Food spoilage
What is wine's major spoilage yeast?
Yeast
Wine tasting Yeast Wine fault Zygosaccharomyces bailii Red wine
Wine_tasting-Connoisseur_wine_tasting Yeast-Food_spoilage Wine_fault-Acetic_acid Zygosaccharomyces_bailii-Physiological_properties Red_wine-Racking
A wine's quality can be judged by its bouquet and taste. The bouquet is the total aromatic experience of the wine. Assessing a wine's bouquet can also reveal faults such as cork taint, oxidation due to age, overexposure to oxygen, or lack of preservatives and wild yeast contamination due to "Brettanomyces" or acetobacter yeasts. Although low levels of "Brettanomyces" aromatic characteristics can be a positive attribute, giving the wine a distinctive character, generally it is considered a wine spoilage yeast. The bouquet of wine is best revealed by gently swirling the wine in a wine glass to expose it to more oxygen and release more aromatic etheric, ester, and aldehyde molecules that comprise the essential components of a wine's bouquet. Sparkling wine should not be swirled to the point of releasing bubbles. Pausing to experience a wine's bouquet aids the wine taster in anticipating the wine's flavors. The "nose" of a wine - its bouquet or aroma - is the major determinate of perceived flavor in the mouth. Once inside the mouth, the aromatics are further liberated by exposure to body heat, and transferred retronasally to the olfactory receptor site. It is here that the complex taste experience characteristic of a wine actually commences. Thoroughly tasting a wine involves perception of its array of taste and mouthfeel attributes, which involve the combination of textures, flavors, weight, and overall "structure". Following appreciation of its olfactory characteristics, the wine taster savors a wine by holding it in the mouth for a few seconds to saturate the taste buds. By pursing ones lips and breathing through that small opening oxygen will pass over the wine and release even more esters. When the wine is allowed to pass slowly through the mouth it presents the connoisseur with the fullest gustatory profile available to the human palate. The acts of pausing and focusing through each step distinguishes wine tasting from simple quaffing. Through this process, the full array of aromatic molecules is captured and interpreted by approximately 15 million olfactory receptors, comprising a few hundred olfactory receptor classes. When tasting several wines in succession, however, key aspects of this fuller experience (length and finish, or aftertaste) must necessarily be sacrificed through expectoration. Although taste qualities are known to be widely distributed throughout the oral cavity, the concept of an anatomical "tongue map" yet persists in the wine tasting arena, in which different tastes are believed to map to different areas of the tongue. A widely accepted example is the misperception that the tip of the tongue uniquely tells how sweet a wine is and the upper edges tell its acidity. 
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 ".
Acetic acid in wine, often referred to as volatile acidity (VA) or "vinegar taint", can be contributed by many wine spoilage yeasts and bacteria. This can be from either a by-product of fermentation, or due to the spoilage of finished wine. Acetic acid bacteria, such as those from the genera "Acetobacter" and "Gluconobacter" produce high levels of acetic acid. The sensory threshold for acetic acid in wine is >700 mg/L, with concentrations greater than 1.2-1.3 g/L becoming unpleasant. There are different opinions as to what level of volatile acidity is appropriate for higher quality wine. Although too high a concentration is sure to leave an undesirable, 'vinegar' tasting wine, some wine's acetic acid levels are developed to create a more 'complex', desirable taste. The renowned 1947 Cheval Blanc is widely recognized to contain high levels of volatile acidity. 
Among the "Zygosaccharomyces" spoilage species, "Z. bailii" possesses the most pronounced and diversified resistance characteristics, enabling it to survive and proliferate in very stressful conditions. It appears that "Z. bailii" prefers ecological environments characterized by high osmotic conditions. The most frequently described natural habitats are dried or fermented fruits, tree exudates (in vineyards and orchards), and at various stages of sugar refining and syrup production. Besides, it is seldom to encounter "Z. bailii" as a major spoilage agent in unprocessed foods; usually the yeast only attains importance in processed products when the competition with bacteria and moulds is reduced by intrinsic factors such as pH, water activity (aw), preservatives, etc. 
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. 