Therefore, it is important to understand that there will be some degree of TCA taint if natural cork is chosen to close a particular bottling. However, this does not mean that wines closed with other types of closures are totally free of any threat of taint. The raw materials used to manufacture synthetics and screw cap liners have adsorbing capabilities for volatile organic compounds (VOCs) which can migrate from packing materials (cardboard and wood pallets), contaminated warehouses, and cargo containers.7,8

Adsorbed VOCs can be a significant sensory threat to food, food packaging, personal care products, and even medications. 9,10 Therefore, it is recommended that synthetics and screw caps warehoused for prolonged periods of time be tested for potential contaminants, the same way natural cork is QC-tested for TCA upon arrival in the U.S.11

Figure 3 gathers and compares OTR values of natural and technical corks, synthetic (extruded and molded), and screw cap (Saran/Tin and Saranex™) closures.12,13 The OTR values in Figure 3 represent only estimated OTR values that were collected from literature references, product specifications, and Cork Supply analysis (mostly on natural and technical corks). The order of closure type (based on OTR values) is more accurate and should be taken into consideration rather than focusing on specific or discrete oxygen transmission values.

It is important to understand that closure OTR values have been determined using various types of analytical techniques causing disagreement between suppliers and scientists. Number of replicates, attention to detail during testing, and closure application on a glass sleeve (if MOCON technique is chosen for measurement) can have a significant impact on the final OTR result. OTR values can be measured based on pure

oxygen or air (20% oxygen). Therefore, measured OTR values based on pure oxygen are five times larger than values obtained with air.

Figure 3 includes a wide range of oxygen transmission rates for natural corks (from about 5.0 x 10-4 to about 1.0 x 10-2 cc/day/closure). As the visual quality of the cork moves from the best to least, the average OTR value will become larger due to a greater incidence of individual outliers (the OTR standard deviation will widen as the number of lenticels and imperfections increases in a lot of natural corks). This is why wine closed in bottles with natural corks from the very same lot might evolve into products with slightly different flavor nuances.

The cork industry is working to develop ways to sort natural corks, not only by visual grades but also by oxygen transmission rates. In the near future, winemakers may see further reduction in cork taint and improved OTR consistency with natural corks.

There are synthetics (extruded and molded), technical corks, and screw caps with Saranex™ liners with decreasing OTR values (values of around 7.0 x 10-3, 3.0 x 10-3, and 1.0 x 10-3 cc/day/closure, respectively). OTR values for technical corks are more discrete (narrow ranges) than natural corks, while

there are wide ranges for synthetic and screw cap (Saranex™ liner) closures that are due not to product inconsistency (just the opposite) but rather due to the wide selection of availabile products (with several OTR values).

On the other extreme of the spectrum are Saran/Tin screw caps with impermeable tin layers. Given the low OTR nature of Saran/Tin liners (approximately 1.0 x 10-4 cc/day/closure), screw cap application requires careful consideration due to the potential for wine reduction to occur during storage and cellaring. Reduction is a chemical process which occurs in the absence of oxygen resulting in unpleasant aromas due to the formation of certain sulfur-containing volatile compounds.

It is important to consider how the screw cap liner is shaped during closure application in order to protect the wine from oxygen. As the closure is put in place during bottling, the aluminum cap presses the wafer-shaped liner against the glass bottle by thinning and stretching the circumference of the liner over and around the rim of the bottle neck in a “reform” process. Depending on how much head block pressure is applied, the density of the polymers sandwiched between glass and aluminum will determine the actual oxygen transmission through the molded barrier.