The process offermentation in winemaking turnsgrape juice into an alcoholic beverage. Duringfermentation,yeasts transform sugars present in the juice intoethanol andcarbon dioxide (as aby-product). Inwinemaking, the temperature and speed of fermentation are important considerations as well as the levels ofoxygen present in themust at the start of the fermentation. The risk ofstuck fermentation and the development of severalwine faults can also occur during this stage, which can last anywhere from 5 to 14 days forprimary fermentation and potentially another 5 to 10 days for asecondary fermentation. Fermentation may be done in stainless steel tanks, which is common with many white wines likeRiesling, in an open wooden vat, inside awine barrel and inside thewine bottle itself as inthe production of manysparkling wines.^[1][2]

The natural occurrence of fermentation means it was probably first observed long ago by humans.^[3] The earliest uses of the word "fermentation" in relation to winemaking was in reference to the apparent "boiling" within the must that came from theanaerobic reaction of the yeast to thesugars in the grape juice and the release of carbon dioxide. TheLatinfervere means, literally,to boil. In the mid-19th century,Louis Pasteur noted the connection between yeast and the process of the fermentation in which the yeast act as catalyst and mediator through a series of a reaction that convert sugar into alcohol. The discovery of theEmbden–Meyerhof–Parnas pathway byGustav Embden,Otto Fritz Meyerhof andJakub Karol Parnas in the early 20th century contributed more to the understanding of the complex chemical processes involved in the conversion of sugar to alcohol.^[4]

In winemaking, there are distinctions made betweenambient yeasts which are naturally present in wine cellars, vineyards and on the grapes themselves (sometimes known as a grape's "bloom" or "blush") andcultured yeast which are specifically isolated and inoculated for use in winemaking. The most commongenera of wild yeasts found in winemaking includeCandida,Klöckera/Hanseniaspora,Metschnikowiaceae,Pichia andZygosaccharomyces. Wild yeasts can produce high-quality, unique-flavored wines; however, they are often unpredictable and may introduce less desirable traits to the wine, and can even contribute to spoilage. Few yeast, and lactic and acetic acid bacterial colonies naturally live on the surface of grapes,^[5] but traditional wine makers, particularly in Europe, advocate use of ambient yeast as a characteristic of the region'sterroir; nevertheless, many winemakers prefer to control fermentation with predictable cultured yeast. The cultured yeasts most commonly used in winemaking belong to theSaccharomyces cerevisiae (also known as "sugar yeast") species. Within this species are several hundred differentstrains of yeast that can be used during fermentation to affect the heat or vigor of the process and enhance or suppress certain flavor characteristics of thevarietal. The use of different strains of yeasts is a major contributor to the diversity of wine, even among the same grape variety.^[6] Alternative, non-Saccharomyces cerevisiae, yeasts are being used more prevalently in the industry to add greater complexity to wine. After a winery has been in operation for a number of years, few yeast strains are actively involved in the fermentation process. The use of active dry yeasts reduces the variety of strains that appear in spontaneous fermentation by outcompeting those strains that are naturally present.^[7]

The addition of cultured yeast normally occurs with the yeast first in a dried or "inactive" state and is reactivated in warm water or diluted grape juice prior to being added to themust. To thrive and be active in fermentation, the yeast needs access to a continuous supply ofcarbon,nitrogen,sulfur,phosphorus as well as access to variousvitamins andminerals. These components are naturally present in the grapemust but their amount may be corrected by adding nutrients to the wine, in order to foster a more encouraging environment for the yeast. Newly formulated time-release nutrients, specifically manufactured for wine fermentations, offer the most advantageous conditions for yeast.Oxygen is needed as well, but in wine making, the risk ofoxidation and the lack of alcohol production from oxygenated yeast requires the exposure of oxygen to be kept at a minimum.^[8]

Upon the introduction of active yeasts to the grape must,phosphates are attached to the sugar and the six-carbon sugarmolecules begin to be split into three-carbon pieces and go through a series ofrearrangement reactions. During this process, thecarboxylic carbon atom is released in the form of carbon dioxide with the remaining components becomingacetaldehyde. The absence of oxygen in thisanaerobic process allows the acetaldehyde to be eventually converted, by reduction, toethanol. During the conversion of acetaldehyde, a small amount is converted, by oxidation, toacetic acid which, in excess, can contribute to the wine fault known asvolatile acidity (vinegar taint). After the yeast has exhausted its life cycle, they fall to the bottom of the fermentation tank as sediment known aslees.^[9] Yeast ceases its activity whenever all of the sugar in must has been converted into other chemicals or whenever the alcohol content has reached 15% alcohol per unit volume; a concentration strong enough to halt the enzymatic activity of almost all strains of yeast.^[10]

Themetabolism ofamino acids and breakdown of sugars by yeasts has the effect of creating other biochemical compounds that can contribute to the flavor andaroma of wine. These compounds can be considered "volatile" likealdehydes,ethyl acetate,ester,fatty acids,fusel oils,hydrogen sulfide,ketones andmercaptans or "non-volatile" likeglycerol, acetic acid andsuccinic acid. Yeast also has the effect during fermentation of releasingglycoside hydrolase which canhydrolyse the flavor precursors ofaliphatics (a flavor component that reacts withoak),benzene derivatives,monoterpenes (responsible for floral aromas from grapes likeMuscat andTraminer),norisoprenoids (responsible for some of the spice notes inChardonnay), andphenols.

