Question: For what reason is pure yeast cultures added to the must?
About the article "Yeast", section "Wine".

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Yeast is used in winemaking, where it converts the sugars present in grape juice (must) into ethanol.
Yeast is normally already present on grape skins.
Fermentation can be done with this endogenous "wild yeast," but this procedure gives unpredictable results, which depend upon the exact types of yeast species present.
For this reason, a pure yeast culture is usually added to the must; this yeast quickly dominates the fermentation.
The wild yeasts are repressed, which ensures a reliable and predictable fermentation.
Most added wine yeasts are strains of "S. cerevisiae", though not all strains of the species are suitable.
Different "S. cerevisiae" yeast strains have differing physiological and fermentative properties, therefore the actual strain of yeast selected can have a direct impact on the finished wine.
Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.
The growth of some yeasts, such as "Zygosaccharomyces" and "Brettanomyces", in wine can result in wine faults and subsequent spoilage.
"Brettanomyces" produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds.
Together, these compounds are often referred to as ""Brettanomyces" character", and are often described as "antiseptic" or "barnyard" type aromas.
"Brettanomyces" is a significant contributor to wine faults within the wine industry.
Researchers from University of British Columbia, Canada, have found a new strain of yeast that has reduced amines.
The amines in red wine and Chardonnay produce off-flavors and cause headaches and hypertension in some people.
About 30% of people are sensitive to biogenic amines, such as histamines.
Pure culture yeasts that are grown in a lab are often freeze dried and packaged for commercial use.
Prior to their addition into must, these yeasts need to be re-hydrated in "starter cultures" that must be carefully monitored (particularly in regards to temperature) to insure that the yeast cells are not killed off by cold shock.
Ideally winemakers want to add enough inoculum to have a viable cell population density of 5 million cells per milliliter.
The exact amount of freeze-dried culture varies by manufacturer and strain of yeast but it is often around 1 gram per gallon (or 25 grams per 100 liters).
Wines that could have potentially problematic fermentation (such as high sugar level late harvest or botryized wines) may have more yeast added.
Similarly, re-hydration procedures will also vary depending on the manufacturer and winery.
Yeast is often inoculated in a volume or water or grape must that is 5–10 times the weight of the dry yeast.
This liquid is often brought to temperature of 40°C (104°F) prior to the introduction of the yeast (though some yeast strains may need temperatures below 38°C (100°F)) 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 15°C (60°F).
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 5–10°C (9–18°F) 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 60% of the culture.
Additionally, surviving cells exposed to cold shock tend to see an increase in hydrogen sulfide production. 
Yeast is the microorganism that is responsible for fermentation in beer.
Yeast metabolises the sugars extracted from grains, which produces alcohol and carbon dioxide, and thereby turns wort into beer.
In addition to fermenting the beer, yeast influences the character and flavour.
The dominant types of yeast used to make beer are the top-fermenting "Saccharomyces cerevisiae" and bottom-fermenting "Saccharomyces uvarum".
"Brettanomyces" ferments lambics, and "Torulaspora delbrueckii" ferments Bavarian weissbier.
Before the role of yeast in fermentation was understood, fermentation involved wild or airborne yeasts.
A few styles such as lambics rely on this method today, but most modern fermentation adds pure yeast cultures. 
Saccharomyces bayanus is a yeast of the genus "Saccharomyces", and is used in winemaking and cider fermentation, and to make distilled beverages.
Saccharomyces bayanus like Saccharomyces pastorianus, is now accepted to be the result of multiple hybridisation events between three pure species, Saccharomyces uvarum, Saccharomyces cerevisiae and Saccharomyces eubayanus.
Notably, most commercial yeast cultures sold as pure S. bayanus for wine making, e.g.
Lalvin EC-1118 strain, have been found to contain S. cerevisiae cultures instead S. bayanus is used intensively in comparative genomics studies .
Based on a computation-based experimental design system, Caudy et al.
generated a rich resource for expression profiles for S. bayanus, which has been used in several comparative studies in yeast systems, including expression patterns and nucleosome profiles .  
It is not known when yeast was first used to bake bread; the earliest definite records come from Ancient Egypt.
Researchers speculate that a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking.
The resulting bread would have been lighter and tastier than the previous hard flatbreads.
It is generally assumed that the earliest forms of leavening were likely very similar to modern sourdough; the leavening action of yeast would have been discovered from its action on flatbread doughs, and would have been either cultivated separately or transferred from batch to batch by means of previously mixed ("old") dough.
Also, the development of leavened bread seems to have developed in close proximity to the development of beer brewing, and barm from the beer fermentation process can also be used in bread making.
Without an understanding of microbiology, early bakers would have had little ability to directly control yeast cultures, but still kept locally interesting cultures by reusing doughs and starters to leaven later batches.
However, it became possible to isolate and propagate favored yeast strains in the same manner as was done in the beer industry, and it eventually became practical to propagate yeast in a slurry with a composition similar to beer wort, usually including malted barley and wheat flour.
Such cultures (sometimes referred to in old American cookery as "emptins", from their origins as the dregs of beer or cider fermentation) would become the ancestors of modern baker's yeast, as, in general, they were carefully maintained to avoid what would later be discovered to be bacterial contamination, including using preservatives such as hops as well as boiling the growth medium.
In the 19th century, bread bakers obtained their yeast from beer brewers, and this led to sweet-fermented breads such as the Imperial "Kaiser-Semmel" roll, which in general lacked the sourness created by the acidification typical of "Lactobacillus".
However, beer brewers slowly switched from top-fermenting to bottom-fermenting yeast (both S. cerevisiae) and this created a shortage of yeast for making bread, so the Vienna Process was developed in 1846.
While the innovation is often popularly credited for using steam in baking ovens, leading to a different crust characteristic, it is notable for including procedures for high milling of grains (see Vienna grits), cracking them incrementally instead of mashing them with one pass; as well as better processes for growing and harvesting top-fermenting yeasts, known as press-yeast.
Refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains.
In 1879, Great Britain introduced specialized growing vats for the production of "S. cerevisiae", and in the United States around the turn of the century centrifuges were used for concentrating the yeast, making modern commercial yeast possible, and turning yeast production into a major industrial endeavor.
The slurry yeast made by small bakers and grocery shops became cream yeast, a suspension of live yeast cells in growth medium, and then compressed yeast, the fresh cake yeast that became the standard leaven for bread bakers in much of the Westernized world during the early 20th century.
During World War II, Fleischmann's developed a active dry yeast for the United States armed forces, which did not require refrigeration and had a longer shelf-life and better temperature tolerance than fresh yeast; it is still the standard yeast for US military recipes.
The company created yeast that would rise twice as fast, cutting down on baking time.
Lesaffre would later create instant yeast in the 1970s, which has gained considerable use and market share at the expense of both fresh and dry yeast in their various applications.