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40 p r a c t i c a l w i n e r y & v i n e ya r d J a n U a r y 2 0 1 5 P A C K A G I N G wines. 11,12 In contrast, these compounds were essentially retained in the outer cork stoppers, preventing the migration of this compound into bottled wines, showing that corks are effective barriers to the transmission of exogenous gases and volatile compounds. 13 Recent results on closure OTR In the past several years numerous changes have occurred in the closure industry with some products discontin- ued, new closures introduced into the market and developments in the manu- facturing process designed to improve the performance of existing products. Given this scenario and the increasing recognition of the importance of closures by the wine industry, it became pertinent to determine the Oxygen Transmission Rate of closures recently introduced to the wine market, and/or update the OTR of existing closures. The colorimetric method developed by P. Lopes and co- workers has been used to monitor and determine the OTR of current commer- cially available wine closures under real- istic conditions. 4,5 Overall results show some material changes in OTR performance, but the general trends by closure type are con- sistent with results in previous studies (Figure 3). Today, the largest synthetic manufac- turer offers a range of closure perme- ability, varying from 3.4 µL of oxygen/ closure/day for the Nomacorc Select 100 to 10.0 µL for the Nomacorc Light. Mechanisms of oxygen ingress into bottles Using the above mentioned colorimetric method P. Lopes and co-workers were also able to determine the main routes of oxygen ingress through synthetic and cork closures. 6 For synthetic clo- sures, the data indicated that after the first month, when oxygen is expelled from the compressed closure, oxygen ingress is a result of gas permeation through the closure's central foam core. This permeation occurs at a constant rate over time. In contrast, the main source of oxygen- ation in bottles sealed with technical and natural corks is from oxygen compressed within the closure's internal structure. This is diffused into a bottle essentially during the first several months of storage. Atmospheric oxygen entering through- out the cork-glass interface or throughout the cork is negligible. Recent studies suggest that oxygen transport within natural corks occurs by diffusion through a gas phase in large spaces such as lenticels, plasmodesma- tas (microscopic channels that make the communication between cells) or inside of empty cells following the Fick's or Knudsen mechanisms. 8,9,10 These results were also confirmed by P. Lopes and co-workers, who showed that exogenous gases and volatile compounds such as TCA and volatile phenols are able to permeate through synthetic and screw cap saranex (more permeable liner) closures into bottled bottles was substantially greater dur- ing bottling than in the following 36 months of storage. This appeared to be due to the insertion of oxygen contained under the screw cap in the bottle headspace at the time of sealing. After the first month, screw caps and "technical" cork stoppers (Twin Top and Neutrocork) exhibited the lowest oxy- gen transfer rates (less than 1.0 µL of oxygen/closure/day) (Figure 1 and 2). In contrast, a synthetic closure such as Nomacorc classic, exhibited the highest oxygen transmission rate, reaching the quantifiable limit of our method (2.5 mL of oxygen) within 7 to 10 months depend- ing on bottle orientation. The oxygen kinetics for natural corks differed from other closures (Figure 1). Generally, oxygen ingress through natu- ral corks decreased over time, reaching a steady state between 18 and 24 months of storage. On average, the best grade of natural cork stoppers ("flor") exhibited lower OTR, 0.6 to 1.4 µL of oxygen/clo- sure/day, than the cork with more lenti- cels (first grade), 0.7 to 6.1 µL of oxygen/ closure/day. In addition, it was observed that stor- age orientation did not significantly impact the oxygen transmission proper- ties for cylindrical closures (Figure 1B). Contact with liquid is an important fac- tor in oxygen transmission through natu- ral corks. But even in upright storage, the relative humidity inside the bottle is maintained at 100%. This is sufficient to keep the cork moisturized. Bottle #1 [left] (control) contains indigo carmine solution after 36 months of horizontal storage. Bottle #2 contains indigo carmine solution sealed with "technical" Neutrocork ® cork stopper after 36 months of horizontal storage. Bottle #3 contains indigo carmine solution sealed with natural cork stopper after 36 months of horizontal storage. Bottle #4 [right] contains indigo carmine solution sealed with Nomacorc Classic ® synthetic closure after 10 months of horizontal storage. 0.000 0.002 0.004 0.006 0.008 0.010 0.012 Oxygen transmission rates (OTR)* (mL/closure/day) Figure 3: Oxygen transmission rates (mL/closure/day) of different closures. OTRs were obtained from colorimetric measurements done during 24 months of horizontal storage at room temperature. The values obtained in the non-steady state (first month) were not included in the calculations. Error bars represent the standard deviation of 10 replicates for technical corks, synthetic and screwcaps, and 100 replicates for natural corks.