Azm

2,4,6-trichlorophenol

2,4,6-trichloroanisole (TCA)

Figure 1. Conversion of 2,4,6-trichlorophenol to 2,4,6-trichloroanisole (TCA).

Biochemical evidence suggests that TCA is synthesized from chlorophenol precursors by a methylation reaction (Alvarez-Rodriguez et al., 2002). Chlorophenols are highly toxic, and this methylation reaction has been thought to be involved in detoxification (TCA, although odorous, is non-toxic). It has recently been reported that cork-associated fungi can perform this reaction and produce TCA in cork, provided 2,4,6-trichlorophenol was present as a substrate (ilvarez-Rodriguez et al., 2002). In this particular study, a strain of Trichoderma longibrachia-tum produced the highest level of TCA (nearly 400 ng/g of cork), and several other fungi produced more than 100 ng/g of TCA. It is important to emphasize that TCA synthesis required 2,4,6-trichlorophenol as a substrate and that no TCA was formed in these experiments when other chlorinated phenols were present. Thus, these investigators concluded that fungi are responsible for TCA and cork taint formation.When TCA is actually made during cork processing is not known, nor is it clear from where the trichlorophenol originates. Chlorinated polyphenols were once used as agricultural herbicides and fungicides and were likely applied in cork-producing areas. It has been suggested that chlorine treatment of harvested cork may further increase the trichlorophenol content, and subsequently, the TCA level.

Despite these recent findings, there is still considerable debate regarding the true incidence of cork-taint in wine, as well as the source of the tainted aroma. Cork producers and some cork researchers have argued that cork taint occurs far less often than the reported frequencies (Silva Pereira et al., 2000).They claim that TCA can arise from other sources—not just cork— and that other non-cork taints continue to be referred to as cork taint. Poor storage and handling of corks can indeed result in other off-flavors; however, these defects are controllable. Cork stoppers, their proponents contend, are still the best way to seal bottles and preserve the quality of wine (Silva Pereira et al., 2000). Finally, some wine writers have suggested that the cork taint problem has been exaggerated, in part by the industry itself and also by other wine writers (Casey, 1999).

Remedies and alternatives

Regardless of whether the frequency of cork taint is 1% or 12% or somewhere in between (2% to 5% is the figure most often quoted), this can hardly be considered an acceptable defect rate. No industry could long tolerate a situation where even one unit out of 100 fails to meet quality standards.The wine industry is no different and that is why non-cork alternatives have become so popular in the past ten years. Efforts by the cork industry to improve cork and prevent cork taint are also under way. For example, supercritical CO2 treatment can reportedly extract TCA from cork to undetectable levels (by sensory analysis). Analytical methods have also been developed to screen corks for TCA so that tainted corks are not used.

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

The main alternatives to corks for bottled wine include cork agglomerates and hybrids, plastic enclosures, and metal screw caps (bag-in-the-box packages might also qualify, but will not be further discussed). Agglomerates consists of cork granules (also called "dust") stuck together with glue and extruded into cork shapes.They have long been used by sparkling wine manufacturers and have many of the desirable features of cork, but there are reports they may also contain TCA.A new generation of agglomerates are now available that reportedly are TCA-free (via supercritical extraction). Hybrid corks are similar to the agglomerates, but contain plastic particles mixed with natural, ground cork. Plastic or synthetic corks are comprised of ethylene vinyl acetate.They have resilience similar to cork, but contain no cork material (and no TCA). Detractors have suggested that plastic corks impart plastic flavors to the wine (i.e., plastic taint).

Metal screw caps for wine were introduced in the 1990s.They are similar to those used for carbonated beverages. Screw caps are easy to open and re-close, and are TCA-free.They have all of the attributes of an ideal enclosure with no real technical downside. Wines from screw-capped bottles always score well in taste tests (although rubber-like flavors, from the enclosure seal, have been reported to migrate into the wine).

Ultimately, the type of enclosure used by a particular winery depends only in part on the performance characteristics of the particular material. Rather, perhaps the more important determining factor is the attitude or perception consumers have toward specific types of wine enclosures. Surveys indicate that consumers prefer cork, that they expect to hear a "pop" when the bottle is opened, and that tradition is a key feature of the wine-drinking experience. Many wine drinkers may not even recognize off-flavors in wines, including cork and other taints. Moreover, plastic corks, and especially screw cap enclosures, are often associated with economy wines, regardless of what is actually in the bottle.

Despite these attitudes, non-cork enclosures continue to gain popularity, and now are used for as much as 50% or more of the wine produced in some countries. For example,Australia and New Zealand have championed the use of screw cap enclosures. Many California wineries use cork only for their premium wines, and synthetic composite or screw caps for their other wines. Some experts have predicted that the trend away from cork will continue.

References

Álvarez-Rodríguez, M.L., L. López-Ocaña,J.M. Lípez-Coronado, E. Rodríguez, M.J. Martínez, G. Larriba, and J.J.R. Coque. 2002. Cork taints of wines: role of the filamentous fungi isolated from cork in the formation of 2,4,6-trichloroanisole by O-methylation of 2,4,6-trichlorophenol.Appl. Environ. Microbiol. 68:5860-5869. Casey,J. 1999T'aint necessarily so.Wine Industry Journal. 14:(6)49-56.

Coque, J.-J.R., M.L. Álvarez-Rodríguez, and G. Larriba. 2003. Characterization of an inducible chlorophenol O-methyltransferase from Trichoderma longibrachiatum involved in the formation of chloroanisoles and determination of its role in cork taint of wines.Appl. Environ. Microbiol. 69:5089-5095. Lee,T.H., and R.F. Simpson. 1993. Microbiology and chemistry of cork taints in wine, p. 353-372. In Fleet,

G.H. (ed.), Wine Microbiology and Biotechnology. Harwood Academic Publishers. Chur, Switzerland. Jackson, R.S. 2000. Wine Science:Principles, Practice, Perception, 2nd Ed.,Academic Press. San Diego, California.

Ribéreau-Gayon, P., D. Dubourdieu, B. Doneche, and A. Lonvaud. 2000. Handbook of Enology, Volume 1:

The Microbiology of Wine and Vinifications. John Wiley and Sons, Ltd.West Essex, England. Peña-Neira,A., B.F. de Simón, M.C. García-Vallejo,T. Hernández, E. Cadahía, and J.S. Suarez. 2000. Presence of cork-taint responsible compounds in wines and their cork stoppers. Eur. Food Res. Technol. 211:2 57-262.

Silva Pereira, C.S., J.J.F. Marques, and M.V.S. Romao. 2000. Cork taint in wine: scientific knowledge and public perception—a critical review. Crit. Rev. Microbiol. 26:147-162. Simpson, R.F., D.L. Capone, and M.A. Sefton. 2004. Isolation and identification of 2-methoxy-3, 5-dimethylpyrazine, a potent musty compound from wine corks.J.Agric. Food Chem. 52:5425-5430.

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