Fermentation in winemaking

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Fermenting must Mthomebrew must.JPG
Fermenting must

The process of fermentation in winemaking turns grape juice into an alcoholic beverage. During fermentation, yeasts transform sugars present in the juice into ethanol and carbon dioxide (as a by-product). In winemaking, the temperature and speed of fermentation are important considerations 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 as in the production of many sparkling wines. [1] [2]

Contents

History

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 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 in the conversion of sugar to alcohol. [4] In the early 2010s, New Jersey based wine tech company GOfermentor invented an automated winemaking device that ferments in single-use liners similar to the single-use bioreactor. [5] [6]

Process

"Bloom", visible as a dusting on the berries Yeast on grapes.jpg
"Bloom", visible as a dusting on the berries

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 genera 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 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, [7] but 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 can 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 is a major contributor to the diversity of wine, even among the same grape variety. [8] 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. [9]

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 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 of oxidation and the lack of alcohol production from oxygenated yeast requires the exposure of oxygen to be kept at a minimum. [10]

Dry winemaking yeast (left) and yeast nutrients used in the rehydration process to stimulate yeast cells. Yeast and nutrients.JPG
Dry winemaking yeast (left) and yeast nutrients used in the rehydration process to stimulate yeast cells.

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. [11] 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. [12]

Other compounds involved

The metabolism of amino acids and breakdown of sugars by yeasts has the effect 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 derivatives, 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 associated with Sauvignon blanc.
Brettanomyces yeasts are responsible for the "barnyard aroma" characteristic in some red wines like Burgundy and Pinot noir. [13]

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. [14]

Winemaking considerations

Carbon dioxide activity is visible during the fermentation process in the form of bubbles in the must. Fermenting.jpg
Carbon dioxide activity is visible during the fermentation process in the form of bubbles in the must.

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. [15] 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 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. [11]

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 ancient Bordeaux practice of placing the fermentation vat atop blocks of ice to sophisticated fermentation tanks that have built-in cooling rings. [16] иьИс

A risk factor involved with fermentation is the development of chemical residue and spoilage which can be corrected with the addition of sulfur dioxide (SO2), although excess SO2 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. [11]

The ethanol produced through fermentation acts as an important co-solvent to the non-polar compound 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. [17]

Other types of fermentation

A California Chardonnay that shows it has been barrel fermented. Barrel fermented California Chard.jpg
A California Chardonnay that shows it has been barrel fermented.

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 where sugar and additional yeast known as liqueur de tirage is added to the wine. This secondary fermentation is what creates the carbon dioxide bubbles that sparkling wine is known for. [18]

Carbonic maceration

The process of carbonic maceration is also known as whole 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 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. [19] 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. [20]

Malolactic fermentation

Instead of yeast, bacteria play 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 very same time as the yeast fermentation. [21] Alternatively, some strains of yeast may be developed that can convert L-malate to L-lactate during alcohol fermentation. [22] For example, Saccharomyces cerevisiae strain ML01 (S. cerevisiae strain ML01), which carries a gene encoding malolactic enzyme from Oenococcus oeni and a gene encoding malate permease from Schizosaccharomyces pombe. S. cerevisiae strain ML01 has received regulatory approval in both Canada and the United States. [23]

See also

Related Research Articles

<span class="mw-page-title-main">Winemaking</span> Production of wine

Winemaking, wine-making, or vinification is the production of wine, starting with the selection of the fruit, its fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over millennia. There is evidence that suggests that the earliest wine production took place in Georgia and Iran around 6000 to 5000 B.C. The science of wine and winemaking is known as oenology. A winemaker may also be called a vintner. The growing of grapes is viticulture and there are many varieties of grapes.

<span class="mw-page-title-main">Oak (wine)</span> Barrel used in wine making

Oak is used in winemaking to vary the color, flavor, tannin profile and texture of wine. It can be introduced in the form of a barrel during the fermentation or aging periods, or as free-floating chips or staves added to wine fermented in a vessel like stainless steel. Oak barrels can impart other qualities to wine through evaporation and low level exposure to oxygen.

<span class="mw-page-title-main">Carbonic maceration</span> Winemaking technique

Carbonic maceration is a winemaking technique, often associated with the French wine region of Beaujolais, in which whole grapes are fermented in a carbon dioxide rich environment before crushing. Conventional alcoholic fermentation involves crushing the grapes to free the juice and pulp from the skin with yeast serving to convert sugar into ethanol. Carbonic maceration ferments most of the juice while it is still inside the grape, although grapes at the bottom of the vessel are crushed by gravity and undergo conventional fermentation. The resulting wine is fruity with very low tannins. It is ready to drink quickly but lacks the structure for long-term aging. In extreme cases such as Beaujolais nouveau, the period between picking and bottling can be less than six weeks.

