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A Technical Guide for Wine Producers
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WHAT'S UP IN THE VINEYARD AND CELLAR
Microbes in wine
Wine cannot be produced without microbes. Yeasts are required to convert grape juice into wine and malolactic bacteria may have a considerable influence on the taste and flavour of wine. It is unfortunately also true that undesirable microbes may occur, with detrimental consequences for the quality of the wine and potentially even spoilage. Although wine microbiology is one of the oldest disciplines in the wine industry, there is often confusion regarding the different microbes. This article presents an overview of the various microbes that may occur in wine.
As a result of the low pH of grape juice, most microbes are unable to grow in it. The formation of alcohol during the alcoholic fermentation process renders the growth environment for microbes even more unfavourable. Pathogens that are able to cause human disease are unable to survive in wine, only a few isolated yeasts and bacteria are able to do so.
Moulds may flourish in vineyards under favourable conditions, causing various vine diseases. In addition to extracting nutrients from the grapes, they are able to release the enzyme laccase, which, together with other compounds, may cause browning in juice and wines, eventually resulting in high sulphur dioxide levels. As a result of the physical damage to the grapes, it also becomes accessible to yeasts and bacteria that may have other negative consequences. Although it is unable to survive at alcohol levels above 0.5%, it is able to grow on wood or cork products, when trichloro anisole (TCA) may be formed which may end up in the wine through migration and impart a mouldy character. A mould such as Botrytis cinerea may have a beneficial impact on grapes under certain conditions, resulting in noble rot, which may be used to great advantage to make noble late harvest wines.
Yeasts may occur in the vineyard in various guises. Some may induce alcoholic fermentations partially or completely, or grow on the surface as a crust. The term "wild yeasts" that are often used for these natural yeast flora, may be deceptive as it may refer to any yeast that is not added to the juice or to all species apart from Saccharomyces. Kloeckera apiculata is the yeast species that most frequently occurs on grapes. It is resistant to low temperatures (10°C) and although normal levels of sulphur dioxide inhibit its growth, it is not eliminated. It is considered undesirable because it may form defective aromas such as ethyl acetate, amyl acetate and acetic acid as well as exhaust the nutrients required by desirable yeasts during fermentation.
Saccharomyces cerevisiae does not commonly occur on grapes, but is frequently found in wine cellars. The species is especially known for its ability to complete alcoholic fermentations. Stuck fermentations, however, may occur if there are stressful conditions such as high alcohol, nutrient deficiencies and temperature shocks etc.. The formation of undesirable sulphur compounds by this species is usually ascribed to nutrient deficiencies, spray residue and other causes. If these occur in wine with residual sugar after fermentation, secondary alcoholic fermentation may take place, which could have serious repercussions, especially in bottled products.
Zycosaccharomyces bailli is an osmotolerant yeast that may consequently occur in grape juice concentrate. It is also resistant to preservatives such as sulphur dioxide, sorbate and benzoate and may therefore cause secondary alcoholic fermentation in products that contain sugar.
Dekkera/Brettanomyces is commonly known as "Brett" and the various species differ considerably with regard to substrate and resistance to alcohol, temperature and sulphur dioxide. It was originally isolated from beer, but is widespread in all wine industries. Most strains form flavourants that have a significant influence on the flavour profile of wines. It is commonly described by flavour descriptives such as "sweaty horse" and "band aid". These yeasts are able to survive on the nutrient residue of other organisms and therefore they do not require significant amounts of res sugars. Widespread occurrence is possible in cellars or in bottled wines.
Aerobic yeasts or bacteria which require oxygen for survival, may also occur in cellars. Candidia and Pichia are the most common aerobic yeasts and may form acetaldehyde from ethanol. As both require oxygen for growth, they often occur as a crust on the surface of wine. Acetobacter or in other words acetic acid bacteria are the best known spoilage bacteria. Due to their aerobic demand, the exclusion of contact with oxygen is the easiest form of control. In the presence of oxygen they grow as a crust on the surface of wine, when the ethanol is converted to acetic acid and ethyl acetate, which is reminiscent of paint remover. Fortunately this reaction cannot occur in bottled wines, except when using a closure that is not airtight.
Lactic acid bacteria that occur in wine belong to three different genera, Oenococcus, Lactobacillus and Pediococcus. They are responsible for the conversion of malic acid to lactic acid during malolactic fermentation (MLF), but may also cause spoilage problems. High pH wines are usually preferred for growth; they may differ with regard to resistance to sulphur dioxide and they may also break down sorbate to products with a prominent geranium flavour. Oenococcus oeni is the species most favoured by cellars and used commercially to inoculate wines. This species grows at a lower pH and forms fewer off-flavours. Various Lactobacillus-species occur, differing in certain respects, but they can basically be divided into two groups, namely those which form acetic acid from the sugar in wine and those which do not. The latter species usually form compounds such as diacethyl (which has a buttery flavour), sauerkraut-flavours or biogene amines, but are generally not as harmful as the former group. The former group prefer pHs = 3.6, low sulphur dioxide and temperatures above 18°C. They usually occur in red wines. Under favourable conditions they may flourish during MLF, forming acetic acid from the fermentable sugars. The acetic acid thus formed may contribute to stuck fermentations. Pediococcus bacteria do not cause the formation of acetic acid, but may cause pentoses or malic acid turbidity in bottled wines as a result of the breakdown of residual sugars. A considerable increase in the viscosity of the wine, known as "ropiness", may also occur. Lactobacillus and Pediococcus species may also break down certain amino acids occurring in grapes or yeast, to biogene amines. Histamine is the best known and may cause headaches, hot flushes and other side-effects in sensitive persons. Several countries already have or are in the process of imposing regulatory limitations.
Reference
Van de Water, Lisa. 2008. Wine Microbes Part 1: Cast of Characters. Meeting Microbes, Good and Bad. Vineyard & Winery Management, January/February 2008: 90 - 96.
This article is also available on the website www.vwm.online.com.
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