Spoilage by fungi

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Fungal growth and spoilage rarely occurs during the wine fermentation, since most fungi are aerobic and sensitive to ethanol. Rather, molds are most important before and after the wine is made. Fungal growth on grapes is one of the most serious problems encountered in grape viticulture, causing considerable loss of crop.If not controlled, rots, mildews, and other fungal diseases can wipe out an entire vineyard. As noted earlier for sweet botryized wines, a fine line sometimes separates spoiled, rotten grapes from desirable, noble rot growth of B. cinerea. Some fungi, such as Penicillium, Aspergillus, Mucor, and Rhizopus, can grow on freshly harvested grapes during transport to the winery. Pesticides, sulfiting agents, and other antimycotics can be applied to help control this problem, but care must be exercised to minimize their impact on the wine and during fermentation.

Post-fermentation problems with mold are usually due to contaminated cork closures. Cork, the bark from the cork tree, can contribute so-called cork taints. This is one of the most serious defects in bottled wine and one which has attracted wide attention among cork producers, wineries, and consumers (Box 10-7).The defect is now thought to occur as a result of growth of various fungi (including Penicillium, Aspergillus, and Trichoderma). Visible mold growth is rarely evident (those corks would not be used), so cork taints occur earlier in the cork making process, when exposure to fungi is common. These fungi then produce musty- or mushroom-smelling compounds that diffuse into the wine after the corks are inserted into the bottles.Wine corks can now be treated to remove the offending taint. However, there has been a strong trend toward the use of plastic and other alternative wine enclosures, the use of which may significantly reduce the incidence of cork-related defects.

Box 10—7. Cork Taints in Wine: Are Corks the Cause?

Imagine you've just purchased an expensive bottle of wine and are anticipating the fine bouquet about to be released from the soon-to-be opened bottle. Upon removing the cork, however, you discover that the wine has an obnoxious musty and moldy aroma that renders the wine undrinkable.As unfortunate as this scenario might be, it is by no means rare. In fact, cork taint, the most likely cause of this particular defect, is considered to be the most common cause of wine spoilage, affecting as much as 12% of all wine (this spoilage rate has been disputed, however, as discussed below).

How often consumers actually return or even reject cork-tainted wine is not known, but it seems likely they will remember their experience with that particular brand when they purchase their next bottle of wine. Not surprisingly, given the frequency with which this problem occurs, cork taint has become one of the most important (and perhaps most controversial) issues facing the wine industry.Annual economic losses of more than $10 billion have been estimated (Pena-Neira et al., 2000).Therefore, understanding what causes cork taint and developing strategies to prevent it are the focus of considerable academic and industry-sponsored research.

Cork technology and microbiology

Corks have been used as closures for wine bottles at least since the 1600s. Not only are cork stoppers effective at preventing leaking, they also permit a very small amount of gas exchange that is believed to promote flavor and aroma development in aged wine. Cork is obtained from the bark of the Quercus suber oak tree, which grows in regions around the western Mediterranean Sea. Portugal is the main producer of cork (and has the biggest stake in the cork taint problem).

The manufacture of cork follows a traditional process that starts with its harvest (Jackson, 2000).The first, or virgin bark, stripped from the tree when it is about twenty to thirty years old, is not used; neither is the second growth obtained after another seven to ten years. Bark is not cork-worthy for another nine years, when it is called reproduction bark (subsequent strippings can occur every nine years). The harvested cork slabs are then boiled, cut into strips, and the cylindric cork units are punched out and washed in water. Next, the corks can be treated either with a hypochlorite solution or with peracetic acid. Both of these treatments inactivate microorganisms, although they are not without important side reactions (see below). The corks are dried, sorted, and packaged.The name of the winery or vintage can also be etched onto the corks.

Among the microorganisms associated with cork, fungi, and Penicillium species, in particular, appear to be the most common (Lee and Simpson, 1993). Other fungi include Trichoderma, Aspergillus,Mucor, and Monillia.Yeast may also be present.The boiling and bleaching steps effectively reduce the microbial load. However, fungi growth is possible prior to these steps or after, if re-contamination occurs. In fact, mold growth on cork has long been thought to be necessary for cork maturation and is, therefore, encouraged as part of traditional cork processing (Silva Pereira et al., 2000). Still, fungi have long been suspected of being involved in wine taint formation, although direct evidence establishing this link has only recently emerged.

Cork Taints

The actual chemical agent responsible for cork taint has been identified as 2,4,6-trichloroanisole or TCA (Figure 1). Not only does TCA have a musty, disagreeable odor, but its threshold for detection is extremely low. Sensitive tasters can detect TCA at levels as low as a few parts per trillion, although for most individuals, higher concentrations must be present before TCA is noticed (Pena-Neira et al., 2000).Although other chloroanisoles, as well as other phenol- and pyrazine-containing compounds, may also contribute to cork taint (Pena-Neira et al., 2000; Simpson et al., 2004),TCA is invariably present in tainted wine (Coque et al., 2003).

Box 10—7. Cork Taints in Wine: Are Corks the Cause? (Continued)

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