<span class="mw-page-title-main">Red wine</span> Wine made from dark-colored grape varieties

Red wine is a type of wine made from dark-colored grape varieties - The color of the wine can range from intense violet, typical of young wines, through to brick red for mature wines and brown for older red wines. The juice from most purple grapes is greenish-white, the red color coming from anthocyan pigments present in the skin of the grape. Much of the red wine production process involves extraction of color and flavor components from the grape skin.

<span class="mw-page-title-main">Malolactic fermentation</span> Process in winemaking

Malolactic conversion is a process in winemaking in which tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation is most often performed as a secondary fermentation shortly after the end of the primary fermentation, but can sometimes run concurrently with it. The process is standard for most red wine production and common for some white grape varieties such as Chardonnay, where it can impart a "buttery" flavor from diacetyl, a byproduct of the reaction.

<span class="mw-page-title-main">Maury AOC</span>

Maury is an Appellation d'Origine Contrôlée (AOC) for fortified vin doux naturel wines made in the Roussillon wine region of France. Almost all wines are red, made from at least 75% Grenache noir (Garnacha). Other permitted grapes are Grenache blanc, Grenache gris, Macabeu (Macabeo), Malvoisie du Roussillon (Tourbat), Syrah, Muscat and other local varieties. Although the grapes are different, they are used and marketed very much like port. It is made in the communes of Maury, Saint-Paul-de-Fenouillet, Lesquerde, Tautavel and Rasiguères. The AOC was granted in 1936.

<span class="mw-page-title-main">Kilju</span> Finnish home made alcoholic beverage

Kilju is the Finnish word for home made alcoholic beverage typically made of sugar, yeast, and water, making it both affordable and cheap to produce. The ABV is around 15–17%, and since it does not contain a sweet reserve it is completely dry. Crude fermented water may be distilled to moonshine. Kilju for consumption is clarified to avoid wine fault. It is a flax-colored alcoholic beverage with no discernible taste other than that of ethanol. It can be used as an ethanol base for drink mixers.

<span class="mw-page-title-main">Maceration (wine)</span> Winemaking process where grape skins and seeds are kept in contact with the juice

Maceration is the winemaking process where the phenolic materials of the grape—tannins, coloring agents (anthocyanins) and flavor compounds—are leached from the grape skins, seeds and stems into the must. To macerate is to soften by soaking, and maceration is the process by which the red wine receives its red color, since raw grape juice is clear-grayish in color. In the production of white wines, maceration is either avoided or allowed only in very limited manner in the form of a short amount of skin contact with the juice prior to pressing. This is more common in the production of varietals with less natural flavor and body structure like Sauvignon blanc and Sémillon. For Rosé, red wine grapes are allowed some maceration between the skins and must, but not to the extent of red wine production.

A wine fault is a sensory-associated (organoleptic) characteristic of a wine that is unpleasant, and may include elements of taste, smell, or appearance, elements that may arise from a "chemical or a microbial origin", where particular sensory experiences might arise from more than one wine fault. Wine faults may result from poor winemaking practices or storage conditions that lead to wine spoilage.

<span class="mw-page-title-main">Stuck fermentation</span>

A stuck fermentation occurs in brewing beer or winemaking when the yeast become dormant before the fermentation has completed. Unlike an "arrested fermentation" where the winemaker intentionally stops fermentation, a stuck fermentation is an unintentional and unwanted occurrence that can lead to the wine being spoiled by bacteria and oxidation. There are several potential causes of a stuck fermentation; the most common are excessively high temperatures killing off the yeast, or a must deficient in the nitrogen food source needed for the yeast to thrive. Once the fermentation is stuck, it is very difficult to restart due to a chemical compound released by dying yeast cells that inhibit the future growth of yeast cells in the batch. Winemakers often take several steps to limit the possibility of a stuck fermentation occurring, such as adding nitrogen to the must in the form of diammonium phosphate or using cultured yeast with a high temperature and alcohol tolerance. These steps will each have their own subtle or dramatic effect on the resulting flavors and quality of the wine.

Secondary fermentation is a process commonly associated with winemaking, which entails a second period of fermentation in a different vessel than the one used to start the fermentation process. An example of this would be starting fermentation in a carboy or stainless steel tank and then moving it over to oak barrels. Rather than being a separate, second fermentation, this is most often one single fermentation period that is conducted in multiple vessels. However, the term does also apply to procedures that could be described as a second and distinct fermentation period.

