Christian E. Butzke, PhD., associate specialist in the department of viticulture and enology at the University of California–Davis College of Agricultural & Environmental Sciences, replies:
Basically that is an off-odor that gets into the wine from the cork—the natural bark taken from the cork oak which has been used for several hundred years, at least, in wine making. Under certain circumstances, the cork itself can become tainted with a moldy, musty aroma that develops during the processing and can get into the wine once the cork is in the bottle. So thats a musty, moldy, off-odor that is relatively easy to recognize. Its mostly caused by one particular chemical component, called TCA or trichloroanisol, a chlorinated component that has a very, very intense odor even at very, very small concentrations. This affects between two and five percent of all wines.
It’s a two-step formation. One is chlorination, and traditionally comes from a chlorine bleach that the corks undergo during the processing. They bleach the corks so they look nicer and lighter and more even, and that is a technique that has been used for many years. Then you have microbial activity from molds, a methyl group, and those molds can turn phenols into anisols. Theres a lot of phenolic material in the cork, so you go from chlorinated phenols and add a methyl group to create a methylated chlorophenol, or chloroanisol, and in particular the trichloroanisol, or TCA.
TCA has an average sensory threshold in wine that is lower than five nanograms per liter, or five quarts per trillion. So it’s there in a very, very diluted form but you can still smell it even at those very low levels. It would only take about a good teaspoon full of pure TCA to spoil all the wine that is made in the U.S.
One way to avoid cork taint is by improving manufacturing practices to eliminate the fungal, microbial step of the formation process of TCA. What we have seen in recent years is the use of anything from ozone to microwaves to beta radiation—which seems to be very effective in killing all the mold. Another way is to eliminate the chlorination step. That is being done by using hydrogen peroxide or ozone or other washes instead of the chlorine bleach—or by not bleaching the cork at all. So you are trying to not get any chlorine into the equation: avoiding chlorinated water, avoiding any other contact of the corks with things that might have been chlorinated in the past.
But corks tend to absorb odors from the environment too, so you always want to make sure that the cork is properly stored and doesnt absorb anything from wood preservatives to moldy shipping conditions. In particular, we tend to have problems because the corks are almost exclusively shipped over here from Portugal. Portugal produces about 75 percent of all the natural corks used in wine bottles. It is pretty much a monopoly and that has also led to a very slow adoption to new processes and improvement in the quality of corks. So if corks are shipped through the Panama Canal, let’s say in a big boat in a container, you can imagine that the possibilities for mold growth are increasing. So to avoid those kinds of conditions, for instance, what one winery does here in California—Mondavi—they bring in just the raw cork and then they process it here in California themselves. So theyre gaining more control over the manufacturing process.
Another thing that can be done, as we see now in quite a few examples, is use alternative closures. The two to five percent of all cork-finished wines that taint affects is obviously quite a substantial number. So we are moving away from the traditional, natural cork, even for some high-end wines. Manufacturers and wine makers have gone to using closures that are synthetic, basically different types of plastic—injection molded plastics, extruded plastics (the newer generation) or different extruded polymers—that are then cut into cork-sized pieces and basically used like a traditional bark cork, and in the bottles you still need a corkscrew to get it out. Then we also see some wineries now even going to screw caps. We happened to have an example of a winery in Napa Valley that has bottled a $135 bottle of wine with screw caps—a very nice package. And so in that situation you eliminate the possibility of cork taint.
Whether screw caps are effective, that is actually one of the things we are currently trying to find out. A screw top may not cause wine taint, but we are trying to accomplish a number of things with a wine bottle closure, and one is protection from oxygen. Oxygen is usually considered harmful, in particular to white wine. Different wines have a much different capacity to handle oxygen that comes in. Red wine, I believe, can take many, many oxygen saturations of many, many times and you wouldn’t see any changes for quite a while. But if you have a delicate white wine that does not have much anti-oxidants, then the oxidation would come about more rapidly. But even for red wine the closure has to last, as high-end wines are aged for many years.
So if a screw cap is providing a seal that is tight, and does not allow for permeation or much permeation of oxygen through the seal, then indeed it would provide a very good closure for a wine bottle. The permeability of a seal depends on the materials that are used, how high the pressure is between the seal and the glass, and so those kind of things are something that still need to be evaluated more. There’s still very little published on that. In fact, theres still little published research on the effectiveness of synthetic closures in general in wine. Like with the polymers or plastics, we dont quite have much experience yet, how those hold up in the long run. With bark corks we know, obviously, for many many decades and centuries, how theyre holding up.