The use of cover crops in a vineyard has been well studied, and is often employed in vineyards around the world. Basically, cover crops are plants (usually grasses) sown in between the rows of vines, which are most often used in areas of the world where there is summer rainfall events or irrigation systems. Many studies to date have examined the effects of cover crops on grapevine growth as a result of water competition between the cover crops and the grapevines, and have found that the water consumption by the cover crops effectively reduces the amount of water available to the grapevine, thus reducing vegetative growth.
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Reduced vegetative growth results in a favorable vegetative to reproductive growth ratio, with more energy being put into the berries instead of the vegetative portions of the plant. Less vegetative growth means a lower canopy density, which effectively improves the microclimate at each grape cluster. Cover cropping also results in reduced must titratable acidity and increased soluble sugar, increased sugar to acid ratios, and increased total phenols and anthocyanin concentrations in the skins of red grapes. All of this results in enhanced grape and wine quality.
Even though there have been many studies examining viticultural and enological effects of cover cropping, very few studies have examined the effects of cover cropping on the aromatic compounds of a wine. It is been shown that cover cropping enhances the flavor and overall favorability of the wine; however, few studies have examined what changes chemically in the aroma of the wine. The study under review today, which was published at the beginning of this year, sought to examine that very question. The overall goal of this study was to determine the influence of a permanent cover crop on the major volatile compounds of the Cabernet Sauvignon grown in a continental monsoon climate, and whether different types of cover crops changed the overall quality of the wines.
Methods
Study Site and Vineyard
The study site was located in the Yangling district of Shaanxi Province in northwest China. This area sees a temperate climate, with a mean precipitation of 580mm per growing season. The soil type in this vineyard was primarily loam, with organic matter of 1.2% and a pH of 8.3. The grapes used were Cabernet Sauvignon (Vitis vinifera L.) and was planted in 2002 (experiment performed 2006-2007). Spacing between rows was 1.5m, and spacing between vines was 1.0m. Rows were oriented in a south-north direction. Vines were trained on a vertical shoot positioning system with a pair of wires, with the shoots trimmed twice (manually) between bloom and veraison to a height of 1m.
Experimental Treatments
There were four treatments which were established in a randomized complete block design with three replicates for each treatment (for those not familiar with stats, this type of a design is very common and effective). Each replicate contained four rows of vines, with 60 vines per row (or 200 vines per replicate; 600 per treatment).
The four treatments (sown in 2005, with the experiment starting in 2006) were as follows:
1) Control: clean tillage between rows
2) Permanent cover crop of white clover (Trifolium repens L.)at 15kg/ha
3) Permanent cover crop of alfalfa (Medicago sativa) at 20kg/ha
4) Permanent cover crop of tall fescue (Festuca arundincea Schreb.) at 30kg/ha
All cover crops were mowed with a flail mower three times per year to a height of 10-15cm. All of the clippings were left on the surface of the soil/cover crop to decompose naturally. Weeds were controlled using a spading machine. All other cultivation practices were constant for all treatments.
Winemaking
Cabernet Sauvignon grapes were harvested at 20oBrix. Grapes were destemmed and crushed, then transferred to stainless steel tanks. Forty liters in three replicates for each treatment were produced. Sixty mg/L of SO2 was added to the must, and 20g/hL of dried active yeast (Saccharomyces cerevisiae) were added according to commercial specifications. Maceration occurred simultaneously with fermentation for a total of 10 days at 25-28oC. After fermentation was complete, wines were transferred to another tank and cold stabilized for three weeks at 4oC before being bottled.
General enological parameters measured were: degrees alcohol, reducing sugar, pH, total acidity, volatile acidity, free SO2, and dry extract. Identification of volatile compounds was completed using mass spectrometry.
Sensory Analysis
Judges were graduate students and teachers at the College of Enology, Northwest A and F University in Shaanxi Province, China. Wines were randomly and blindly distributed to the judges, and sensory characteristics evaluated were visual aspects, aroma, taste and balance (“harmony”).
Results
General Enological Parameters
- All general enological parameters were within the standards, though compared to the control, all cover crop treatments showed a significant decrease in total acidity.
- Cover crops treatments also showed a significant increase in wine pH and dry extract compared to the control, though no difference between the different cover crop treatment types.
Volatile Compounds
- The majority of volatiles found in the wines were: higher alcohols, ethyl esters, acetate esters, and fatty acids. Minor compounds found were: terpenes, norisoprenoids, volatile phenols, sulfur compounds, and furan.
- Concentrations of total volatiles of the four treatments ranged from 39.8-88.1mg/L.
o Wines from the alfalfa cover crop had the highest levels of volatiles, while the control had the lowest.
§ This suggests that cover crops may enhance wine quality.
- Four acetate esters were detected in the wines of study, with concentrations ranging from 5.8-10.2mg/L (or 14.2-22.9% of the total volatiles).
o Acetate esters detected: ethyl acetate (fruity, sweet), isoamyl acetate (fresh, banana), hexyl acetate (pleasant fruity, pear), and heptyl acetate (pear, apricot, almond). Higher levels of these esters are considered to improve the quality of young wines.
o Wines from the alfalfa cover crop showed the highest levels of acetate esters, with the tall fescue coming in second. The lowest levels of acetate esters belonged to the wine produced from the control (no cover crop) treatment.
