Iowa State University Extension and Outreach
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Midwest Grape and Wine Industry Institute

Research Winemaking Focus

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Figure 1. Marquette wines in bottle from 2014 to 2019 

Ongoing research: Evolution of Marquette and Frontenac red wines during aging

How long a red wine can age? Several years for Vitis vinifera red wines, but it depends on many factors including how much it would be protected from oxidation. 

The goal of the wine industry is to produce high quality wines that could be enjoyed by consumers at the time of purchase and up to a few years after. Wines in Iowa and in the Midwest are produced from cold-hardy grapes that differ from Vitis vinifera, the most commonly grown grapes in the world. Cold-hardy red wines, such as Marquette and Frontenac grape varieties, tend to have low concentration of tannins, phenolic compounds which help protect wine against oxidation. Because the chemistry of red wines produced from cold-hardy grapes is not well known, it is a challenge to determine the best winemaking practices to produce a high quality wine capable of aging for a few years from those cold-hardy grapes. As per a previous wine quality study conducted on Iowa wines, the main faults found in most of the red wines were oxidation-related, most likely due to a lack of the recommended amount of sulfur dioxide. 

Sulfer Dioxide

Sulfur dioxide is a preservative that is used during all the steps of the winemaking process to protect wine against oxidation and microbial spoilage. Its action is both on the inactivation of enzymes, which would oxidize polyphenols and lead to a browning reaction, and on damaging the bacteria and yeasts cells as well. Sulfur dioxide is a compound difficult to manage in winemaking because different forms exist with different actions. In wine, the molecular form of sulfur dioxide is the most effective form, but is present in the least amount under acidic conditions (wine pH). In order to add the appropriate concentration of sulfur dioxide in wines, chemical parameters need to be checked including: 

  • wine pH,  
  • level of free sulfur dioxide already present in the wine, and  
  • wine compounds that could bind with sulfur dioxide and reduce its efficacy (Carrascón et al., 2018).  

Sulfur dioxide addition is especially important in wine at bottling to protect wines and limit the development of wine faults during aging, meaning until the wines are consumed.  

One piece of information often left off the wine label is the vintage, i.e., the year of wine production. This information is not mandatory on wine labels but provides an indication to the consumers about the age of the wine. The longer a wine is aged, the more complex or oxidized it can be if the levels of sulfur dioxide and the level of antioxidant compounds such as tannins have not been well managed at bottling.  

In one of my labs’ current research and extension project, funded by the Iowa Department of Agriculture and Land Stewardship in 2021-2022 (award ID: 025244-00001), we chose to evaluate the amount of sulfur dioxide and oxidation-related compounds of bottled red wines made from Frontenac and Marquette grapes from 2013 to 2020. The goal of this study was to determine the relationship between oxidation-related fault, wine age, and management of sulfur dioxide. 

We have worked with three Iowa wineries who had more than 3 wines from the same variety for at least 3 consecutive years in bottle. We have gathered 14 Frontenac red wines and 9 Marquette red wines of vintages from 2013 to 2020. The basic chemistry was analyzed including pH, titratable acidity, color intensity and hue. Other analysis done include the amount of free and total sulfur dioxide (SO2) remaining in those wines as well as the concentration of phenolic compounds (tannins and anthocyanins) and acetaldehyde, which is one marker of oxidation, as it is the product of oxidation of ethanol.  

Overall, the pH of Frontenac wines ranged from 3.29 to 3.75 and the pH of Marquette wines ranged from 3.43 to 3.77. The hue of Frontenac wines significantly decreased from 2013 to 2020, suggesting that more browning was observed in 2013 Frontenac wines than in 2020 Frontenac wines. The amount of phenolic compounds, especially tannins, did not follow any trend throughout aging in bottle, but were still very low compared to the average content of tannins in Pinot noir wines, a Vitis vinifera variety known to be low in tannin content. 

The amount of free SO2 was evaluated in the wines and compared to free SO2 and pH values at bottling, which the involved wineries provided. Based on the calculations made from the provided wine pH for 0.8 ppm molecular SO2 (this level tends to be for whites that do not contain high content of phenolic compounds but was selected for those red wines as the content of tannins was low), 11 wines out of 23 showed an amount of free SO2 below the expected amount. After considering an extra 40% of free SO2 at bottling, which would bind with other compounds, still based on the pH for 0.8 ppm molecular SO2, 20 wines out of 23 wines did not have enough free SO2 at bottling.  

These results show the importance of an appropriate management of sulfur dioxide to protect the wines against oxidation and maintain a good wine quality that consumers will appreciate. A follow up study will start in October 2022 to evaluate the rationale use of sulfur dioxide in cold-hardy red wines during aging. 

Figure 2. Dr. Aude Watrelot hosting the workshop and presenting sulfur dioxide management, March 2022.

Some preliminary data were presented during the workshop on sulfur dioxide management offered in March 2022 at Iowa State University as well as at the American Society for Enology and Viticulture –Eastern Section in July 2022. Those results will be submitted for publication in a scientific peer-reviewed journal. 

If you have any suggestions or questions about research and winemaking, feel free to contact me at watrelot@iastate.edu.  

References to learn more:  

Carrascón, V., Vallverdú-Queralt, A., Meudec, E., Sommerer, N., Fernandez-Zurbano, P., & Ferreira, V. (2018). The kinetics of oxygen and SO2 consumption by red wines. What do they tell about oxidation mechanisms and about changes in wine composition? Food Chemistry, 241, 206–214. https://doi.org/10.1016/j.foodchem.2017.08.090