Corks and screw caps: Can wine consumers taste the variation?

University of California, Davis
September 22, 2012

UC Davis wine chemist Andrew Waterhouse says exposure to oxygen is the most important factor in wine aging but too much can cause a loss of color, flavor and aroma. (Joe Proudman/UC Davis photo)

To help winemakers determine the best caps for their wine bottles, researchers at the University of California, Davis, are studying the performance -- specifically the variability -- within different types of closures.

Their goal is to determine whether consumers can taste the difference in wines that are bottled and capped exactly the same -- a difference that could be attributed only to variation among each type of wine closure.

The researchers -- including a wine chemist, a medical radiologist and a biomedical engineer -- are evaluating 600 bottles of Sauvignon Blanc wine, each sealed with one of three different types of

closures: natural cork, screw caps or synthetic cork. The study will monitor changes in the wine during aging, culminating in a sensory evaluation to determine if wine experts and consumers can taste the different levels of oxidation that occur in the wine due to variability within each type of closure.

Oxidation, or exposure to oxygen, is the most important factor in wine aging, according to wine chemist Andrew Waterhouse, a professor in the UC Davis Department of Viticulture and Enology. But too much oxidation can cause a loss of color, flavor and aroma.

"Our goal in this study is to determine if individual bottles might be getting a lot more or less oxygen -- and therefore aging at different rates -- as a result of the variation in the closures used to seal the bottle," said Waterhouse, who is carrying out the study with UC Davis undergraduate student Jillian Guernsey.

"Ultimately, when all of the data are in, we won't be declaring that one type of closure is superior to another. Rather we'll be giving winemakers information about the variability of each type so that they can determine which is most appropriate for use in bottling their wines," Waterhouse said. "If variation is high enough for consumers to notice a difference, we will work with the industry to help find ways to manage the variation so that consumers receive the wine as it was intended."

The Department of Viticulture Enology, the largest and most comprehensive university wine program in the United States, has been at the forefront of international grape and wine innovation for 130 years. It continually partners with industry to develop practical solutions to problems that are of concern to winemakers and consumers.

The researchers have included a novel step in this study, using medical imaging technology to obtain a baseline evaluation of each of the corks. To do this, they teamed up with John Boone, a radiology professor in the UC Davis School of Medicine and an internationally known expert in designing and improving computed tomography scans for breast imaging. Boone, who also leads the cancer imaging research initiative for the UC Davis Comprehensive Cancer Center, used a new CT scanner, which he had invented, to obtain images of each of the 200 natural and synthetic corks.

David Fyhrie, a professor of biomedical engineering who also holds the David Linn Chair in Orthopaedic Surgery in the UC Davis School of Medicine, will analyze the images to look for differences in the internal structure of the corks.

All of the 600 bottles of Sauvignon Blanc wine are being stored in a temperature-controlled wine cellar at 20 degrees C (68 degrees Fahrenheit). Each of the bottles will be tested for darkening in wine color at three-month intervals during the 12-month study. Earlier research in Australia has demonstrated that the color of white wine is a reliable indicator of the degree of oxidation.

The researchers will use specially modified spectrophotometer devices to analyze the wine color in the bottles without opening them. Clear glass bottles are being used to facilitate color-based monitoring of oxidation based on color change.

At the study's end, the 600 bottles will be divided into three groups, based on whether they show high, average or low color change. The wines with the most and least color change will be opened for chemical analysis and sampling by members of a sensory panel, who will try to identify differences in taste and aroma between the most- and least-oxidized wines. That sensory analysis is expected to take place in during the summer of 2013.

Waterhouse said that there is also considerable interest in the comparative sustainability of wine bottle closures.

"All wine closures are made with sustainable practices, and to date I have not seen data showing a definitive difference between them," he said. "It's important for wine consumers to remember that the bottle closure is a very small part of the wine package's environmental footprint," he said.

The study is supported in part by CADE Winery of The PlumpJack Group.  PlumpJack Winery pioneered the use of screw-cap closures for ultra-premium wine more than 10 years ago.

Media contact(s):

* Andrew Waterhouse, Viticulture and Enology, (530) 752-4777, alwaterhouse@ucdavis.edu

* Pat Bailey, UC Davis News Service, (530) 752-9843, pjbailey@ucdavis.edu