- Based on odor thresholds, ethyl acetate and isoamyl acetate had the most impact on odor of the wines, and were higher in the cover crop treatments than in the control treatment.
- Fourteen ethyl esters (which contribute to the fruity aroma of young wines) were found in the sample wines in concentrations ranging from 7.2-14.7mg/L (or 10.8-20.1% of the total volatiles).
o Wines in the cover crop treatments had significantly higher levels of ethyl esters than the control (no cover crop) treatment.
§ The tall fescue cover crop showed the highest levels of ethyl esters, with the alfalfa cover crop coming in second (again, with the control containing the lowest levels).
o The most abundant ethyl esters in the sample wines were: ethyl octanoate (sweet, fruity, anise), ethyl decanoate (fruity, fatty, pleasant vinegar), ethyl hexanoate (green apple, strawberry, anise), and phenylethyl acetate (pleasant, floral).
§ All cover crop treatment wines showed higher levels of these ethyl esters than the control (no cover crop) treatment, which indicates higher quality wine.
- The two fatty acids of higher alcohol found in samples wines were: isoamyl octanoate and isoamly decanoate.
o These fatty acids were higher in the alfalfa and tall fescue cover crops than all other treatments.
- In regards to total esters, wines made from the alfalfa cover plot contained the highest levels, followed by tall fescue, and the control last.
o Acetate esters and ethyl esters are the main aromatic contributors in young wines, giving off fruity and floral aromas, therefore wines that have been cover cropped in the vineyard should have greater fruity and floral aromas than wines that had come from non-cover cropped vineyards, which ultimately results in higher quality wines.
- Higher alcohols were the most abundant volatiles found in the sample wines. Concentrations ranged from 24.1-54.9mg/L (or 58.4-62.3% of the total aromatic compounds).
o Higher alcohols detected were: isoamyl alcohol (alcohol, harsh, bitter), isobutyl alcohol (fusel alcohol), 1-hexanol (green, grass), and phenylethanol (rose, pollen, flowery).
o Cover crops showed significantly higher levels of these higher alcohols (alfalfa treatment the highest), which contributed in a positive manner to the aroma of the wines.
- Six terpene and norisoprenoids were detected in the sample wines with concentrations ranging from 0.5-0.7mg/L (or 0.7-1.2% of the total aromatic compounds).
o The alfalfa treatment wines contained the highest levels of these compounds, including citronellol (close, anise), β-damascenone (bark, canned peach, baked apple, dried plum), and trans-nerolidol (muscat, flowery, fruity).
- Other compounds found in the sample wines included: 3-methylthio-1-propanol, 5-amyl-dihydro-2(3H)-furan, and 2,4-di-tert-butyl-phenol.
o Wines with the cover crop treatments showed higher levels of 5-amyl-dihydro-2(3H)-furan than the control.
Sensory Analysis
- Cabernet Sauvignon wines from cover crop treatments were evaluated better/higher than the control (no cover crop) in regards to visual aspects, aroma, taste, and balance.
- The best olfactory characteristics came from the alfalfa treatment and the tall fescue, followed by the white clover and finally, the control.
- Out of 100 points, the best valued wine was the alfalfa treatment wine, with a total of 83.7 points.
o 2nd place: tall fescue treatment: 81.1 points “very good”
o 3rd place: white clover treatment: 76.4 points “good”
o Last place: Control/no cover crop treatment: 68.2 points “regular”
Conclusions
Based on the results of this study, it is clear that cover crop treatments play a very important role in the aroma of wine, resulting in an increased quality of cover crop treated wines. Out of the three cover crops studied, it appears as though alfalfa provides the greatest levels of aromatic volatiles, and thus the greatest quality wine, with the tall fescue coming in a close second.
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For many of these compounds found in the sample wines, the levels of the aromatic compounds were found to be able threshold, thus result in significant effects of the aroma of the wines created from the different treatment grapes. Wines from cover crop treatments contained higher levels of all aromatic compounds, thus it can be inferred that cover crops should contribute positively to the aroma and ultimately overall quality of wine.
It would be interesting to see if this sort of viticulture practice still had positive effects on wine aged in oak barrels, of which contribute specific aromatic compounds that are separate and different from wines produced in stainless steel tanks (as was done in this experiment). I would also be curious to see if this type of viticulture practice will result in increased aromatic quality of wines from grapes grown in different climate types, as opposed to the one climate type that was studied in this experiment. This study tells us that wines produced from grapes grown in temperate climates and produced in stainless steel tanks will see increased wine quality when cover cropping is employed, however, what about other climates or barrel types? Will the results still hold?
I’d love to hear what you all think! Please feel free to comment below!
Source= doi: 10.1016/j.foodchem.2011.01.033
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!