Some strains of yeasts can generate volatilethiols which contribute to the fruity aromas in many wines such as thegooseberry scent commonly associated withSauvignon blanc.
Brettanomyces yeasts are responsible for the "barnyard aroma" characteristic in some red wines likeBurgundy andPinot noir.^[11]

Methanol is not a major constituent of wine. The usual concentration range is between 0.1 g/liter and 0.2 g/liter. These small traces have no adverse effect on people and no direct effect on the senses.^[12]

During fermentation, there are several factors that winemakers take into consideration, with the most influential to ethanol production being sugar content in the must, the yeast strain used, and the fermentation temperature.^[13] The biochemical process of fermentation itself creates a lot of residualheat which can take the must out of the ideal temperature range for the wine. Typically, white wine is fermented between 18–20 °C (64–68 °F) though a wine maker may choose to use a higher temperature to bring out some of the complexity of the wine. Red wine is typically fermented at higher temperatures 20–30 °C (68–86 °F). Fermentation at higher temperatures may have adverse effect on the wine in stunning the yeast to inactivity and even "boiling off" some of the flavors of the wines. Some winemakers may ferment their red wines at cooler temperatures, more typical of white wines, in order to bring out more fruit flavors.^[9]

To control the heat generated during fermentation, the winemaker must choose a suitable vessel size or else use a cooling device. Various kinds of cooling devices are available, ranging from the ancientBordeaux practice of placing the fermentation vat atop blocks of ice to sophisticated fermentation tanks that have built-in cooling rings.^[14]

A risk factor involved with fermentation is the development of chemical residue and spoilage which can be corrected with the addition ofsulfur dioxide (SO₂), although excess SO₂ can lead to a wine fault. A winemaker who wishes to make a wine with high levels ofresidual sugar (like adessert wine) may stop fermentation early either by dropping the temperature of the must to stun the yeast or by adding a high level of alcohol (likebrandy) to the must to kill off the yeast and create afortified wine.^[9]

The ethanol produced through fermentation acts as an important co-solvent for the non-polar compounds that water cannot dissolve, such as pigments from grape skins, giving wine varieties their distinct color, and other aromatics. Ethanol and the acidity of wine act as an inhibitor to bacterial growth, allowing wine to be safely kept for years in the absence of air.^[15]

In winemaking, there are different processes that fall under the title of "Fermentation" but might not follow the same procedure commonly associated with wine fermentation.

Bottle fermentation is a method ofsparkling wine production, originating in theChampagne region where after thecuvee has gone through a primary yeast fermentation the wine is then bottled and goes through a secondary fermentation where sugar and additional yeast known asliqueur de tirage is added to the wine. This secondary fermentation is what creates the carbon dioxide bubbles that sparkling wine is known for.^[16]

The process ofcarbonic maceration is also known aswhole grape fermentation where instead of yeast being added, the grapes fermentation is encouraged to take place inside the individual grape berries. This method is common in the creation ofBeaujolais wine and involves whole clusters of grapes being stored in a closed container with the oxygen in the container being replaced with carbon dioxide.^[17] Unlike normal fermentation where yeast converts sugar into alcohol, carbonic maceration works by enzymes within the grape breaking down the cellular matter to formethanol and other chemical properties. The resulting wines are typically soft and fruity.^[18]

Instead of yeast,bacteria play a fundamental role inmalolactic fermentation which is essentially the conversion ofmalic acid intolactic acid. This has the benefit of reducing some of the tartness and making the resulting wine taste softer. Depending on the style of wine that the winemaker is trying to produce, malolactic fermentation may take place at the very same time as the yeast fermentation.^[19] Alternatively, some strains of yeast may be developed that can convert L-malate to L-lactate during alcohol fermentation.^[20] For example,Saccharomyces cerevisiae strain ML01 (S. cerevisiae strain ML01), which carries a gene encoding malolactic enzyme fromOenococcus oeni and a gene encoding malate permease fromSchizosaccharomyces pombe.S. cerevisiae strain ML01 has received regulatory approval in both Canada and the United States.^[21]

• Co-fermentation

• Fermentation in food processing
• Winemaking

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