Definition: Cork processing

Are we really on the verge of eliminating cork taint? A new decontamination method using supercritical carbon dioxide should soon protect wines from cork taint. The "true cork taint", which used to be all too common, is caused by a fungus, Armillaria mellea. Today, appropriate treatments have virtually eliminated it. However, another unpleasant taste also spoils wine. Less well known, it is caused by various microorganisms (bacteria and yeasts) that transform certain organic molecules into volatile and foul-smelling substances (chloroanisoles). Guy Lumia, from the CEA in Pierrelatte, has found a way to decontaminate corks using supercritical carbon dioxide. His method is so effective at removing unpleasant odours from corks that it could replace boiling, the traditional process using steam and various oxidising additives. Carbon dioxide becomes supercritical when its temperature exceeds 31°C under a pressure of 74 atmospheres, i.e. above its critical point. Neither gas nor liquid, this fluid has such low viscosity that it penetrates deep into most porous materials; it is very unreactive and solubilises many organic molecules. For this reason, it is increasingly used to extract natural flavours, dry clean clothes and remove oil from machined parts. At the request of Sabaté Diosos, a company that produces corks, G. Lumia attempted to use supercritical carbon dioxide to eliminate the microorganisms responsible for cork taint. In an initial experiment, he first pressurised the corks in an autoclave, but when he removed them, they were in poor condition. These pressure-damaged corks were analysed and found to be perfectly clean. This decontamination is particularly effective since the total amount of chloroanisoles in a cork does not exceed a few tens of nanograms per gram. These chloroanisoles are all the more difficult to remove because the molecules are dispersed throughout the volume. After passing supercritical carbon dioxide through the cork, the residual proportion of chloroanisoles is so low that it is below the detection threshold of the best gas chromatographs. Even oenologists are unable to detect chloroanisoles in cleaned cork during tasting. However, a trained human palate is significantly more sensitive than a chromatograph. The treatment also has a bacteriostatic effect (i.e. it stabilises the microbial fauna) and an antifungal effect (it reduces the fungal population). To avoid damaging the cork during treatment, G. Lumia and his colleagues increase the pressure in stages, allowing the mechanical stresses to be distributed throughout the entire volume of the cork. In this way, the corks are purified without being degraded. This process, based on the use of a common and inexpensive gas, is expected to grow, preventing the contamination of several hundred million of the approximately 20 billion bottles corked each year worldwide. See cork taint.