There are many things that can have an effect on the quality and stability of wine. Specifically, both enzymatic and non-enzymatic oxidative reactions can significantly influence the aromatic and structural quality of a wine, which without appropriate control, can run rampant and cause off-aromas and flavors in the wine, thus spoiling the beverage. Spoilage microorganisms (such as certain yeasts and bacteria), as well as compounds naturally present in wine can cause oxidation reactions or become oxidized themselves, resulting in undesirable sensory characteristics in the finished wine.
In order to control these spoilage microorganisms and undesirable chemical reactions, winemakers have historically (and currently) employed SO2 (sulfur dioxide) during the winemaking process. In recent years, however, there has been a strong push for winemakers to reduce their usage of SO2, stemmingpartly from the finding that SO2 exposure could be a health risk for certain individuals. Currently, there isnâ€™t one compound or product that can completely replace SO2 in winemaking, though research is ongoing and has already found that reducing the amount of SO2 used in conjunction with another alternative technology could protect the finished wine just as well as higher levels of SO2 used alone. These alternative technologies include hydrostatic pressure, pulsed electric fields, ultrasound irradiation, and UV (ultraviolet) irradiation.
The study presented today aimed to examine the use of UV irradiation as a protective agent against wine spoilage, while comparing with SO2 and no treatment controls. The measure for determining if any of the treatments protected against wine spoilage or oxidation was polyphenol oxidase activity. Polyphenol oxidase is partially responsible for the browning of white wines after oxidation, so theoretically, if polyphenol oxidase levels are decreased, the wine is not oxidized (or is less oxidized) than a wine with higher polyphenol oxidase levels.
Two white grapes were used in this study: Xarel-lo and Parellada (from vineyards in Spain). Grapes were processed in a home juicer and then pressed. To remove any solids, juice samples were centrifuged then the liquid removed. The juice was then split, with half remaining as fresh juice and the other half going into a freezer. For both fresh and frozen juice, samples were split into the following treatments: 1) SO2 addition (50 mg/L potassium metabisulphite) priorto winemaking; 2) UV irradiation treatment prior to winemaking; and 3) winemaking without SO2 or UV treatments (control).
UV irradiation occurred in a dark chamber that housed the juice and a lamp. The irradiation treatment lasted 3 hours and 30 minutes, with samples taken at 0, 60, 120, 180, and 210 minutes.
Winemaking occurred at the Raimat winery in Lleida, Spain. 500mL of each treatment must were placed into a separate glass bottle. Fermentation occurred within these glass bottles using a commercial yeast.
The following analyses were performed on the samples: pH of the must before and after UV irradiation; pH of the wine; soluble solids content; color of musts and wines; tartaric acid content; alcohol content; volatile acidity; and polyphenol oxidase activity.
â€˘ In untreated musts, Xarel-lo musts had higher levels of soluble solids than Parellada musts.
â€˘ Polyphenol oxidase activity was decreased in all UV irradiation samples.
o The level of polyphenol oxidase activity remained at 18% +/-1 of its original activity in Xarel-lo samples and at 30% +/- 1 in Parellada samples after UV treatment.
â€˘ Polyphenol oxidase activity was completely deactivated by SO2 treatment.
â€˘ Polyphenol oxidase activity inactivation was the same for both fresh and frozen samples of grape varieties (i.e. no statistical differences in inactivation between fresh and frozen samples).
â€˘ Inactivation constants were higher for Xarel-lo samples than Parellada samples, which the authors attributed to an increased denaturation ability of the enzyme in Xarel-lo samples compared with Parellada samples.
o Inactivation constants were statistically similar between fresh and frozen samples of both grape varieties.
â€˘ UV irradiation alone did not cause any change in brightness (color) in any sample.
o In Xarel-lo musts, fermentation caused a decrease in brightness in samples that had been previously irradiated, but not for those treated with SO2.
o In Parellada musts, fermentation caused a decrease in brightness in samples that had been previously irradiated and in untreated samples.
o Frozen musts were less bright after thawing than fresh musts.
â€˘ In terms of redness, the freezing and thawing process increased the redness of the sample.
o Wines from UV treated musts were redder than wines from SO2 treated musts.
â€˘ In terms of yellowness, untreated frozen samples showed an increase in yellowness (the trend was not seen in UV treated samples).
o In Xarel-lo wines, fermentation of frozen and thawed musts resulted in a decrease in yellowness that was not found in any other treatment.
o In Parellada wines, UV treated musts created less yellow wines than wines made from SO2 treated musts.
â€˘ There were no significant differences between any of the treatments in regards to absorbance spectra.
â€˘ UV irradiation treatment samples fermented at a slower rate than SO2 treatment samples, which the authors attributed to the decrease in natural microflora caused by UV irradiation.
â€˘ There were no significant differences found between any of the samples in regards to density evolution through the winemaking process.
â€˘ In terms of enological parameters:
o Freezing and thawing resulted in Xarel-lo wines with lower pH, higher tartaric acid, and higher volatile acidity.
o In Parellada wines, pH was higher in wines made from frozen and thawed musts as well as UV treated musts, however, no in wines made from SO2 treated musts.
The authors of this study reported that based on the results, they can conclude that UV irradiation can partially decrease polyphenol oxidase activity in the white wines Xarel-lo and Parellada. It is important to note that UV treatment did not completely eliminate polyphenol oxidase activity as it did with SO2 treatment, but as the authors mentioned, it could potentially be utilized in concert with reduced SO2 levels.
Though the results of this study are interesting, the results only raise more questions for me. First of all, polyphenol oxidase, while a very important contributor to wine spoilage (in terms of oxidation and browning), itâ€™s not the only one. The authors themselves mentioned a couple other enzymes responsible for wine spoilage, including laccase and peroxidase, though we donâ€™t know how UV irradiation would affect the activity of these browning enzymes.
The authors also mention toward the end of the paper that UV light has historically been shown to have negative consequences on wine, but that those papers only looked at UV exposure on finished wine, and not on the musts prior to fermentation. Thatâ€™s all well and good, but why would you bother saying that and not present us with the comparison? Wine goes bad with UV exposure, sowhy wouldnâ€™t the same thing happen with the musts-turned-wine? The authors give no explanation to why they think treating the musts wouldnâ€™t yield the same negative consequences on the wine as it does when exposed to finished wine. Maybe the mechanism and effects are different, but one really shouldnâ€™t make a statement like that unless they are prepared to step up with some results.
After all this talk of wine spoilage, browning, and protection against spoilage and off-aromas or flavors, the study is strongly lacking in any sort of sensory analysis of the finished wines for each treatment. If the primary outcome is strongly tied to aromatic quality, why not have a sensory analysis? Iâ€™d be very curious to see how UV irradiation affected the flavor and aroma of the finished wine, particularly after the authors made the statement that UV exposure of finished wines has been shown to have negative sensory effects.
Overall, this study is a good start; however, there are several issues that should be addressed in a follow-up before I am convinced UV irradiation is an appropriate treatment for combating oxidation or spoilage in wines.Â Don’t give your wines a tan just yet!
What do you all think of this study? Do you see any other problems with the study? Am I being too harsh? What sorts of follow-up experiments or studies would you like to see?
Source: Falguera, V., Forns, M., and Ibarz, A. 2013. UV-vis irradiation: An alternative to reduce SO2 in white wines? LWT â€“ Food Science and Technology 51: 59-64.