Question: Why are pure yeast cultures added to the must?
About the article "Yeast", section "Wine".

Candidate passages:
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. 
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.
Eukaryotic cell culture.
Isolation of pure cultures.
For single-celled eukaryotes, such as yeast, the isolation of pure cultures uses the same techniques as for bacterial cultures.
Pure cultures of multicellular organisms are often more easily isolated by simply picking out a single individual to initiate a culture.
This is a useful technique for pure culture of fungi, multicellular algae, and small metazoa, for example.m/M;Developing pure culture techniques is crucial to the observation of the specimen in question.
The most common method to isolate individual cells and produce a pure culture is to prepare a streak plate.
The streak plate method is a way to physically separate the microbial population, and is done by spreading the inoculate back and forth with an inoculating loop over the solid agar plate.
Upon incubation, colonies will arise and single cells will have been isolated from the biomass. 
The most familiar yeast in food production, "Saccharomyces cerevisiae", has been used in brewing and baking for thousands of years."S.
cerevisiae" feeds on the sugars present in the bread dough and produces the gas carbon dioxide.
This forms bubbles within the dough, causing it to expand and the bread to rise.
Several different yeasts are used in brewing beer, where they ferment the sugars present in malted barley to produce alcohol.
One of the most common is "S. cerevisiae".
The same strain of "S. cerevisiae" which can also be used in breadmaking is used to make ale-type beers.
It is known as a top-fermenting yeast because it creates a foam on the top of the brew.
Bottom-fermenting yeasts, such as "S. pastorianus", are more commonly used to make lagers.
They ferment more of the sugars in the mixture than top-fermenting yeasts, which gives a cleaner taste.
The alcohol in wine is formed by the fermentation of the sugars in grape juice, with carbon dioxide as a by-product.
Yeast is naturally present on grapeskins, and this alone can be sufficient for the fermentation of sugars to alcohol to occur.
A pure yeast culture, most often "S. cerevisiae", is usually added to ensure the fermentation is reliable.
Other yeast cultures like "Pichia", "Torulaspora" and "Kluyveromyces" are naturally present or added to create special flavours in the wine.
Sparkling wine, including champagne, is made by adding further yeast to the wine when it is bottled.
The carbon dioxide formed in this second fermentation is trapped as bubbles.
Yeasts are also used to produce kefir products, semi-soft ripened cheeses and fermented soy drinks.