yeast in winemaking yeast in winemaking the role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. in the absence of oxygen, yeast converts the sugars of the fruit into alcohol and carbon dioxide through the process of fermentation. 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. sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. this can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy 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. the process of fermentation at work on pinot noir. as yeast consume the sugar in the must it releases alcohol and carbon dioxide (seen here as the foaming bubbles) as byproducts. the most common yeast associated with winemaking is saccharomyces cerevisiae which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and sulfur dioxide 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 from cultured stock, s. cerevisiae is rarely the only yeast species involved in a fermentation. grapes brought in from harvest are usually teeming with a variety of "wild yeast" from the kloeckera and candida genera. 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. 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 of alcohol on the yeast cells physiology while the more alcohol tolerant saccharomyces species take over. in addition to s. 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 as ports and varieties such as zinfandel and syrah harvested at high brix sugar levels. another common yeast involved in wine production is brettanomyces whose presence in a wine may be viewed by different winemakers as either a wine fault or 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 the history 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 the etymology of the word "yeast" itself which essentially means "to boil". in the mid-19th century, the french scientist louis 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 of microbiology", 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. 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. some of these difference include the "vigor" or speed of fermentation, temperature tolerance, the production of volatile sulfur compounds (such as hydrogen sulfide) and other compounds that may influence the aroma of the wine. 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. the primary role of yeast is to convert the sugars present (namely glucose) 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 as glycolysis) but will rely on other pathways such as reduction of acetaldehyde 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 leaving 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 includes glycerol which is produced when an intermediate of the glycolysis cycle (dihydroxyacetone) 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 balance. as glycerol contributes increased body 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 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. other by-products of yeast include: methanol – caused by the demethylation of pectins in the must by enzymes of the yeast. more commonly found in red wines than white but only in very small amounts between 20–200 mg/l. fusel oils – formed by the decomposition of amino acids by the yeast. this includes 2,3-butanediol which is formed by yeast-consuming diacetyl, the compound that gives chardonnay and other wines a "buttery" aroma, reducing it first to 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 – 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 – considered a main component of volatile acidity 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 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. the distinctive "aldehydic" notes of sherry wines are caused by special yeast native to the jerez wine region. acetaldehyde – while most of the acetaldehyde produce gets reduced to ethanol or is bound by sulfur dioxide, concentrations between 50–100 mg/l can remain in the wine. the flor yeast strains that produce the spanish wine sherry 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 aromas. hydrogen sulfide – often produced by yeast during fermentation because of a nitrogen deficiency in the must. this can be done by a reduction of sulfates or sulfites 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 rackings. in the presence of alcohol, hydrogen sulfide can react with ethanol to form ethyl mercaptans and disulfides 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 – along with acetaldehyde, this compound can react with anthocyanins extracted from contact with grape skins to create a more stable color pigment (pyranoanthocyanin) that can enhance the color of some red wines. various esters, ketones, lactones, phenols and acetals. 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 insoluble tartrates, 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 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. 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 and described by cato the elder in the 2nd century bc. today the practice is widely associated with any red wines that are barrel fermented, muscadet, sparkling wine champagne as well as chardonnay 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 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 and hydrogen sulfide that can appear if the lees layer is more than 10 cm (4 inches) thick and undisturbed for more than a week. most of the benefits associated with lees contact deals with