For certain wines, infections of the fungus Botrytis cinerea (grey mould) are sought after and desired. This “Noble Rot” refers to the stage of Botrytis cinerea infection that results in a positive impact on the quality of wines, most notably in wines such as Tokaji Aszu, Sauternes, and Trockenbeeren auslese. This noble rot can also affect wines that are traditionally not botrytised, which comes into play for wines developed from withered grapes, which can be susceptible to Botrytis cinerea infection under the right climatic conditions.
One example of a wine created from withered grapes, which is the focus of the study presented today, is Amarone wine, a dry red wine made from Corvina and Rondinella grapes. The withering process can occur two different ways: the more traditional approach of letting nature take its course; and the more modern approach, which uses conditioned chambers with forced ventilation and highly controlled humidity and temperature. The modern approach allows more control over potential mold attacks, rendering the berries less susceptible to Botrytis cinerea infection. At this point in time, most Amarone wine is produced in the traditional method.
There have been very few studies examining the effects of noble rot infections on Amarone wine, though one study showed some variations in aromatic compounds between “healthy” wines and those wines infected by Botrytis cinerea. However, this being the only study (that we know of at this point in time) examining the effects of noble rot on Amarone wine, much more research focusing on the molds’ impact on quality must be done.
The goal of the study presented today was to examine any differences in the chemistry behind aroma and sensory characteristics of Amarone wine produced by health grapes and botrytised grapes 4.5 years after bottling.
Grapes were withered under the traditional method (natural conditions) for four months, using the traditional procedure of the Valpolicella area in Verona, Italy. Grape bunches were manually harvested from the same block in San Pietro in Cariano (Verona, Italy) during the 2006 vintage. Bunches were selected by visually assessing the presence or absence of Botrytis cinerea. The developmental stage of Botrytis cinerea was classified as pourri plein (grapes entirely botrytised, but not desiccated). Three batches of healthy berries (100kg each) and three batches of 50% healthy and 50% botrytised grapes (100kg each) were randomly selected and processed.
The must yield was approximately 54% and 52% in healthy and botrytised grapes, respectively. The final volume of the wines was 50L. Each wine was produced in triplicate. Before adding commercial yeasts, 50mg/L SO2 was added. Wines were produced in a local Italian winery with no precise temperature control, resulting in temperatures ranging between 11 and 15oC during the experiment. Alcoholic fermentation was considered complete when residual sugars were less than 3.0g/L. Wines did not go through malolactic fermentation. After completion of fermentation, wines were kept in a stainless steel tank at 6-8oC for one year, then bottled in 0.75L dark glass bottles, filled to the top, and sealed with screwcaps. Wines were stored at 16oC for 3.5 more years.
For microbiological analysis, Botrytis cinerea was grown counted for each wine, as well as other molds and yeasts.
Enzymatic activity of laccase, esterase, and β-glucosidase was measured.
For wine musts, the following were measured and analyzed: reducing sugar content, glucose and fructose concentrations, ethanol content, as well as gluconic acid and glycerol levels.
For wine aroma, aromatic components were analyzed by solid phase extraction and gas chromatograph-mass spectrometry.
Healthy vs Botrytised Grapes
- Musts from botrytised grapes had higher levels of glycerol and gluconic acid than healthy grapes.
o These are well-known markers of Botrytis cinerea infections.
- Botrytised grapes had higher laccase activity and higher numbers of fungi present, compared to healthy grapes.
- Botrytised grapes showed higher esterase activity and higher β-glucosidase activity than healthy grapes.
o These results are likely due to the contribution of enzymes of fungal origin.
- Glucose and fructose in healthy wines was 1.63g/L and 1.83g/L in Botrytised wines.
- Ethanol content in healthy wines was 15.2% v/v and 15.9% v/v in Botrytised wines.
o Since neither sugars nor ethanol content significantly differed between the wines, fermentation kinetics was the same.
- 80 different aromatic compounds were detected in the wines, with 40 of them being different between healthy and botrytised wines.
- 1-octen-3-ol was 10 times higher in botrytised wines than healthy wines, and was above the sensory threshold.
o This produces an off-flavor, similar to mushroom, which suggests a strong influence of Botrytis cinerea on Amarone wine.
