By Aad van der Werf

Paulo Lopes et al. (Faculté d’Oenologie de Bordeaux) publish their pioneering work on different closures in the Journal of Agricultural and Food Chemistry. A summary:

Main Routes of Oxygen Ingress through Different Closures into Wine Bottles

Paulo Lopes et al. Faculté d’Oenologie de Bordeaux

The main routes of oxygen ingress into wine bottles through “technical” cork stoppers (Neutrocork), natural cork stoppers, and synthetic closures (Nomacorc) were investigated. A comparison was made among closures left uncovered (controls), closures with the closure-glass interface covered, and closures completely covered with a polyurethane impermeable varnish. The oxygen ingress into the bottles was measured by a nondestructive colorimetric method.

Introduction

During bottle aging, oxygen ingress into wine bottles depends on the sealing effectiveness of the closures, which differ in their oxygen barrier properties. Recently, we have shown that only a bottle sealed with a glass closure by flame (bottle ampoule) is completely airtight, while more commercial closures are permeable to oxygen.

Generally, oxygen ingress through closures into wine bottles is much more important during bottling and in the first month than in the following months of storage. For natural corks, this is followed by a gradual decline in oxygen ingress rates for the remainder of the first year (2-6 µL/day) and a very low rate of oxygenation in the 24 months thereafter (0.1-2 µL/day). Screw caps and “technical” cork stoppers display a consistently low level of oxygen permeation during storage (less than 1µL/day). In contrast, synthetic closures, Nomacorc and Supremecorc, continue to exhibit high oxygen permeation rates, 6 and 13 µL/day, respectively.

Given their high oxygen permeation rates, the use of synthetic closures resulted in wines with a tendency to lose fruit attributes and develop oxidized, “wet wool”, and toasty aromas prematurely. In contrast, too little oxygen has been linked with the presence of undesirable struck flint/rubber (reduced) aroma characters, more noticeable in screw-cap-sealed wines. Generally, wine sealed with natural corks displayed intermediate performances.

This study complements and extends the results of previous research on oxygen barrier properties of closures using a nondestructive (i.e., without opening the bottles) colorimetric method. This method screens oxygen ingress through closures into indigo carmine bottled solutions that gradually changes color from yellow to indigo as oxygen reacts with the reduced indigo carmine. Our purpose was to investigate the different routes of oxygen ingress through different cork stoppers and synthetic closures during bottle aging.

 Materials and methods

For each closure type three different “treatments” were set up. Indigo carmine bottled solutions were sealed normally (i.e., uncovered). For another set of closures the closure-glass interface was covered with an impermeable polyurethane varnish. A thin layer of varnish was applied to the internal surface of the bottleneck before closure insertion; the external closure-glass interface was then also covered with varnish. In this way, oxygen ingress between the closure and the bottleneck was prevented and only the permeation throughout the closure’s body was measured. Finally, another set of closures were completely covered with polyurethane varnish and glass (20 x 20 mm). Therefore, only oxygen within the closures able to ingress into the bottles was measured.

All bottles were left upright for 24 h and then stored horizontally for 24 months under a constant temperature of 20 ± 1 ºC and a relative humidity of 65 ± 1%.

Results and discussion

The analytical data obtained for oxygen ingress into wine bottles during 24 months of horizontal storage showed significant differences among the natural corks, technical corks, and synthetic closures tested.

After 24 months of horizontal storage, oxygen ingress amounts through uncovered, interface-covered, and fully covered Neutrocork technical corks were 0.7, 0.7, and 0.6 mL of oxygen, respectively, without significant differences between the three treatments.
The results obtained in this study seem to show that oxygen diffuses from the Neutrocork internal structure due to the compression in the bottleneck, mainly during the first month.

Uncovered, interface-covered, and fully covered natural corks displayed oxygen ingress of 1.4, 1.3, and 1.3 mL over 24 months of storage, respectively. There were no significant differences between the three treatments.
When natural cork stoppers are compressed in the bottleneck immediately after bottling, the air pressure in the cells ranges from 0.6 to 0.9 MPa. Therefore, air at atmospheric pressure (0.101 MPa) is unable to enter into the bottles through the cork or between the cork-bottleneck interface.
Theoretically, natural corks (44 mm length and 24 mm diameter) contain 3.4-3.6 mL of oxygen within their structure. We have shown that 1.3 mL of oxygen diffuses from the natural corks into the bottles, which represents 36-38% of the theoretical total oxygen within their cell structure.

With regard to the Nomacorc synthetic closures, the results showed that uncovered and interface-covered closures exhibited high oxygen permeation, reaching 2.5 mL of oxygen (limit of quantification for the method) within approximately 8 months. In contrast, closures fully covered with impermeable polyurethane varnish allowed ingress of 1.4 mL of oxygen during 24 months.
Uncovered and interface-covered Nomacorc closures were clearly much more permeable than those fully covered. These data clearly indicate that atmospheric oxygen can ingress throughout Nomacorc synthetic closures, mainly after the first month in the bottle.

Summary by
Aad van der Werf

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