Probably the most common and most disastrous types of microbial spoilage of wine are those caused by bacteria.Two distinct groups are of importance: the acetic acid bacteria and the lactic acid bacteria, both of which contain species able to tolerate the low pH, high ethanol conditions found in wine.These bacteria are responsible acidic and other end products that seriously affect wine quality.
The acetic acid bacteria that are most important in wine spoilage belong to one of three genera: Acetobacter, Gluconoacetobac-ter, and Gluconobacter. The main species involved in wine spoilage are Acetobacter aceti, Acetobacter pasteurianus, and Gluconobacter oxydans. They are Gram negative, catalase-positive rods capable of oxidizing alcohols to acids. These bacteria also are considered as obligate aerobes; however, it now appears that limited growth and metabolism can occur even under the mostly anaerobic conditions that prevail during wine making. Although acetic acid bacteria are generally found at relatively low levels in vineyards and in must (<100 cells per g), moldy or bruised grapes can contain appreciably higher levels. If the ethanol fermentation occurs soon after harvesting and crushing, then growth of these organisms, especially G. oxydans, is inhibited and numbers may actually decline.When the fermentation is complete and the wine is drawn off and subsequently transferred and racked, aeration inevitably occurs, activating growth of these bacteria.
Aging in oak barrels may also promote growth of acetic acid bacteria, in part because barrels may actually be contaminated with these bacteria, but also because oxygen diffusion into small oak barrels can be significant. Under these conditions, then, acetic acid bacteria can oxidize ethanol in the wine, producing enough acetic acid (>0.7 g/L) to give the wine a pronounced vinegar flavor and aroma. Although lesser amounts can sometimes be tolerated, other end products resulting from growth of acetic acid bacteria, including ethyl acetate, acetaldehyde, and dihydroxyacetone, may also contribute to spoilage defects. Low sulfur dioxide levels will further enhance growth of acetic acid bacteria.
The other group of bacteria associated with microbial spoilage of wine are the lactic acid bacteria. This cluster of Gram positive, facultative rods and cocci consists of twelve genera, however, only four, Lactobacillus, Oenococcus, Leuconostoc, and Pediococcus, are involved in wine spoilage. Due to their saccharolytic metabolism, it is not surprising that lactic acid bacteria are found on intact grapes, albeit at low populations (102/g to 103/g) that eventually increase ten- to 100-fold during harvesting and crushing. Although most lactic bacteria do not grow during the ethanolic fermentation (the exception are the malolactic bacteria), some strains are tolerant of high ethanol concentrations and are able to grow later during post-fermentation steps. High temperature and pH and low SO2 concentrations favor growth of lactic acid bacteria. Growth of these bacteria in wine can result in several spoilage conditions, including acidification and deacidification, as well as production of various metabolites that cause off-flavors, aromas, and other defects.
As described in Chapter 2, lactic acid bacteria have either a homofermentative or hetero-fermentative metabolism, or in some strains, the metabolic capacity for both. When ho-mofermentative lactic acid bacteria, such as Pediococcus and Lactobacillus plantarum, grow in wine and ferment glucose, they produce lactic acid. This acid causes the wine to become excessively sour. In contrast, metabolism of pentose sugars, present in the grapes as well as extracted from oak barrels, occurs via the pentose phosphate pathway (also known as the phosphoketolase pathway), yielding acetic acid, ethanol, and CO2. These same end-products are also produced by several heterofermentative lactic acid bacteria found in wine, including Leuconostoc mesen-teroides, Oenococcus oenus, and Lactobacil-lus brevis. Although these bacteria produce considerably less acetic acid than Acetobacter or Gluconobacter, enough may be produced to impart a detectable vinegar-like flavor to the wine.
Several lactic acid bacteria have the capacity to convert malic acid to lactic acid via the malolactic pathway. In musts containing high malic acid concentrations and having low pH, the malolactic bacteria perform a desirable, even essential function, by deacidification of the wine. However, if the must acidity is already low and the pH high, the malolactic fermentation may result in an increase in wine pH such that too little acidity remains. The wine also will be more susceptible to spoilage since a major barrier to microbial growth, low pH, is diminished.
Spoilage by lactic acid bacteria can also be caused by other metabolic end products. Metabolism of fructose by heterofermentative L. brevis, for example, can cause mannitol to form, and generate acetic acid as a side reaction. In a fermentation analogous to the malo-lactic pathway, in terms of effect, L. brevis can metabolize tartaric acid, causing an increase in wine pH and formation of volatile acids. Mousy taints, similar to those produced by the spoilage yeast Brettanomyces, can also be produced by Lactobacillus sp., Leuconostoc sp., and other heterofermentative lactic acid bacte-ria.A flowery off-odor referred to as geranium taint also may be produced by lactobacilli.
Glycerol oxidation by Leuconostoc mesen-teroides, Lactobacillus brevis, and other lacto-bacilli generates acrolein, which reacts with tannins and anthocyanin phenolics to form bitter compounds. This defect, referred to as amertume, is more common in red wines, due to the higher tannin and phenolic levels in red wines. Its microbiological cause was first noted by Pasteur. Several lactic acid bacteria, including strains of Pediococcus and Lactobacillus, produce diacetyl, which imparts a buttery aroma in wine that, at high concentrations (generally above 5 mg/L), is considered undesirable. Finally, the formation of glucose-containing polysaccharides, such as dextrins and glucans, can give an oily, viscous and objectionable mouth feel. Ropiness usually occurs only in sweet wines and is caused by Pediococcus, Oenococcus, and Leuconostoc spp.
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