As we’ve seen many times here on this blog and on many other sources, red wine is chock full of phenolic compounds that not only contribute to the sensory characteristics of the wine, but also provide numerous health benefits for those that consume the beverage. These antioxidant polyphenols have been shown to play a role in protection against cardiovascular disease, inflammation, and some cancers. One polyphenol in particular, resveratrol (trans-3,5,4’-trihydroxystilbene), has been frequently found to possess health benefiting characteristics. Resveratrol, which is a polyphenolic compound produced by grape vines in response to fungal infection, is believed to be beneficial for human health because of its ability to; inhibit the oxidation of low-density lipoprotein, block platelet aggregation, and activate vasodilatation.
Resveratrol, in addition to many other polyphenols, is found in the skins and seeds of grapes, and the final concentrations of which depend on a variety of factors, including (but not limited to) the type of grape, harvest year, climate, the extraction technique, and winemaking practices used. Studies have shown that different winemaking practices can influence the phenolic composition of wine. Specifically, fermentation temperature, thermovinification, must freezing, and extended maceration techniques have all been shown to increase polyphenol levels in wine. On the other hand, certain treatments such as the addition of certain SO2 levels and cold-soak techniques have been shown to have little to no influence on polyphenol content in wine, while some studies actually showed a decrease.
Polyphenol composition in wines is extremely variable, with some factors more controllable than others. Certain factors, such as harvest year and climate, cannot be controlled by the winemaker, however, factors such as winemaking technique most certainly can be controlled, therefore the winemaker is able to adjust his/her technique accordingly to achieve the desired goals of the final wine in regards to not only sensory characteristics but also polyphenol levels. The goal of the study presented today was to examine the influence of grape variety and winemaking technique on the polyphenol composition of red wine by using the techniques: “double pasta” (addition of extra pomace during fermentation or separation of must from pomace) and thermovinification (destroys cell membranes of grape skins, releasing components into the must).
Wines were made at the enological station Radmilovac of the Faculty of Agriculture in Zemun, Belgrade. The grape varieties used were Merlot, Cabernet Sauvignon, Pinot Noir, and Prokupac. After the grapes were destemmed, they were crushed and sulfur dioxide was added (10gK2S2O5/100kg crushed grapes). At this point, different winemaking treatments were applied.
Thermovinification (2 treatments). Crushed grapes were heated either at 60oC for one hour, or 80oC for three minutes. Both treatments were then cooled at 27oC and inoculated with a pure yeast strain. Maceration and fermentation were done using the microvinification method, at an average temperature of 25oC for 14 days. At the end of fermentation, wines were separated.
Separation of must from crushed grapes (2 treatments). For this treatment, either 30% or 50% of the must was separated from the crushed grapes. The remainder of the crushed grapes were then inoculated with a pure yeast, and the microvinification method was applied just as in the thermovinification treatment.
Control (1 treatment). Grapes were destemmed and crushed, and SO2 was added. After this, the must was inoculated with the pure yeast, and the microvinification method was applied just as in the other treatments.
These treatments resulted in the creation of five different types of wine for each grape variety, for a total of 20 wine samples.
The following parameters were measured: trans-resveratrol, cis-resveratrol, total phenolic compounds, and radical scavenging activity.
- The amount of trans- and cis-resveratrol varied greatly among the wines studied, depending on the grape variety and the winemaking treatment used.
o Total resveratrol content ranged from 0.35 to 4.85mg/L.
- The antioxidant capacity (measured by radical scavenging activity) ranged from 0.58 to 2.91μL/mL.
- Merlot wines had the highest level of trans-resveratrol, while Prokupac wines had the lowest level.
- Cis-resveratrol (not natural, uncertain of how it develops in wine) was identified in all wine samples, though only measured numerically for 14 of them. Levels ranged from 0.02 to 2.90mg/L, with an average of 1.12mg/L.
- Merlot wines had the highest level of cis-resveratrol.
- Wines made from Pinot Noir grapes were split into two different groups (in regards to their resveratrol content), depending upon which winemaking technique was used.
o Thermovinification treatments showed similar levels to each other, while must separation treatments showed similar levels to each other (both treatment types significantly different).
- There was no significant correlation between winemaking treatment and resveratrol content in wines.
o Samples from the vinification technique showed the highest levels of total resveratrol in the Merlot and Pinot Noir wines.
o Samples from the 50% must separation technique showed the highest levels of total resveratrol in the Cabernet Sauvignon and Prokupac wines.
§ Different characteristics for each grape variety may lead to variations in extraction efficiency during winemaking.
Total Phenolic Composition and Antioxidant Capacity
- Merlot wines showed the highest level of polyphenols.
- Prokupac wines showed the lowest level of polyphenols.
- There was no significant correlation between total resveratrol and antioxidant capacity, or total phenol levels.
o Antioxidant capacity of wine depends on phenolic compounds other than resveratrol.
- Wines undergoing the thermovinification treatments showed higher levels of phenolic compounds than the control.
o Wines treated at lower temperatures for longer (60oC for one hour) resulted in higher levels of phenolic compounds than wines treated at higher temperature for shorter (80oC for three minutes).
§ The length of time of skin contact (one hour versus three minutes) between the pomace and the juice is important in the extraction of polyphenols into the wine.
o Thermovinification allowed greater extraction of polyphenols since the heat destroys the grape skin cells, which releases the pigments, polyphenols, and other components into the wine.
- The must separation treatment did not significantly increase the total phenolic content or antioxidant capacity of the wine.
o Must separation tended to decrease the antioxidant potential, likely a result of the loss of phenolic compounds caused by earlier removal of the skins than the control.
What does this all mean?
The results of this study show that grape variety plays a huge role in the phenolic composition and antioxidant capacity of red wine. As a result of this large variability, one cannot simply apply a particular winemaking technique to any grape variety and come up with the same outcome every time. The results of this study show that each grape variety reacts different to different winemaking technique, and one must have a good understanding of the entire system before making any decisions on which winemaking technique to use for a particular grape variety. Merlot and Pinot Noir wines showed highest levels of polyphenols using the thermovinification technique, while Cabernet Sauvignon and Prokupac wines showed the highest levels of polyphenols using the must separation methods.
For the must separation treatment, even though Cabernet Sauvignon and Prokupac displayed the highest levels of polyphenols, they did not differ significantly from the controls, thereby indicating that this winemaking technique may not be the best option for the particular grape varieties that were studied.
The results were clear, however, in that the thermovinification technique did increase polyphenol levels (including resveratrol) in all wine samples, though some types showed a greater increase than others (with Merlot and Pinot Noir).
If the ultimate goal is to create a wine with higher levels of health beneficial polyphenols and antioxidants, the results of this study make it clear that first, grape variety choice is extremely important, and that second, winemaking technique choice is also important. Out of the two techniques studied, it appears as though thermovinification methods (longer time at a slightly cooler temperature) may be more effective in increasing polyphenol and antioxidant content than traditional methods alone.
Ultimately, more research needs to be done regarding more grape varieties, more winemaking techniques, and also sensory analysis for potential changes in wine quality. Perhaps a large-scale study examining multiple varieties of grapes under multiple winemaking techniques will allow a greater understanding of phenolic composition of wine, and how a winemaker may use this knowledge to produce a quality wine with the desired levels of polyphenols for their health-conscious consumers.
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Source: Atanacković, M., Petrović, A., Jović, S., Gojković-Bukarica, L., Bursać, M., and Cvejić, J. 2012. Influence of winemaking techniques on the resveratrol content, total phenolic content and antioxidant potential of red wines. Food Chemistry 131: 513-518.
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!