<span class="mw-page-title-main">Outline of wine</span> Alcoholic drink made by fermentation of grapes or other fruits and foods

The following outline is provided as an overview of and topical guide to wine:

<span class="mw-page-title-main">Lychee wine</span> Chinese dessert wine

Lychee wine is a full-bodied Chinese dessert wine made of 100% lychee fruit. This wine has a golden colour and rich, sweet taste. It is usually served ice cold, either straight up or on the rocks with food. Lychee wine is believed to pair better with shellfish and Asian cuisine than with heavier meat dishes. This refreshing beverage can also be used as a cocktail mixer paired with other spirits.

Governo is a winemaking technique reportedly invented in Tuscany in the 14th century to help complete fermentation and stabilize the wine. The technique involves saving a batch of harvested grapes and allowing them to partially dry. If fermentation of the main batch starts to slow or appears to be nearing stuck fermentation, the half dried grapes are added to the must which then gives the yeast cells a new source of sugar to enliven the batch. From there, the must can be fermented dry or stopped with the wine having a higher level of residual sugar. The process was widely used in the Chianti zones until the advent of temperature controlled fermentation tanks. From Tuscany the technique spread to Marche and Umbria where it is sometimes used today. In the Marche the technique is most often used on wines made from the Verdicchio grape to counteract the grape's natural bitterness and to add some sweetness and frizzante qualities.

<span class="mw-page-title-main">Sugars in wine</span>

Sugars in wine are at the heart of what makes winemaking possible. During the process of fermentation, sugars from wine grapes are broken down and converted by yeast into alcohol (ethanol) and carbon dioxide. Grapes accumulate sugars as they grow on the grapevine through the translocation of sucrose molecules that are produced by photosynthesis from the leaves. During ripening the sucrose molecules are hydrolyzed (separated) by the enzyme invertase into glucose and fructose. By the time of harvest, between 15 and 25% of the grape will be composed of simple sugars. Both glucose and fructose are six-carbon sugars but three-, four-, five- and seven-carbon sugars are also present in the grape. Not all sugars are fermentable, with sugars like the five-carbon arabinose, rhamnose and xylose still being present in the wine after fermentation. Very high sugar content will effectively kill the yeast once a certain (high) alcohol content is reached. For these reasons, no wine is ever fermented completely "dry". Sugar's role in dictating the final alcohol content of the wine sometimes encourages winemakers to add sugar during winemaking in a process known as chaptalization solely in order to boost the alcohol content – chaptalization does not increase the sweetness of a wine.

<span class="mw-page-title-main">Acids in wine</span>

The acids in wine are an important component in both winemaking and the finished product of wine. They are present in both grapes and wine, having direct influences on the color, balance and taste of the wine as well as the growth and vitality of yeast during fermentation and protecting the wine from bacteria. The measure of the amount of acidity in wine is known as the “titratable acidity” or “total acidity”, which refers to the test that yields the total of all acids present, while strength of acidity is measured according to pH, with most wines having a pH between 2.9 and 3.9. Generally, the lower the pH, the higher the acidity in the wine. There is no direct connection between total acidity and pH. In wine tasting, the term “acidity” refers to the fresh, tart and sour attributes of the wine which are evaluated in relation to how well the acidity balances out the sweetness and bitter components of the wine such as tannins. Three primary acids are found in wine grapes: tartaric, malic, and citric acids. During the course of winemaking and in the finished wines, acetic, butyric, lactic, and succinic acids can play significant roles. Most of the acids involved with wine are fixed acids with the notable exception of acetic acid, mostly found in vinegar, which is volatile and can contribute to the wine fault known as volatile acidity. Sometimes, additional acids, such as ascorbic, sorbic and sulfurous acids, are used in winemaking.

This glossary of winemaking terms lists some of terms and definitions involved in making wine, fruit wine, and mead.

<span class="mw-page-title-main">Clarification and stabilization of wine</span> Wine clarification and stabilisation

In winemaking, clarification and stabilization are the processes by which insoluble matter suspended in the wine is removed before bottling. This matter may include dead yeast cells (lees), bacteria, tartrates, proteins, pectins, various tannins and other phenolic compounds, as well as pieces of grape skin, pulp, stems and gums. Clarification and stabilization may involve fining, filtration, centrifugation, flotation, refrigeration, pasteurization, and/or barrel maturation and racking.

<span class="mw-page-title-main">Yeast in winemaking</span> Yeasts used for alcoholic fermentation of wine

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.