- Methionol levels decreased in Botrytised wines, though remained under the sensory threshold.
o This compound is associated with potato and cabbage notes.
o This decrease could be the result of activated fungal oxidases in the presence of oxygen during the aging process, though more work would need to confirm this.
- Botrytised wines showed lower levels of ethyl esters, fatty acids, ethyl 3-and 4-hydroxybutyrate and isoamyl acetate.
o These compounds are usually associated with fruity notes.
o Differences in ester components between botrytised and healthy wines are likely due to higher esterase activity in botrytised wines.
o Lower levels of fatty acids may be due to the presence of other fungi associated with Botrytis cinerea infection, which typically exhibit high lipid degradation activity.
- Ethyl phenylacetate and diethyl 2-hydroxyglutarate increased in botrytised wines compared with healthy wines.
o This difference could be due to different microflora composition between the botrytised and healthy grapes.
- Furaneol and phenylacetaldehyde was higher in botrytised wines than healthy wines.
o These compounds are known to affect the aroma of botrytised wines in Bordeaux.
o Phenylacetaldehyde is known for flower and honey-hyacinth notes, and increased 2.5 times over the sensory threshold in botrytised wines, compared to healthy wines.
- Benzaldehyde concentrations were lower in botrytised wines than healthy wines, though both were below sensory threshold levels.
o This compound is commonly associated with Botrytis cinerea infection.
- Benzyl alcohol was significantly higher in botrytised wines than healthy wines.
o This could be the result of fungal enzymatic activity.
- α-terpinol, 4-terpinol, trans- and cis-furan linalool oxide, cis-pyran linalool oxide and ho-diendiol I differed between botrytised and healthy wines, though none contribute to the aroma of Amarone wines since they were below sensory thresholds.
- γ-Nonalactone and 4-carbethoxy-γ-butyrolcatone increased in botrytised wines compared to healthy wines.
o γ-Nonalactone was over the sensory threshold, and is known for coconut and sweet notes in wine.
o 4-carbethoxy-γ-butyrolcatone was over the sensory threshold, contributing to the aroma of botrytised Amarone wines.
§ This increase may be due to Flor yeast activity, similar to what is found in sherry wine, though more work needs to be done to confirm this.
- γ-Butyrolactone and sherry lactones 1 and 2 decreased in botrytised wines compared to healthy wines.
- N-(3-methylbutyl)acetamide increased in botrytised wines compared to healthy wines.
o This compound is known for vinegar or pungent notes.
o This increase may be due to a higher microbial and wild yeast contamination in botrytised grapes.
Overall, this study found that Botrytis cinerea/noble rot infections of grape varieties producing Amarone wines has a significant effect on the aromatic composition of the final wine, many compounds increasing above the sensory threshold. Some desirable compounds increased, as well as some undesirable, which begs the question if noble rot actually increases or decreases the quality of the Amarone wine after 4.5 years of bottle aging.
One thing that would have made this study more complete, in my opinion, is a sensory evaluation of the two different types of wine. Chemically, the wines are very different, with some desired aromatic compounds increasing above threshold, while some undesired aromatic compounds also increasing above threshold (with some of both decreasing as well). How does this infection actually change the flavor and taste of the wines? I don’t think we really know without a sensory analysis.
I’d also like to see a study examining a various levels of Botrytis cinerea infection. This study only looked at a wine created from 50% botrytised and 50% healthy grapes. It’d be interesting to see a range of infection levels, say 25% botrytised and 75% healthy grapes compared to 50% botrytised and 50% healthy grapes, compared to 75% botrytised and 25% healthy grapes, and finally compared to 100% botrytised grapes. Perhaps one combination is more desirable than another, in regards to Amarone aromatic quality.
Another thing I’d like to see done is how these aromatic and sensory characteristics change when the wine is aged in oak barrels instead of stainless steel, the former of which is the more practiced aging method for Amarone wines. While this study is fascinating in that it shows Botrytis cinerea infections have a significant effect on the aromatic composition of wines, much more work needs to be done.
I’d love to hear what you all think! Please feel free to leave your comments (no html tags, please) below!
Source: Tosi, E., Fedrizzi, B., Azzolini, M., Finato, F., Simonato, B., and Zapparoli, G. 2012. Effects of noble rot on must composition and aroma profile of Amarone wine produced by the traditional grape withering protocol. Food Chemistry 130: 370-375.
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!