Why are wine closures important? First, wine closures keep the wine in the bottle until you want to let it out. Second, they control (or manage) the interaction of the atmosphere outside the bottle, mainly the oxygen, with the wine on the inside. As has been said, after a month in the bottle, a wine will never be the same again!
What happens as a wine ages? Typically, white wines age in a way different from red wines. They have lighter, fruitier flavors, and don’t have the tannins and other chemicals that are extracted from the grape skins during red wine production. Oxygen typically accelerates aging in both types of wine, and even very little oxygen in a white wine can cause it to age too quickly, while in a red wine aging influenced by oxygen is often considered to be maturation, as many higher rated red wines are not considered near optimum taste until at least four years in the bottle.
(The oxygen being discussed comes from four main sources: the atmosphere during bulk storage and transfers, permeation through barrels during primary aging, microoxygenation by the winemaker, and nanooxygenation through the wine bottle closure.)
White wines have delicate flavors. Properly bottled and sealed, which historically has meant using a cork stopper, white wines can last for ten years or more. One of the key fruity/tropical fruit flavor aroma compounds is a chemical called 3-mercaptohexanol, or 3-MH. This chemical tends to degrade over time, with degradation rates greater with higher oxygen levels. If a natural antioxidant, glutathione, is present in white wines, the rate of degradation of this delicate flavor chemical can be reduced by 70%. So how do we increase the amount of glutathione in a wine since it can’t be added directly: we ensure adequate nitrogen levels for yeast, select an appropriate yeast stain, use more lees aging, and minimize oxygen exposure during transfers and post-fermentation storage.
In addition to increasing the rate of flavor degradation, too much oxygen can result in the formation of off flavors and prematurely aged wine. Sotolon, an off aroma, was found at higher levels in wines exposed to higher oxygen levels.
So, for white wines, let’s just keep out oxygen! Not so fast. Low oxygen wine is considered to be in a “reductive” environment. What happens here is that some off-flavor sulfur containing compounds, such as hydrogen sulfide and methyl mercaptan, can develop, overwhelming the delicate flavors of the white wines. “Burnt rubber” and “struck flint” can’t be looked upon as positive wine attributes. A little oxygen is generally good. But note that this problem can be minimized by viticulture and enology practices, such as controlling the use of sulfur in the vineyard, and using copper fining on fermented wine.
Red wines have slightly different issues with oxygen than white wines. The compounds extracted from the skins lead to a higher flavor and aroma base. As a result, the formation by oxidation of some compounds, such as phenylacetaldehyde and 3-(methylthio) propionaldehyde contribute to red wine flavor complexity at low levels, but at higher levels still lead to off aromas such as “farm feed” and “woody”. Again, some oxygen is good, but too much is bad.
Another major flavor component of red wines is polyphenols. These anthocyanin flavonoids are responsible for red wine color and astringency. As the wines age (by exposure to oxygen), these compounds tend to polymerize, i.e., form longer and longer chains. As the polymers form, the color of red wines changes from mostly red to a deeper purple hue. Also, with polymerization, the level of astringency tends to decrease, allowing for more of the subtle fruit flavors to “emerge”.
Oxygen is a major factor affecting the rate of aging in a wine. For years, actually centuries, cork stoppers were used to seal wine bottles, and winemaking practices were adjusted to its properties. A number of other wine bottle closures have been developed, partly as a result of the 1 – 4% contamination of natural cork by the chemical trichloroanisole (TCA), and each of these closures has its own oxygen transmission characteristics. These are:
Closure Relative Oxygen Transmission Rate
Glass ampoule (perfect seal) 0.0
Screw cap, with saran or saranex 0.5
“Technical” cork (agglomerated cork) 0.8
Screw cap, with LDPE/PET/foam (VinPerfect) 1.0
Natural cork 1.6
Plastic cork #1 2.2
Plastic cork #2 3.0
Screw caps are made of metal, and are theoretically impervious to oxygen, but they are manufactured with a liner that allows a small amount of oxygen to enter the wine during storage. The earlier versions allowed very little oxygen transmission, and wine stored under them retained free SO2 better than wines stored under corks. These wines also had less browning, and more varietal thiol aromas, but sometimes the lack of oxygen resulted in the formation of unpleasant hydrogen sulfide and mercaptans. With better winemaking control these off flavors can generally be avoided, making them suitable for consideration with white wines.
A screw cap with a different liner, the VinPerfect cap, is now available. It allows oxygen transmission rates just below those of natural cork, which may make it suitable for use with red wines.
There are three issues to contend with when considering the use of screw cap closures. First, current liners tend to degrade significantly by the fifth year after bottling, making them undesirable for white wines with long intended storage periods. Second, the capping equipment is more costly than that required for traditional corks. And third, the tin metal of the screw caps is susceptible to denting, which can deform the threads, allowing unwanted high oxygen levels in, and even some wine to leak out.
Synthetic corks were developed to address shortages of natural cork and to overcome the risk of “cork taint” contamination of natural cork. They look a lot like natural cork, but are actually a lot firmer. They tend to be more difficult to insert, almost impossible to re-insert, and because they are less flexible, can let oxygen leak in between the side of the cork and any irregularities in the inside of the wine bottles. From the table above, it is seen that synthetic corks have a higher oxygen transmission rate than natural corks. This is due to the larger size of the internal cells in current synthetic corks, as shown in the table below.
Closure Internal cell diameter range (μm)
natural cork 37 – 74
technical cork 51 – 88
synthetic stopper (Italy) 300 – 400
synthetic stopper (Australia) 100 – 200
synthetic stopper (France) 100 – 200
synthetic stopper (Italy) 300 – 400
The higher oxygen transmission rates found with synthetic corks leads to premature browning of white wines, and to oxidative aromas in both red wines and white. Most wineries now use synthetic corks for wines that will be consumed in 12 – 18 months.
“Technical cork,” shown in both tables above, is derived from natural cork. It is made of a mixture of microspheres, bonding agents, and natural cork bits that have been ground down, washed, heated, and even steam distilled. The mixture is formed in the shape of a natural cork, and has many similar properties, but is only one fifth as likely to have cork taint. It has made some inroads into the natural cork market.
And now we get to natural cork. Excluding inexpensive, poor quality product, natural cork can provide an effective bottle seal for extended periods (up to 25 – 30 years before recorking). Oxygen transmission is controlled at manageable levels preferred for many wines. But there is still the issue of 1 – 5% incidence of cork taint. A Portuguese cork supplier has developed a process to test each cork for the presence of TCA and eliminate those corks that are contaminated, leaving TCA-free product.
What is a winemaker to do? It’s a winemaker’s choice. Consider the cap when deciding on some of the winemaking practices. Then consider your options. Screwcaps, when properly protected from denting, seem suited for white wines with shorter expected consumption times. Synthetic corks seem more suited to red wines with shorter expected consumption times. Natural corks remain the go to general purpose product, especially if cork taint can indeed be controlled.