Appearing in the March 2020 edition of Wine Business Monthly
The oxygen transmission rate (OTR) of a wine closure is just one of several factors that contribute to the total package oxygen (TPO) in a bottle of wine. According to Dr. Paulo Lopes who conducts research and development at Santa Maria da Fiera-based Amorim & Irmãos, S.A. and has extensively studied the OTR of natural corks, closures are the least variable aspect when considering TPO. “We know precisely how much oxygen a closure will provide to the wine but only by accurately measuring oxygen during the bottling process are we able to make precision additions during winemaking.”
Lopes’ current research illustrates the oxygen release of natural cork over time, a measurement that is particularly relevant in the context of an oxygen audit designed to measure total package oxygen – the combination of the oxygen contained within the closure combined with the presence of atmospheric and headspace oxygen during bottling and the dissolved oxygen in the wine.
Oxygen Dynamics of Natural Cork
Not surprisingly, different grades of cork contain different amounts of oxygen; a longer, higher-quality Grade A cork with fewer lenticels will release less oxygen. “Longer corks are much more homogeneous in oxygen release,” said Lopes. “Also, due to the [sloping] shape of the bottle neck, the cork is less compressed and thus releases less oxygen.” To that effect, Amorim has created an online application which makes the OTR rates of it closures readily available.
Lopes is also researching the contribution of cork phenolics to wine. “Phenols from cork in low amounts can help shape the oxygen reduction potential of a wine by polymerizing some compounds to reduce astringency and bitterness,” he said. In effect, they provide extra protection against oxidation. “We’re working to understand the relationship between cork length and different kinds of wine. By using the same approach as the barrel industry we’ll be able to identify the optimal pairing between wine and cork.”
On average, a natural cork will release up to one mg of oxygen during the first six months in bottle and then continuously micro-oxygenate at just over one mg from its cellular structure over a period of 60 months of storage. Although it’s impermeable to atmospheric oxygen, oxygen from the cell structures of the cork travels through the plasmodems and lenticels in to the wine.
Corks used to seal wine bottles have a lifespan of about 25 years, after which they begin to lose elasticity and can start to let atmospheric air into the bottle along their sides. “After ten years, a cork will lose only one to two percent of its elasticity,” said Lopes. “And if stored in contact with the wine, it will absorb about three millimeters of wine. “
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