that serve as important growth and survival factors. Among the other nutritional needs of wine yeast:[4]
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-Ferm andFerm-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 wine regions is theripasso method of adding the leftoverpomace from thepressing of other wines into a newly fermenting batch of wine as an additional food source for the yeast.[4]
Saccharomyces cerevisiae can assimilate nitrogen from both inorganic (ammonia andammonium) and organic forms (amino acids, particularlyarginine). As yeast cells die, enzymes within the cells beginautolyzing 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 acidcysteine) which can combine with other molecules and react with alcohol to create volatilethiols that can contribute to a "stinky fermentation" or later development into various wine faults.[4]
Yeasts arefacultative anaerobes meaning that they can exist in both the presence and absence of oxygen. While fermentation is traditionally thought of as ananaerobic 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 asergosterol andlanosterol. Thesesterols are important in maintaining theselective permeability of the yeast cell membrane which becomes critical as the yeast becomes exposed to increasingosmotic pressure 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.[4]
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.[4]
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.[4]
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 ofKloeckera andCandida. Even the common wine yeastSaccharomyces cerevisiae can be behind some wine faults with some strains of the yeast known to produce higher than ideal levels ofacetic acid,acetaldehyde and 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.[4]
In the presence ofoxygen several species ofCandida andPichia can create afilm 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 othermicrobial attack. The presence of these yeasts is often identified by elevated levels ofvolatile acidity, particularly acetic acid. Some strains ofPichia will metabolize acetic acid (as well asethyl acetate andisoamyl acetate that may also be produced) with the side-effect of substantially decreasing thetitratable 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 theflor sherry yeast that are usually welcomed by winemakers in producing the delicate fino-style wines.[4]
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 favorableSaccharomyces yeast kick in, will often chill their must, such as the practice of "cold soaking" the must during a pre-fermentationmaceration at 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.[5]
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 noir fromBurgundy), a limited amount of these compounds could be considered a positive attribute that adds to the complexity of wine.[4] To other winemakers and with other wine styles (such asRiesling from theMosel), the presence of any Brett will be considered a fault.[15]Fruit flies are commonvector in the transfer ofBrettanomyces between tanks and even nearby wineries.[5]
As a fermentation yeast,Brettanomyces can usually ferment a wine up to 10–11% alcohol levels before they die out. SometimesBrettanomyces already present in a wine that has been inoculated withSaccharomyces cerevisiae will out compete theSaccharomyces strain for nutrients and even inhibit it due to the high levels of acetic acid,decanoic acid andoctanoic acid that many strains ofBrettanomyces can produce.[5]
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 ofBrettanomyces can 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.[4] 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.[5]
| Harvest | |
|---|---|
| Pressing | |
| Maceration | |
| Fermentation | |
| Aging | |
| Other steps | |
| Related | |
