Screw caps extend sauvignon shelf life

Screw caps are better than corks at preserving the fruity bouquet of sauvignon blanc wines, report researchers in New Zealand.     

The team studied the composition of two-year old wines from the wine growing region of Marlborough. They compared bottles of wine that had been sealed either with corks or with screw caps. HPLC analysis revealed that levels of two volatile thiols with a fruity aroma - 3-mercaptohexyl acetate (with an aroma of passion fruit or box tree) and 3-mercaptohexanol (with a fruity aroma, particularly of grapefruit) - were up to 23 per cent lower in cork-stoppered bottles.     

Corks and screw caps appeared equally successful at preventing oxygen ingress, and the researchers suspect that the cork may have absorbed some of the volatiles during storage. A panel of 12 trained wine tasters failed to pick up any significant differences in the wines but identified more passion fruit notes in wines with higher thiol concentrations.   

New Zealand’s sauvignon blanc is noted for its ’powerful tropical fruit aroma harmoniously mixed with grassy vegetal aromas,’ said lead researcher Laura Nicolau at the University of Auckland. The thiols, which contribute to the tropical aroma, are very susceptible to oxidation, Nicolau added. Which explains why sauvignon blanc can lose its fruity character after one or two years.     

The news that corks could affect a wine’s aroma came as no surprise to UK wine journalist Malcolm Gluck. ’Corks are notoriously inept at retaining real fruit characters compared with screw caps,’ Gluck told Chemistry World.     

Gluck stressed the importance of retaining the ’tropicality’ of a new world sauvignon blanc. The wine is supposed to be drunk young, he said, but the arrival of screw caps might mean wines can be kept longer.   

He once drank a Czech sauvignon blanc bottled in 1947. ’The maker had walled much of his wine up, not to conceal it from the Nazis, who had been recent occupants, but from the Russians who were poised to take over,’ recalled Gluck. ’He was wise. The wine was drinkable but extremely quaint, and the cork had lignified so the immediate fruit character had been unusually retained. It lasted ten minutes in the glass.’     

Nicolau’s team in Auckland plans to identify how significant the differences in thiol concentrations need to be before a panel of tasters can distinguish them. The researchers are also trying to understand exactly how the compounds contribute to the sauvignon blanc bouquet and are studying the long-term stability of those compounds. Emma Davies