Science proves how closures work

"Cork stoppers play an important role in the development of wine in bottle, through managed micro-oxygenation," according to the latest Bordeaux University research to be verified and published in the Journal of Food and Agriculture Chemistry.

While studies of the effect oxygen has on wine have been around for over a century – in 1871 Louis Pasteur wrote “oxygen makes a wine, which ages under its influence” – this is the key research to provide scientific knowledge of the  impact commercial closures have on wine after bottling.

Taking bottles of a Bordeaux Sauvignon Blanc, sealed with different, commercially available stoppers – plastic, natural and technical corks, glass and screwcap with two different liners – the research programme studied the effect of oxygen, and the performance of each closure, over a two-year maturation period.

Previous work on closure permeability over three years (published 2004, 2005, 2006 and 2007) proved that aluminium sealed bottles contained a minimal amount of oxygen, present since bottling; the cork-closed ones initially transferred oxygen from their internal cells, but levelled off after a few months; and that the plastic-stoppered bottles showed high and rapid oxygen ingression, soon rising off the graph measure. Thus, aluminium provided the tightest seal, plastic the loosest owing to its rigidity, and cork – with its flexible cells – remained a steady influence between the two.

Having proved where the oxygen came from, this time the scientists wanted to find out how that oxygen affected the bottled wine, in terms of taste and colour, using both chemical and sensory analysis.  They measured the bottles – lying uniformly flat - at intervals of 2, 12 and 24 months.

To summarise briefly, at the end of the two-year period it was found that the ascorbic acid and sulphur dioxide (which are added to wine at bottling to prevent oxidation) were practically depleted in the plastic-stoppered bottles and the wine colour had developed deeply  – indicating high levels of oxygen ingress affecting the fruit characters of the wine.   Conversely, both levels in the screwtopped bottles were still high, and the wine colour remained light, as expected with such a tight seal. 

Meanwhile, bottles with natural cork stoppers – whole, colmated and technical – showed remaining levels of ascorbic acid and sulphur dioxide balanced between plastic and screwcap, with a similar result in colour analysis. Thus, oxygen transmission was active but slow and minimal.

Then it was the turn of the sensory team of experienced tasters from Bordeaux University’s Department of Oenology.  While they found that the plastic-stoppered wines lost their fruity attributes and developed oxydised aromas, they also identified some ‘rotten egg’ or ‘putrefaction’ characters in the bottles sealed hermetically with either glass or aluminium. The screwcap with a saranex liner, however, was able to minimise these reduced-like aromas so the levels of H2S were not high enough to spoil the wine.

In the case of cork, the sensory team concluded that cork stoppers played an intermediate role, minimising both reductive and oxidative characters, retaining the varietal fruit characters typical of a Sauvignon Blanc.

Paolo Lopez, author of the published paper, sums up:  "It is important for winemakers to understand how oxygen can benefit – or spoil – a wine.  So their choice of stopper is crucial for the right oxygen management during development.  It is hoped that the results of this study will help to elucidate, with scientific evidence, the role of oxygen on wine during the post-bottling period."

The full scientific paper can be found at J. Agric. Food Chem. 2009, 57, 10261-10270. 

Abstract:

This work outlines the results from an investigation to determine the effect of the oxygen dissolved at bottling and the specific oxygen barrier properties of commercially available closures on the composition, color and sensory properties of a Bordeaux Sauvignon Blanc wine during two years of storage. The importance of oxygen for wine development after bottling was also assessed using an airtight bottle ampule. Wines were assessed for the antioxidants (SO2 and ascorbic acid), varietal thiols (4-mercapto-4-methylpentan-2-one, 3-mercaptohexan-1-ol), hydrogen sulfide and sotolon content, and color throughout 24 months of storage. In addition, the aroma and palate properties of wines were also assessed. The combination of oxygen dissolved at bottling and the oxygen transferred through closures has a significant effect on Sauvignon Blanc development after bottling. Wines highly exposed to oxygen at bottling and those sealed with a synthetic, Nomacorc classic closure, highly permeable to oxygen, were relatively oxidized in aroma, brown in color, and low in antioxidants and volatile compounds compared to wines sealed with other closures. Conversely, wines sealed under more airtight conditions, bottle ampule and screw cap Saran-tin, have the slowest rate of browning, and displayed the greatest contents of antioxidants and varietal thiols, but also high levels of H2S, which were responsible for the reduced dominating character found in these wines, while wines sealed with cork stoppers and screw cap Saranex presented negligible reduced and oxidized characters.

1 January 2010 - Felicity Murray