5 January 2005, Tyson Stelzer, The WineStar Journal
Corks, screw caps and oxygen ingress… an answer at last?
The debate over whether oxygen ingress through the closure is a requirement for proper bottle development has been ongoing ever since Louis Pasteur announced in 1863 “it is the oxygen that maketh the wine.” In recent times, Bordeaux wine scientists Jean Ribéreau-Gayon and Emile Peynaud have argued the case that wine development is a reductive, not an oxidative process and that the quantities of oxygen that normally penetrate into the bottle are negligible, anyway. Hence, reactions that take place as bottled wine matures do not require oxygen. The counter-argument to this theory has been put forward by Californian wine scientist Roger Boulton, who asserts that wine maturation is dependent upon small amounts of oxygen seeping through the cork.
The debate has been stirred with the revival of screw cap closures in recent years, with the screw cap lobby pushing the Ribéreau-Gayon/Peynaud line. In my book “Screwed for good?” I spent five pages tossing around the arguments. The issue has become one of the biggest questions surrounding the maturation of wine under screw cap.
I was recently a guest at the International Screw Cap Symposium in New Zealand, where wine scientist Peter Godden of the Australian Wine Research Institute presented some fascinating results on oxygen ingress which just might settle the case once and for all. And, in contrast to five pages, I can now answer the entire 142-year-old debate in just a few paragraphs:
In a trial of the technical performance of various wine closures conducted by the Australian Wine Research Institute (AWRI), the oxygen permeation through the closure was measured approximately three years post-bottling. The measurements revealed that 45mm reference 2 corks permitted between 0.0001 and 0.1227mL of oxygen to enter the bottle per day, with an average of 0.0179mL. By comparison, screw caps permitted 0.0002 to 0.0008mL, with an average of 0.0005mL.
This demonstrates two important factors relating to the performance of corks and screw caps. First, the consistent performance of screw caps is revealed by the range of 0.0002 to 0.0008mL, meaning that the difference between the best and the worst screw cap is a factor of four, and both represent negligible levels, anyway. Looking at the same comparison for cork, the factor is a whopping 1227!
Second, and more important in the current debate, is the comparison between cork and screw cap. The very best cork in the trial offers an ingress of 0.0001mL, while all screw caps in the trial had ingresses between 0.0002 and 0.0008mL. What this is saying is that the oxygen permeability of the screw cap is essentially identical to that of the very best corks, and in both cases it is negligible. The screw cap thus accurately replicates the maturation conditions provided by the very best corks.
It is universally accepted that the greatest old wines are those with the best corks and the smallest ullages. These are the corks with the very lowest oxygen permeability. The screw cap replicates their performance, not in the odd instance of the occasional “best bottle,” but consistently, for every bottle.
It appears that the AWRI measurements finally settle the debate, confirming that oxygen ingress through the best corks is indeed negligible, verifying the case presented by Ribéreau-Gayon and Peynaud. Many screw cap advocates have been arguing this line for some time, but now there is solid, scientific, quantitative evidence, to support this case.
Measurements of such minute levels of oxygen ingress as were made in this trial had not been performed until now. Using the latest technology, scientists at the AWRI developed new techniques which made these results possible. All credit to the AWRI!