Dissecting Current Research Related to Wine
September 1, 2011
Manipulating the "Peppery" Aroma of Wines: The Characterization of Rotundone
There are many compounds in wine that contribute to the aroma and flavor of each varietal, though only a small number of those actually have a distinctive aromatic characteristic assigned to them individually. Most aromatic qualities are a result of many different types of compounds working together, with very few individual compounds being responsible all by themselves for a particular aroma or flavor. Isoprenoids, or more specifically, monoterpenes, are some of those compounds that are responsible for particular aromatic characteristics of wine, which are considered to be important for identifying individual varietals as well as individual clones of a particular varietal.
One monoterpene family in particular, the sesquiterpenes, has been studied by some in Baga, Syrah, Riesling, and Cabernet Sauvignon grapes, though the exact aroma contribution is somewhat unknown. Recently, a specific type of sesquiterpene ketone called rotundone, has been identified as being responsible for the peppery character in not only wine, but herbs and spices as well. Rotundone, a guaiane-like compound, has the very distinctive aroma of pepper, with a very low sensory threshold (16ng/L in red wine and 8ng/L in water), therefore is a very important compound in wine that warrants further study. Initial studies have found rotundone concentrations up to 145ng/L in Australian Syrah, Mourvèdre, and Durif wines, while another recent study found rotundone concentrations as high as 561ng/L in European red wines Schioppettino and Vespolina, and white wine Gruener Veltliner.
The concentration of rotundone in a grape can depend on many factors, including the cultivar, region, and climate. There is an assumption that varying viticulture and winemaking techniques may be able to alter the concentrations of rotundone in the final wine (and thus, the aroma), though this is more speculation, as little work has been done in this field. The current study under review, which was published earlier this year, sought to; first, develop a method for analyzing the rotundone concentrations in grapes (which I’ll skim past so your eyes don’t glaze over); second, to analyze the kinetics and accumulation of rotundone during ripening; and third, to determine the accumulation site within the grape, and it’s extractability during the winemaking process.
The grapes used in this study were Vespolina grapes, which are Italian red grapes known for their peppery character. The experimental vineyard belonged to the Azienda Agricola Bisi, which is located in Cascina San Michele at San Damiano al Colle, Italy (latitude 45o 1ʹN, longitude 09o 20ʹE; average 200m in altitude). Grapes were sampled during the 2009 and 2010 seasons. The vineyard is located on a 30% slope, having east-west rows, clay-calcareous soils, with spontaneous cover crops between rows, with spring weeding occurring on the rows. Cordon techniques were employed, with a planting density of 6000 vines/ha.
Samples of 500g of grapes were randomly collected from different vines at various times during ripening. In 2009, the sampling times were at 50% veraison, 100% veraison (i.e. when all the grapes changed color), 21 days after 100% veraison, and at harvest time. In 2010, the same sampling times were used, with a few additional sampling times including two extra times between 100% veraison and harvest (at weekly intervals), and an overripe sample 14 days after harvest. Since the experimental vineyard was located partially on a hill slope and partially on a plain at the foot of the hill, ripening times varying between grapes at the top and the bottom, thus sampling times were different for those particular portions of the vineyard. After collection, grapes were stored at -20oC until analysis was completed.
For comparison, three Gruener Veltliner clones (white grapes), which are also known for their peppery character, were used. These grapes were provided by the Walek winery and were grown in the Thermen region of Austria in 2009. These vines were in north-south rows, using Guyot techniques with a planting density of 4,000 vines/ha.
Grapes were destemmed and crushed, and underwent standard winemaking techniques. During fermentation, caps were manually punched down three times per day for the first three days, then every 12 hours during the following days. Maceration ended after 9 days. Malolactic fermentation was allowed to occur. After 12 days, wines were separated and filtered. For rotundone analysis of the must, samples were collected during the fermentation process at: prefermentation, cap formation, end of fermentation, wine separation, and the final bottled wine.
If you’d like more details on exactly how the winemaking process occurred, I have that information and can pass it along to you should you contact me about it. (It was left out of this post for space considerations).
For the must, rotundone was measured after undergoing solid phase extraction, using tandem mass spectrometry in multiple reaction monitoring mode, which is a protocol that has been successfully employed by other studies (for red and white wines) in the past. As another method of measurement, the SPME GC-MS/MS method (in short, gas chromatography) was also used.
Rotundone was extracted from the whole grape, the grape skins, and the grape flesh (without the skins) and was measured using the SPME GC-MS/MS method as used previously.
o 2010 was cooler and rainier than 2009.
o Though not enough data was collected to prove the following statement, it is possible that higher levels of rotundone are associated with cooler vintages/sites, which warrants further study.
o Hillside rotundone levels in 2009 reached concentrations of 2.77 +/- 0.57μg/kg and in 2010 reached concentrations of 5.44 +/- 0.33μg/kg.
o Foot of the hill (plain) rotundone levels in 2009 reached concentrations of 1.42 +/- 0.53μg/kg and in 2010reached concentrations of 3.67 +/- 0.12μg/kg.
o The overripe stage in 2010 reached rotundone concentrations of 6.13 +/- 0.16μg/kg.
o Clone A1-2 reached rotundone concentrations of 0.86 +/-0.06μg/kg.
o Clone A1-3 reached rotundone concentrations of 0.54 +/- 0.04μg/kg.
o Clone A1-5 reached rotundone concentrations of 1.91 +/- 0.013μg/kg.
§ This 4-fold difference between clones that were grown at the same vineyard at the same time indicates that there are important clonal differences in regards to rotundone concentrations, and should be further studied.
Location of Rotundone in Grapes
Kinetics of Rotundone Before/During/After Fermentation
o The amount of rotundone detected at the end of fermentation was in the range of 9.6-9.8% of the total rotundone content present in the grapes themselves, prior to winemaking.
o Since rotundone is hydrophobic in nature, it is possible that the presence of alcohol improves rotundone formation, thus explaining the increase in concentrations from cap formation to the end of fermentation.
o After separation from the wine skins, 10-30% of the rotundone was lost, likely as a result of being bound to the skin/marc and lees particles that were subsequently removed.
o Additional rotundone losses occurred during wine filtration, which brought the rotundone content in the final wine to 50-60% of the amount extracted during the fermentation process.
o Overall, the rotundone concentration in the final wine was only 5.0-6.1% of the total rotundone concentration of the grapes before winemaking.
The results of this study how rotundone, the compound responsible for the peppery aroma in wine (in addition to herbs and spices), accumulates in grapes during ripening, and how the concentration changes during the winemaking process. Since this study has shown that rotundone is almost exclusively located in the skins of the grapes, it is likely that extended maceration techniques, where the skin is left in contact with the juice for a longer period of time, would likely increase the rotundone concentration of the final wine, and thus increase the peppery character of the final wine. Further research, however, needs to be done on aged wine, to monitor how the rotundone concentrations change over time.
This study has also shown that concentration of rotundone is a clonal trait, meaning that some clones of the same grape will contain higher concentrations of rotundone than others. Determining what type of a wine is the ultimate goal may determine which clonal variety to plant in the vineyard. It appears that both viticulture and enological techniques may be used to manipulate the peppery character of wine, and this research provides fascinating insights into how that manipulation may be carried out. Since so much rotundone is lost during the winemaking process, different types of separation and filtering techniques should be analyzed to determine which results in more or less rotundone in the final wine.
This is just the first study of many more to come, that will allow viticulturists and winemakers to manipulate and achieve the desired amount of peppery character of a finished wine.