Air ingress through bottle crowns.

Teumac, F.N., Ross, B.A. and Rassouli, M.R.

Abstract
Investigations into the oxidation of bottled beer as a result of oxygen diffusing through the crown cap liner are described. Analysis of the headspace gas from bottles fitted with caps of various types, which were marked to ensure identification and recovery before being fed into the capping machine of a commercial beer bottling line, showed that the proportion of oxygen in the total gas diffusing into the bottle varies according to the properties of the crown liner. The normal commercial crown cap is lined with polyvinyl chloride (PVC) which in its pure state is very hard and must be combined with so called "plasticisers" to give it the correct consistency. Experiments using different plasticisers and with special low permeability polymers showed that the latter (alone or blended with PVC) could reduce oxygen intake from the normal rate of 0.002 ml/day to 0.001 ml/day, but as they are too expensive and difficult to process for commercial use, an alternative solution was needed. New lining materials have now been developed, combining PVC with special compounds which react with oxygen so as to form a selective barrier to its ingress. It is claimed that they can totally prevent oxygen diffusing through the cap, thereby improving flavour stability, and are otherwise similar to conventional lining materials. However, because they are at the time of writing in the process of being patented in the USA, their composition remains a trade secret until the patent application process is completed.
Keywords : beer closure diffusion glass bottle oxidation oxygen polyvinyl chloride stability