Wine Flaws and Aromas
What Can Cause Wine Defects
TCA (2,4,6 Trichloroanisole) is a naturally occurring haloanisole compound. When present in wine, it is associated with the “moldy”, “musty”, “wet cardboard” aromas often described as “cork taint”. There are several forms of haloanisoles that can create musty aromas, but TCA is the only one associated with cork stoppers.
The origin of haloanisoles is often attributed to the biodegradation of halophenols by microorganisms (molds and soil bacteria). Trichlorophenol (TCP)is converted to trichloroanisole (TCA). Likewise, tetrachlorophenol (TeCP), pentachlorophenol (PCP) and tribromophenol (TBP) are converted to TeCA, PCA and TBA respectively. TCP in wineries is often traced to the former use of bleach as a sanitizer. TeCP, PCP and TBP are wood preservatives. TBP, also used in flame retardants, can be present in plastics and other polymers. It might also result from the use of bromine as a sanitizer. Each of these compounds, or combinations of them, can be found in winery environments.
Sensory detection of TCA can occur at extremely low concentrations and is often measured within a range of nanograms per liter of wine. One nanogram per liter is often expressed as one part per trillion (ppt) and is roughly equivalent to a grain of sand placed in an Olympic size swimming pool.
Individual detection of TCA in wine depends on a great number of variables including the wine matrix, the sensory environment and individual sensitivity to TCA. Tests of trained personnel under sensory controlled conditions suggest a range of sensory detection from 2ppt–6ppt in white wines.
Oxidation occurs when wine is overexposed to air. It shows itself by a stale off-odor, and the loss of fruity aromas in the wine. Natural cork assists in the prevention of oxidation with its ability to seal the wine bottle. Natural cork has the ability to be greatly compressed and then quickly expand back to its original size. These compression/expansion properties, in combination with the natural cork coating’s interaction with the glass of the bottle, promotes a complete seal.
Natural Cork – Diffusion
Under proper circumstances, the bulk of OTR (oxygen transmission rate) from natural corks takes the form of diffusion. A single cork contains up to 800 million tiny cells. Together, the cells contain approximately 3.5ml of air. When the cork is compressed the internal air pressure increases to between 6 and 9 atmospheres. This establishes a pressure imbalance that is solved by the gradual equalization of gasses between cork and headspace.
The exchange of gasses explains why studies of oxygen ingress show that bottles with natural cork “pick up” a small amount of oxygen over the first 6-9 months of aging. After that, oxygen ingress is no longer significant (the referenced study ran for 36 months).
Variations in oxygen diffusion between corks appear in the first six months of storage and likely reflect differences in cellular structures. After the initial diffusion period, additional variation was rarely observed.
Alternative Closures – Permeation
Artificial closures provide oxygen ingress primarily through permeation. Oxygen passes directly through the closure from the outside air. This can happen at a controlled rate, but unlike diffusion, the permeation does not stop. Oxygen continues to enter the bottle at whatever rate is determined by the closure.
Most synthetic closures will exhibit significant oxygen permeation within 18 months. Some manufacturers now advertise optional products with reduced permeation rates. Recent developments with screwcap manufacturers show an interest in fitting screwcaps with a permeable seal, so that more oxygen can be introduced into the wine.
The extra permeation is desired so that oxygen will be available to deter reductive aromas and to assist the wine’s development over time. Proper permeation levels are difficult to anticipate, because unlike the temporary oxygen ingress by diffusion on corks, permeable transfer occurs continuously, with a steady oxygen ingress until the bottle opened.
In cases where wine is oxidized prematurely, the cause is usually related to bottling activity. The closure can be damaged ore improperly fitted during sealing, but more likely are problems with internal bottle pressure. Liquid expands or contracts with changes in temperature, and if bottling temperatures are not properly adjusted to storage temperatures, significant problems can occur. You can see the effect by placing a wine in the refrigerator overnight. A standard 750ml bottle taken from 68°F to 40°F will show a reduction in fill height by 15mm. When wineries bottle at cold temperatures, a similar expansion will occur during cellar storage. Pressure problems are exacerbated if the original fill height is too close to the cork.
Reduction occurs when there is a lack of oxygen within a sealed bottle of wine. Most sulfur compounds that naturally form during the production of wine require an interaction with oxygen within the bottle. Without this interaction off characteristics can occur. Sulfur-like off (SLO) aroma compounds (SLO) such as the smell of rotten eggs, cooked cabbage, or sewer gases. Natural cork prevents this reduction by seeping small amounts of oxygen out of the natural cork over time. This is one reason why screwcaps often include a permeable membrane in the seal.