Question: What is the major spoilage yeast of wine?
About the article "Yeast", section "Food spoilage".

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Once the MLF is complete, the red wine is usually racked (decanted) off its lees (dead yeast cells and other solids), and has sulphur dioxide preservative added to avoid oxidation and bacterial spoilage. 
4-Ethylguaiacol, often abbreviated to 4-EG, is a phenolic compound with the molecular formula C9H12O2.
It is produced along with 4-ethylphenol (4-EP) in wine and beer by the spoilage yeast "Brettanomyces".
When it is produced by the yeast to concentrations greater than the sensory threshold of >600 µg/L, it can contribute bacon, spice, clove, or smoky aromas to the wine.
On their own these characters can be quite attractive in a wine, however as the compound usually occurs with 4-EP whose aromas can be more aggressive, the presence of the compound often signifies a wine fault.
The ratio in which 4-EP and 4-EG are present can greatly affected the organoleptic properties of the wine. 
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. 
In winemaking, there are distinctions made between "ambient yeast"s 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.
It should be noted that few yeast, and lactic and acetic acid bacterial colonies naturally live on the surface of grapes, 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.
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.
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.
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.
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. 
Yeasts are able to grow in foods with a low pH (5.0 or lower) and in the presence of sugars, organic acids, and other easily metabolized carbon sources.
During their growth, yeasts metabolize some food components and produce metabolic end products.
This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled.
The growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as syrups and jams.
The yeast of the "Zygosaccharomyces" genus have had a long history as spoilage yeasts within the food industry.
This is mainly because these species can grow in the presence of high sucrose,ethanol, acetic acid, sorbic acid, benzoic acid, and sulphur dioxide concentrations, representing some of the commonly used food preservation methods.
Methylene blue is used to test for the presence of live yeast cells.
In oenology, the major spoilage yeast is "Dekkera bruxellensis ".