Wine Trial: Closure Consistency on Sauvignon Blanc
In the decade-long debate over the most effective closure type, there have been numerous studies and reports detailing varying oxygen transfer rates, consumer perceptions, TCA amounts and more. Wine Business Monthly has surveyed winemakers about their closures preferences for more than a decade, and we recently released our latest results in the June 2015 issue. Closure companies have taken these studies to heart; constantly working to improve their products. When it comes to consistency in the oxygen transfer rate post-bottling, a UC Davis study showed high consistency rates in natural corks, screw caps and synthetic closures. The results were presented at the 2015 ASEV annual meeting in Portland, Oregon in June.
Working with PlumpJack Wine Group and CADE Winery in the Napa Valley, a group of UC Davis researchers tested oxidation and browning in the winery’s 2011 Sauvignon Blanc over 27 months. The two organizations worked together four years ago in a similar study on CADE’s Cabernet Sauvignon (for more information on that research, read “Seminar Investigates Screw Caps, Corks—PlumpJack Winery’s use of screw caps for fine wines highlighted at Cade Winery event”, in the September 2011 issue of Wine Business Monthly). The PlumpJack/CADE Sauvignon Blanc project was run by Dr. Andrew Waterhouse, professor of enology at UC Davis.
“We wanted to know how the same white wine is aging chemically different within these three commonly applied closures,” said Annegret Cantu, assistant specialist with the UC Davis Department of Viticulture & Enology at the ASEV annual meeting. “We also wanted to know: Can the consumer taste the difference within one closure due to possible inconsistencies of the product?”
To research the effects of post-bottling oxidation on a white wine, UC Davis enlisted the help of CADE Winery, and its 2011 Sauvignon Blanc, which is usually closed with a Saranex-lined screw cap.
In the middle of the bottling run, the researchers were able to fit 600 bottles closed with three different kinds of closures: 200 with Amorim’s high-end quality natural cork, 200 with the Nomacorc Select Series synthetic closure and 200 with the Amcor screw cap—the closure that the winery typically uses for the wine. The bottles were made of clear glass flint in order to best view any browning. The different closures were applied in the middle of the bottling run in order to have a very consistent headspace volume. The screw caps were applied first, followed by the synthetics and the natural corks. Once the wine was bottled, it traveled from the Napa Valley winery to UC Davis’ research winery and stored in a dark room at room temperature for the duration of the trial.
Using the browning of the wine as a proxy for oxidation, researchers used spectrophotometers in the lab to measure the browning at 420 nanometers. Each bottle was used as its control, as the team collected five data points over a period of 27 months. Some bottles had high browning, others had medium and low levels of oxidation.
In addition, the wines were tested for sensory qualities. With the help of Tragon in Redwood City, California, a quantitative descriptive analysis was done on a total of nine wines (one bottle of each closure with high, medium, and low browning). An 11-member panel came up with a set of 33 sensory terms: four for appearance, six for aroma, nine for flavor, four for mouthfeel and 10 for aftertaste/after-effect. The study was created as a complete balance block design and everything was done in triplication.
At the time of the sensory study, free and total SO2 were tested.
“The free and total SO2 numbers showed us a good coloration of the browning,” said Cantu. “The very high browning bottles had low SO2—they were around 4 mg per liter, more or less—and the low browning bottles had higher SO2 which was around 16 to 18 mg per liter.”
“So what did the sensory tell us? Between closures, actually, there were no differences in oxygen levels, with a 95 percent confidence,” said Cantu. Within its own closure group, screw caps, synthetics and natural corks all showed no significant differences in oxygen ingress.
However, there were some differences in 11 attributes for each closure and level of browning, which were green apple, puckering, citrus, fruity, golden, buttery flavor, bubbles, sweet aroma, burning, sour and bitter.
“We had two significant terms that saw a significant impact, which were bitter flavor and fruity flavor. The medium natural cork had the most fruity flavor, and the most bitter flavor was for the screw cap medium,” she said.
The researchers turned to a statistician to make sense of the chemical analysis, as they had a total of 600 bottles tested for five data points over a period of 27 months.
“The statistician was looking at the linear curve, and if the browning was going straight, if nothing was happening, etc. and what she actually found at the end was that the best function fitting for this data is a negatively accelerated exponential function,” said Cantu. What that implied was that the browning change over time was quick and then leveled off.
“She looked also at the difference within the closures and there were no significant differences within closure for screw cap and synthetic corks. That means that the product has a very good consistency,” said Cantu. “Then she also found very little difference within the natural cork , but the variation is about 8 percent and its so its very little so that you can even not visually see it, that difference.”
In terms of oxygen ingress, each of the closures fared well, and while there have been various studies that have shown differences in SO2 levels, this particular study showed that there were still some SO2 levels present—most likely protecting the wine.
When it came to consistency: “Natural corks had the highest variation, with 8 percent, but it still had good consistency. Screw caps and synthetic show very consistent results when applied correctly,” said Cantu.
This article first appeared in the September 2015 issue of Wine Business Monthly