Wine Spoilage Microorganisms
Spoilage by Yeasts in Wine Prodcution
Some yeast genera can be considered as spoilage microorganisms can cause nutritional and sensory quality degradation and consequently lead to major economic losses. The yeast genera that are often found in must and wine include Brettanomyces, Candida, Hansenula, Pichia, Zygosaccharomyces, and Saccharomyces.
Brettanomyces
Of the spoilage yeasts, Brettanomyces bruxellensis are probably the most notorious. Yeasts of the genera Brettanomyces, commonly referred to as Brett, is a spoilage yeast commonly found in fermenting musts and in red wine, especially barrel-matured red wines. Brett can occur in white wines too but not as common. Chardonnay is the white variety most frequently affected by Brett characters. The main aroma compounds produced by Brettanomyces are 4-ethylphenol (4-EP), 4-ethylguaiacol (4-EG), and 4-ethylcatechol (4-EC). 4-EP has Band-Aid, plastic and medicinal characters, 4-EG has smoky, spicy, sweaty characters, and 4-EC has bacon, spice, clove, or smoky characters.
Conditions Favoring Brettanomyces
Brettanomyces bruxellensis is most likely to taint wines that are high in alcohol and low in acidity, with excess nitrogen and with inadequate levels of free sulfur dioxide. Brettanomyces is capable of growing in dry wines with glucose+fructose levels less than 0.2 g/L. In addition to glucose and fructose it may use many of the other wine components as a carbon source. These include the nonfermentable sugars, glycerol, ethanol, and malic acid. In barrels it may also use cellobiose.
Monitoring Wines for Brettanomyces
The key stages of winemaking that require monitoring for Brettanomyces bruxellensis are before alcoholic fermentation, during the so-called cold soak of red musts (Section 3.4), and at the end of alcohol fermentation, and before the onset of the malolactic fermentation. The period between alcoholic and malolactic fermentation is the most critical phase of winemaking to counter the detrimental consequences of Brettanomyces growth in a wine.
Management Strategies
Brettanomyces bruxellensis is particularly difficult to control because its presence may go unnoticed until the wine is permanently tainted. The growth of Brettanomyces in wine is affected by a range of factors, some of which are interlinked. It is important to have an integrated strategy that takes into account the interdependence of diverse wine parameters, such as grape quality, sulfur dioxide, pH, wine temperature, nutrients, oxygen, barrel condition, and oenological practices.
Surface-Film Yeasts
Some yeasts, called film surface-film yeasts, grow oxidatively forming a film layer on top of stored. The species of the genera Candida, Pichia, and Hansenula among others have been associated with these traits (Figure 15.2). Initially, the yeasts can appear as floating flowers. If allowed to continue, growth may rapidly develop into a film of chalky or filamentous white substance that is dry enough to appear dusty. The growth of surface, or film yeasts is reduced during fermentation because they prefer oxidative conditions, however, significant growth may occur during the early stages of fermentation.
Management Strategies
The most important aspect of preventing film yeasts is the proper topping of tanks and barrels, as these yeasts depend on the presence of oxygen. If tanks can be kept only partially full, frequent (bi-weekly) sparging with nitrogen or argon is advised to keep oxygen out of the headspace. The suspected organisms are often part of the airborne microflora and will contaminate the wine's surface whenever possible.
Zygosaccharomyces
Zygosaccharomyces causes spoilage by forming gas, sediment, and/or cloudiness in bottled wines. Given the visual nature of the spoiling effect it is a greater concern in white wines. The yeast can become a problem in wineries using poor sanitation at bottling. Zygosaccharomyces is particularly a problem in wineries that sweeten wines with grape concentrate at bottling and once contamination of the bottling line has occurred, it is very difficult to eliminate.
Management Strategies
Unlike most wine microbes which are inactivated by 0.8 mg/L free molecular sulfur dioxide, Zygosaccharomyces is resistant to free molecular sulfur dioxide levels up to 3 mg/L. Populations as low as 1 to 2 cells/250 mL can cause refermentation. Dry wines with no residual glucose+fructose (less than 1g/L) are microbiologically stable with respect to Zygosaccharomyces activity.
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