Everything about Fermentation Wine totally explained
The process of
fermentation in wine is the
catalyst function that turns
grape juice into an
alcoholic beverage. During fermentation
yeast interact with
sugars in the juice to create
ethanol, commonly known as ethyl alcohol, and
carbon dioxide (as a
by-product). In
winemaking the
temperature and speed of fermentation is an important consideration as well as the levels of
oxygen present in the
must at the start of the fermentation. The risk of
stuck fermentation and the development of several
wine faults can also occur during this stage which can last anywhere from 5 to 14 days for
primary fermentation and potentially another 5 to 10 days for a
secondary fermentation. Fermentation may be done in stainless steel tanks, which is common with many white wines like
Riesling, in an open wooden vat, inside a
wine barrel and inside the
wine bottle itself like in
the production of many
sparkling wines.
History
While there may never be evidence to pinpoint the moment when fermentation was first observed-the natural occurrence of fermentation, with little need for human interaction, makes it likely that fermentation was observed quite early in human history. The earliest uses of the word "Fermentation" in relation to winemaking was in reference to the appearance "boiling" within the must that came from the
anaerobic reaction of the yeast to the sugars in the grape juice and the release of carbon dioxide. The
Latin fervere 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 the
Embden–Meyerhof–Parnas pathway by
Gustav Embden,
Otto Fritz Meyerhof and
Jakub Karol Parnas in the early 20th century contributed more to the understanding of the complex chemical processes involved the conversion of sugar to alcohol.
Process
In winemaking there are distinctions made between
ambient yeasts which are naturally present in wine cellars, vineyards and on the grapes themselves (sometimes known as a grape's "bloom" or "blush") and
cultured yeast which are specifically isolated and inoculated for use in winemaking. The most common
genus of wild yeasts found in winemaking include
Candida, Klöckera/Hanseniaspora, Metschnikowiaceae, Pichia and
Zygosaccharomyces. Wild yeasts can produce high-quality, unique-flavored wines; however, they're often unpredictable and may introduce less desirable traits to the wine, and can even contribute to spoilage. Traditional wine makers, particularly in Europe, advocate use of ambient yeast as a characteristic of the region's
terroir; nevertheless, many winemakers prefer to control fermentation with predictable cultured yeast. The cultured yeasts most commonly used in winemaking belong to the
Saccharomyces cerevisiae (also known as "sugar yeast") species. Within this species are several hundred different
strains of yeast that be used during fermentation to affect the heat or vigor of the process and enhance or suppress certain flavor characteristics of the
varietal. The use of different strains of yeasts are a major contributor to the diversity of wine, even among the same grape variety.
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 the must. To thrive and be active in fermentation, the yeast needs access to a continuous supply of
carbon,
nitrogen,
sulfur,
phosphorus as well as access to various
vitamins and
minerals. These components are naturally present in the grape must but their amount may be corrected by adding nutrient packets to the wine, in order to foster a more encouraging environment for the yeast.
Oxygen is needed as well but in wine making the risk of
oxidation and the lack of alcohol production from oxygenated yeast requires the exposure of oxygen to be kept at a minimum.
Upon the introduction of active yeasts to the grape must,
phosphates are attached to the sugar and the six-carbon sugar
molecules begin to be split into three-carbon pieces and go through a series of
rearrangement reactions. During this process the
carboxylic carbon atom is released in the form of carbon dioxide with the remaining components becoming
acetaldehyde. The absence of oxygen in this
anaerobic process allows the acetaldehyde to be eventually converted, by reduction, to
ethanol. During the conversion of acetaldehyde a small amount is converted, by oxidation, to
acetic acid which, in excess, can contribute to the wine fault known as
volatile acidity (vinegar taint). After the yeast has exhausted its life cycle they fall to the bottom of the fermentation tank as sediment known as
lees.
Other compounds involved
The
metabolism of
amino acids and breakdown of sugars by yeasts has the affect of creating other biochemical compounds that can contribute to the flavor and aroma of wine. These compounds can be considered "
volatile" like
aldehydes,
ethyl acetate,
ester,
fatty acids,
fusel oils,
hydrogen sulfide,
ketones and
mercaptans) or "non-volatile" like
glycerol, acetic acid and
succinic acid. Yeast also has the effect during fermentation of releasing
glycoside hydrolase which can
hydrolyse the flavor precursors of
aliphatics (a flavor component that reacts with
oak),
benzene derivities,
monoterpenes (responsible for floral aromas from grapes like
Muscat and
Traminer),
norisoprenoids (responsible for some of the spice notes in
Chardonnay), and
phenols.
Some strains of yeasts can generate volatile
thiols which contribute to the fruity aromas in many wines such as the
gooseberry scent commonly associates with
Sauvignon blanc.
Brettanomyces yeasts are responsible for the "barnyard aroma" characteristic in some red wines like
Burgundy Pinot noir.
Winemaking considerations
During fermentation there are several factors that winemakers take into consideration. The most notable is that of the internal temperature of the must. The biochemical process of fermentation itself creates a lot of residual
heat which can take the must out of the ideal temperature range for the wine. Typically white wine is fermented between 64-68
°F (18-20
°C) 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 up to 85
°F (29 °C). 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.
A risk factor involved with fermentation is the development of chemical residue and spoilage which can be corrected with the addition of
sulfur dioxide (SO
2), although excess SO
2 can lead to a wine fault. A winemaker who wishes to make a wine with high levels of
residual sugar (like a
dessert 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 (like
brandy) to the must to kill off the yeast and create a
fortified wine.
Carbonic maceration
The process of
carbonic maceration is also known as
whole grape fermentation where instead of yeast being added to grape must fermentation is encouraged to take place inside the individual grape berries. This method is common in the creation of
Beaujolais wine and involves whole clusters of grapes being stored in a closed container with the oxygen in the container being replaced with carbon dioxide. Unlike normal fermentation where yeast converts sugar into alcohol, carbonic maceration works by enzymes within the grape breaking down the cellular matter to form
ethanol and other chemical properties. The resulting wines are typically soft and fruity.
Malolactic fermentation
Instead of yeast,
bacteria plays a fundamental role in
malolactic fermentation which is essentially the conversion of
malic acid into
lactic 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 same time as the yeast fermentation.
Further Information
Get more info on 'Fermentation Wine'.
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