Sam Chafe, Australian wine research scientist who has now retired from the CSIRO, has sent the following thoughts on the likely effects of using screwcaps on wines designed for bottle maturation.
The defect, ‘corked’ wine has prompted a number of winemakers to employ Stelvin [screw-] caps or plastic corks. These ‘artificial’ closures obviate corked wine and have the further advantage of keeping the wine fresher over long periods. However, their ability to ensure that wine matures in bottle in the normal way has been questioned. The reason for this has not been explored, although the nature of wine maturation in bottle and the anatomy of cork could well provide the answer.
Cork comprises the periderm of the cork oak, Quercus suber, and is composed of cells with cell walls of inordinately low permeability. The spaces enclosed by the cell walls, the cell lumina, are ‘vacant’ but must contain gas of some sort, probably air, and it is here that speculations may be made.
It is fairly common to find in old wines a level of ullage in the bottle significantly greater than that normally seen in young wines. The usual explanation for this is that the cork has leaked, perhaps under the influence of too much heat at some stage in the wine's development.
There are times, however, when there is little or no evidence of leaking when the capsule is removed, yet the ullage may be significant. Some suggest that small volumes of air migrate along the cork-glass interface which, as well as contributing to the maturation process, allows the ullage space to enlarge. But if the bottle has been lying on its side, any air entering this interface would inevitably be replaced by wine and, as has been postulated here, there is no evidence of leaking.
There is another possibility. Examination of the cork in old wines will often reveal that the bottom part, i.e. that abutting the wine, is extensively saturated. And, sometimes, almost the entire cork can be so described. Although cork is regarded as impermeable, it is only comparatively so and, given sufficiently long exposure, will absorb liquid. In turn, the absorption of wine by the cork means that the ullage must increase, thereby producing the effect often observable in older wines.
In the bottle, for the cork cells to take up wine they must give up the gas in their lumina, and it is this gas which is ultimately absorbed by the wine. As the gas is probably air, an explanation is provided for the gradual maturation of wine in bottle, via access to oxygen in the air, without resorting to the extreme improbability of air passing through the cork from the outside. And it provides the reason why wine sealed in bottles by impermeable Stelvin caps or plastic corks show little or retarded maturation.
Although non-cork closures will protect the wines from becoming ‘corked’, if the normal development of wine in bottle is the objective, cork may be still the best sealant. And until the problem of cork-taint is finally solved, the occasional corked bottle amongst the finely matured may continue to be the necessary evil of long-term cellaring.