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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 ethanol and carbon dioxide through the process of fermentation.Jeff Cox "From Vines to Wines: The Complete Guide to Growing Grapes and Making Your Own Wine" pp. 133–36 Storey Publishing 1999 The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness.D. Bird "Understanding Wine Technology" pp. 67–73 DBQA Publishing 2005 Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with . 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.J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pp. 778–80 Oxford University Press 2006
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.K. Fugelsang, C. Edwards Wine Microbiology Second Edition pp. 3–28 Springer Science and Business Media, New York (2010) While additions of sulfur dioxide (often added at the crusher) may limit some of the wild yeast activities, these yeasts will usually die out once the alcohol level reaches about 15% due to the toxicity 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.B. Zoecklein, K. Fugelsang, B. Gump, F. Nury Wine Analysis and Production pp. 281–90 Kluwer Academic Publishers, New York (1999)
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.J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pgs 267 & 508 Oxford University Press 2006
The yeast species commonly known as Saccharomyces cerevisiae was first identified in late 19th century enology text as Saccharomyces ellipsoideus due to the Ellipse (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 differences 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.
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.
Other by-products of yeast include:
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 thick and undisturbed for more than a week.
Most of the benefits associated with lees contact deals with the influence on the wine of the mannoproteins released during the autolysis of the yeast cells. Composed primarily of mannose 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.
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:
In addition to Saccharomyces cerevisiae, other species within the genus Saccharomyces that are involved with winemaking include:
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 disgorging. 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.
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 ). 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.
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 (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 attribute for sweet late-harvest wines), development of surface film on the wine (positive for flor 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.
Similarly, re-hydration procedures will also vary depending on the manufacturer and winery. Yeast is often inoculated in a volume of water or grape must that is 5–10 times the weight of the dry yeast. This liquid is often brought to temperature of prior to the introduction of the yeast (though some yeast strains may need temperatures below ) to allow the cells to disperse easily rather than clump and sink to the bottom of the container. The heat activation also allows the cells to quickly reestablish their membrane barrier before soluble cytoplasmic components escape the cell. Re-hydration at lower temperatures can greatly reduce the viability of the yeast with up to 60% cell death if the yeast is re-hydrated at . The culture is then stirred and aerated to incorporate oxygen into the culture which the yeast uses in the synthesis of needed survival factors.
The temperature of the starter culture is then slowly reduced, often by the graduated addition of must to get within of the must that the culture will be added to. This is done to avoid the sudden cold shock that the yeast cells may experience if the starter culture was added directly to the must itself which can kill up to 60% of the culture. Additionally, surviving cells exposed to cold shock tend to see an increase in hydrogen sulfide production.
Many of these nutrients are available in the must and skins of the grapes themselves but sometimes are supplemented by winemakers with additions such as diammonium phosphate (DAP), freeze-dried micro-nutrients (such as Go-Ferm and Ferm-K) and even the remnant of dead or extracted yeast cells such that the fermenting yeast can break down to mine for available nitrogen and nutrients. One historical winemaking tradition that is still practiced in some Italian wine regions is the ripasso method of adding the leftover pomace from the pressing of other wines into a newly fermenting batch of wine as an additional food source for the yeast.
Saccharomyces cerevisiae can assimilate nitrogen from both inorganic (ammonia and ammonium) and organic forms (amino acids, particularly arginine). As yeast cells die, enzymes within the cells begin autolyzing by breaking down the cell, including the amino acids. This autolysis of the cell provides an available nitrogen source for the still-fermenting and viable yeast cells. However, this autolysis can also release sulfur-link compounds (such as the breakdown of amino acid cysteine) which can combine with other molecules and react with alcohol to create volatile thiols that can contribute to a "stinky fermentation" or later development into various wine faults.
Cultured yeasts that are freeze-dried and available for inoculation of wine must are deliberately grown in commercial labs in high oxygen/low sugar conditions that favor the development of these survival factors. One of the reasons that some winemakers prefer using inoculated yeast is the predictability of fermentation due to the high level of survival factors that cultured yeast are assured of having without necessarily needing to expose the wine to additional levels of oxygen. Winemakers using "ambient" yeasts that are resident in their winery may not have this same assurance of survival factors and may need to compensate with other winemaking techniques.
Wild non- Saccharomyces yeasts often need a much greater exposure to oxygen in order to build up survival factors which is why many of these yeasts are often found living oxidatively as "film yeast" on the surface of wines in tanks or barrels.
In the presence of oxygen several species of Candida and Pichia can create a Biofilm on top of the wine in the tank of barrel. Allowed to go unchecked, these yeasts can rapidly deplete the available free sulfur compounds that keeps a wine protected from oxidation and other microbial attack. The presence of these yeasts is often identified by elevated levels of volatile acidity, particularly acetic acid. Some strains of Pichia will metabolize acetic acid (as well as ethyl acetate and isoamyl acetate that may also be produced) with the side-effect of substantially decreasing the titratable acidity and shifting the pH of wine upwards to levels that make the wine prone to attack by other spoilage microbes. Commonly called "film yeast", these yeasts are distinguished from the flor sherry yeast that are usually welcomed by winemakers in producing the delicate fino-style wines.
Growth of many unfavorable wild yeasts is generally slowed at lower cellar temperatures, so many winemakers who wish to inhibit the activities of these yeasts before the more favorable Saccharomyces yeast kick in, will often chill their must, such as the practice of "cold soaking" the must during a pre-fermentation maceration at temperatures between . Though some species, such as Brettanomyces, will not be inhibited and may even thrive during an extended period of cold soaking.
As a fermentation yeast, Brettanomyces can usually ferment a wine up to 10–11% alcohol levels before they die out. Sometimes Brettanomyces already present in a wine that has been inoculated with Saccharomyces cerevisiae will out compete the Saccharomyces strain for nutrients and even inhibit it due to the high levels of acetic acid, decanoic acid and octanoic acid that many strains of Brettanomyces can produce.
Once Brett is in a winery, it is very difficult to control even with strict hygiene and the discarding of barrels and equipment that has previously come into contact with "Bretty" wine. This is because many species of Brettanomyces can use a wide variety of carbon sources in wine and grape must, including ethanol, 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.
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