the influence on the wine of the mannoproteins released during the autolysis of the yeast cells. composed primarily of mannose and proteins, with some glucose, mannoproteins are often bound in the cell wall of yeast with hydrophobic 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 and protein stability, help enhance the body and mouthfeel of the wine as well as decrease the perception of bitterness and astringency of tannins. secondary fermentation the production of champagne and many sparkling wines requires a second fermentation to occur in the bottle in order to produce the carbonation 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 carbon 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 of vin jaune from the jura wine region of france. yeast taxonomy 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 (or "imperfect" form) while others may be classified by their sexual teleomorph (or "perfect" form). a common example of this is brettanomyces (or "brett") that is usually referenced in wine and viticulture text under its asexual classification though some scientific and winemaking texts may describe specific species (such as dekkera bruxellensis) under its sporulating 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. other genera of yeast that can be involved in winemaking (either beneficially or as the cause of potential wine faults) include: brettanomyces (teleomorph dekkera) candida (teleomorphs for different species from several genera including pichia, metschnikowia, issatchenkia, torulaspora and kluyveromyces) kloeckera (teleomorph hanseniaspora), usually the most common "wild yeast" found in the vineyard. some species are known as "killer yeast" that produce inhibitory levels of ethyl acetate and acetic acid that can kill off sensitive strains of saccharomyces cerevisiae saccharomycodes schizosaccharomyces, the only wine yeast that reproduced by fission whereas most wine yeast reproduce by budding. 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 60 brix) and is very resistant to sulfur dioxide. aureobasidium, particularly the "black yeast" species of aureobasidium pullulans found in moist cellars that can contaminate aging wine in barrels. saccharomyces saccharomyces cerevisiae as seen under a differential interference contrast (dic) microscope. the yeast genus saccharomyces (sugar mold) is favored for winemaking (for both grapes as well as other fruit wines in addition also 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 glucose, sucrose and raffinose and metabolize glucose, sucrose, raffinose, maltose and ethanol. however, saccharomyces cannot ferment or utilize pentoses (such as arabinose) 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 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 cerevisiae was the first single-celled, eukaryotic organism to have its entire genome sequenced. this sequencing helped confirm the nearly century of work by mycologists and enologists in identifying different strains of saccharomyces cerevisiae that are used in beer, bread and winemaking. 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 hydrogen 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 the varietal flavors of certainly grape varieties such as sauvignon blanc and sémillon. it is believed that these wines can be influenced by thiols produced by the hydrolysis of certain cysteine-linked compounds by enzymes that are more prevalent in particular strains. other aromatic varieties such as gewürztraminer, riesling and muscat may also be influenced by yeast strains containing high levels of glycosidases enzymes that can modify monoterpenes. similarly, though potentially to a much smaller extent, other varieties could be influenced by hydrolytic enzymes working on aliphatics, norisoprenoids, and benzene derivatives such as polyphenols in the must. in sparkling wine production some winemakers select strains (such as one known as épernay named after the town in the champagne wine region of france and california champagne, also known as uc-davis strain 505) that are known to flocculate well, allowing the dead yeast cells to be removed easily by riddling and disgorgement. in sherry production, the surface film of yeast known as flor used to make the distinctive style of fino 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 saccharomyces 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 natural flora 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 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). 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 and of the grapes themselves. anywhere from 160 to 100,000 colony 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 by fruit flies). the most common wild yeasts found in the vineyard are from the genera kloeckera, candida and pichia 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) 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. 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-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 in the wine. in wine regions such as bordeaux, classified and highly regarded estates will often tout the quality of their resident "chateau" strains. to this extent, wineries will often take the leftover pomace and lees from winemaking and return them to the vineyard to be used as compost 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 volatile acidity 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-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 or dimethyl sulfide produce a minimum amount of residual pyruvate, acetic acid and acetaldehyde produce minimum foaming during fermentation which may create difficulties for cap management during maceration or cause bungs to pop out during barrel fermentation. have high levels of flocculation and lees compaction that makes racking, fining and filtering 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 bordeaux, burgundy, napa valley and the barossa 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