Information about Fermentation (wine)

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 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. The risk of stuck fermentation and the development of several wine faults can also occur during this stage which can last 5 to 10 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.^{[1] [2]}

History

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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.^[3] 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, literrally, 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 mediate 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.^[4]

Process

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In winemaking there is distinctions made between ambient yeasts which are naturally present in wine cellars, vineyards and on the grapes themselves (sometimes known as a grape's "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 are typically unpredictable and unstable and could introduce less desirable traits to the wine and even contribute to spoilage. While many traditional wine makers, particularly in Europe, advocate for the use of ambient as a characteristic of the region's terroir many winemakers prefer to use cultured yeast that are much more predictable and controlled during fermentation. The cultured yeasts most commonly used in winemaking belong 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.^[5]

The addition of cultured yeast normally occurs with the yeast first in a dried or "inactive" state and needing to be reactivated in warm water or diluted grape juice prior to being added to the must. In order to thrive and be active in fermentation the yeast needs access to a continuous supply of carbon, nitrogen, sulfur, phosphorous as well as access to various vitamins and minerals. Some of these components are naturally present in the grape must but some winemakers prefer to add nutrient packets to the wine to foster a more encouraging environment for the yeast. Oxygen is needed as well but in winemaking the risk of oxidation and the lack of alcohol production from oxygenated yeast requires the exposure of oxygen to be kept at a minimum.^[6] 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 to ethanol alcohol. During the conversion of acetaldehyde a small amount is converted in acetic acid which, in excess, can contribute to the wine fault known as Volatile acidity. After the yeast has exhausted its life cycle they fall to the bottom of the fermentation tank as sediment known as lees.^[7]

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. This 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. ^[8]

Winemaking considerations

During fermentation there are several factors that winemakers take into consideration during the process. The most notable is that of the internal temperature of the must. The chemical 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 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. 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. To control the heat generated during fermentat ion many winemakers will use cooling devices of various sorts from the ancient Bordeaux traditions of placing the fermentation vat on top of blocks of ice to todays modern use of sophisticated fermentation tanks with built in cooling rings.^[9]

A risk factor involved with fermentation is the development of chemical residue and spoilage which can be corrected with the addition of sulfur dioxide those excess of that chemical can lead to a wine fault. If a winemaker wishes to make a wine with high levels of residual sugar (like a dessert wine) they may wish to stop fermentation early either by dropping the temperatures to 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.^[7]

Other types of fermentation

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

Bottle fermentation is a method of sparkling wine production originating in the Champagne region where after the cuvee has gone through a primary yeast fermentation the wine is then bottled and goes through a secondary fermentation known as liqueur de tirage where sugar and additional yeast is added to the wine. This secondary fermentation is what creates the carbon dioxide bubbles that sparkling wine is known for.^[10]

Carbonic maceration

The process of Carbonic maceration is also known as whole cluster 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. The wild ambient yeasts naturally present on the grape skins start the fermentation process and, with the pressure of the carbon dioxide gas, pass into the grape berry and cause this form of internal fermentation. The resulting wines are typically soft and fruity.^[11]

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.^[12]

References