<span class="mw-page-title-main">Yeast assimilable nitrogen</span> Form of nitrogen available to wine yeast to use during fermentation

Yeast assimilable nitrogen or YAN is the combination of free amino nitrogen (FAN), ammonia (NH3) and ammonium (NH4+) that is available for a yeast, e.g. the wine yeast Saccharomyces cerevisiae, to use during fermentation. Outside of the fermentable sugars glucose and fructose, nitrogen is the most important nutrient needed to carry out a successful fermentation that doesn't end prior to the intended point of dryness or sees the development of off-odors and related wine faults. To this extent winemakers will often supplement the available YAN resources with nitrogen additives such as diammonium phosphate (DAP).

References

  1. ↑ Jancis Robinson (ed): "The Oxford Companion to Wine" Third Edition, pp. 267–69. Oxford University Press 2006 ISBN   0198609906.
  2. ↑ Jancis Robinson: Jancis Robinson's Wine Course Third Edition, pp. 74–84. Abbeville Press 2003 ISBN   0789208830.
  3. ↑ H. Johnson: Vintage: The Story of Wine p. 16. Simon and Schuster 1989 ISBN   0671687026.
  4. ↑ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition, p. 267. Oxford University Press 2006 ISBN   0198609906.
  5. ↑ Nigro, Dana. "Taking the Water out of Winemaking". Wine Spectator, 2 October 2015.
  6. ↑ Cockcroft, Marlaina. "It's in the Bag". New Jersey Monthly, 15 March 2017.
  7. ↑ Gemma Beltran, Maria Jesus Torija, Maite Novo, Noemi Ferrer, Montserrat Poblet, Jose M. Guillamon, Nicholas Rozes, and Albert Mas. “Analysis of Yeast Populations During Alcohol Fermentation: A Six Year Follow-up Study”. pp. 3–4 Systematic and Applied Microbiology 25.2 (2002): 287–93.
  8. ↑ Jancis Robinson (ed): "The Oxford Companion to Wine" Third Edition, pp. 778–79. Oxford University Press 2006 ISBN   0198609906.
  9. ↑ Gemma Beltran, Maria Jesus Torija, Maite Novo, Noemi Ferrer, Montserrat Poblet, Jose M. Guillamon, Nicholas Rozes, and Albert Mas. “Analysis of Yeast Populations During Alcohol Fermentation: A Six Year Follow-up Study”. Systematic and Applied Microbiology 25. February 2002: 287–93.
  10. ↑ Jancis Robinson (ed): "The Oxford Companion to Wine", Third Edition p. 779. Oxford University Press 2006 ISBN   0198609906.
  11. 1 2 3 Jancis Robinson (ed): "The Oxford Companion to Wine", Third Edition p. 268. Oxford University Press 2006 ISBN   0198609906.
  12. ↑ "fermentation." Oddbins Dictionary of Wine. London: Bloomsbury Publishing Ltd, 2004. Credo Reference.
  13. ↑ J. Robinson (ed) The Oxford Companion to Wine Third Edition, p. 780. Oxford University Press 2006 ISBN   0198609906.
  14. ↑ Jackson, Ronald S. Wine Science Principles and Applications, p. 277. San Diego, California: Academic Press, 2008.
  15. ↑ Jackson, Ronald S.; Wine Science Principles and Applications, p. 276. San Diego, California: Academic Press, 2008. [ ISBN missing ].
  16. ↑ Jancis Robinson: Jancis Robinson's Wine Course, Third Edition, p. 82. Abbeville Press 2003 ISBN   0789208830.
  17. ↑ Jackson, Ronald S.: Wine Science Principles and Applications, p. 276. San Diego, California: Academic Press, 2008 [ ISBN missing ].
  18. ↑ K. MacNeil: The Wine Bible, pp. 168–69. Workman Publishing 2001 ISBN   1563054345.
  19. ↑ K. MacNeil: The Wine Bible, pp. 33–34. Workman Publishing 2001 ISBN   1563054345.
  20. ↑ D. Bird: "Understanding Wine Technology", pp. 89–92, DBQA Publishing 2005 ISBN   1891267914.
  21. ↑ K. MacNeil: The Wine Bible, p. 35. Workman Publishing 2001 ISBN   1563054345.
  22. ↑ "Wine Research Centre at UBC - Malolactic yeast ML01 – the Facts". Archived from the original on 2012-03-16. Retrieved 2012-03-05.
  23. ↑ "New substances: risk assessment summary EAU-224". Ec.gc.ca. October 2018. Retrieved 17 November 2018.
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