The article presented today is mainly a short test of methods, though it could have some interesting and potentially important applications in the wine world.
There are several ways to create wines with higher levels of residual sugar, one of which is the process of dehydration. In particular, the dehydration process is a critical step in creating Andalusian sweet wines such as Pedro Ximénez wine. The dehydration process significantly alters the chemical composition of grapes, including sugar concentration, volatile compounds, phenolic compounds, and enzyme activities. This process of dehydration can take anywhere from 7 to 10 days, after which the grapes are pressed and the remaining must undergoes the winemaking process.
Quality of wine made from dehydrated grapes is determined by many factors, including grape variety, enzymatic activity, and the length of time allotted for dehydration. In an attempt to further control wine quality, winemakers attempt to control the drying time of the grapes by monitoring water loss and sugar concentrations. If the grapes are allowed to dehydrate for too long, the aromatic quality of the wine can be compromised. Therefore, according to the authors of the study presented today, an easy tool that utilizes a sensor that can detect small metabolic changes during grape dehydration could help winemakers determine the optimal drying time for grapes based on conditions other than just sugar concentrations, which is very important for controlling wine quality for these types of wines.
One such tool that is already in use in other parts of the food industry is the Electronic Nose (E-nose). In regards to wine, the E-nose has been used for vintage or variety determination (employed most often when trying to find counterfeit wines), quality characteristic discrimination, and identifying Brettanomyces contaminations.
The short paper presented today examined the evolution of aromatic compounds during the dehydration process of Pedro Ximénez grapes and analyzed the use of an electronic nose to determine if the tool could be a reliable mechanism for analyzing aromatic quality of wines created from these dehydrated grapes.
Pedro Ximénez grapes were randomly sampled from 15 different locations in the Montilla-Moriles region of Spain after 0, 2, 4, 6, and 9 days of drying. Each sample contained 25kg of healthy, dried grapes and pressed in a vertical laboratory press. Musts were clarified using centrifugation and the preserved until analysis.
Sugars (oBrix), titratable acidity, pH, and volatile compounds were measured in the wine. Volatile compounds were measured by gas chromatography/mass spectrometry (GC-MS).
The electronic nose used was created and assembled at the University of Rome Tor Vergata. The nose was based on an array of 8 quartz microbalances, which are electromechanical resonators with a resonance frequency that changes proportionally to that mass which is absorbed on the sensors’ surface.
- Drying after 9 days multiplied oBrix by 1.95.
- In the first 48 hours of drying, grape dehydration reached 11.2% and diminished 7.6% over the 48 hours following.
o Grape dehydration continued to reach 3.45% per day until the 6th day.
o After the 6th day, dehydration continued to reach 1.5% per day and finished the process at 30.2%.
- As the dehydration process continued, pH decreased and titratable acidity increased.
o This is likely due to the loss of water during the drying process.
- The loss of water and likely the cellular structural damage that occurred during the drying process resulted in a general increase of volatile compounds in the wine.
o Most volatile compounds reached a maximum concentration when the percentage of dehydration was 18.8%.
o The only two volatile compounds analyzed that decreased during the dehydration process were (E)-2-hexen-1-ol and (E)-2-hexanal.
o Acetoin reached a maximum when grape dehydration was at 11.2%.
o Hexanoic and octanoic acids reached maximums after 4 days of drying.
o At the end of the dehydration process, the following volatiles showed increases in concentrations (likely due to the loss of water): isobutanol, isoamyl alcohol, 2-phenylethanol, benzyl alcohol, 1-pentanol, ethyl lactate, and 1,1-diethoxyethane.
- Dehydration conditions (i.e. high temperatures) favored the formation of Maillard reaction products, which give rise to toasty aromas with notes of coffee and/or chocolate.
- Volatiles grouped into chemical families significantly changed concentrations during the dehydration process.
o Carbonyl compounds and carboxylic acids were lower in concentration at the end of the drying process.
o Alcohols, esters, and acetals significantly increased by the end of the drying process and reached maximum concentrations between 18.8% and 25.7% dehydration of the grapes.
- Since monitoring a high number of volatile compounds is extremely difficult, cluster analysis was used to monitor similar groups of volatiles.
o Samples taken on day 0 and 2 of drying were very similar to each other and different from the rest.
o Samples taken on day 4 and 6 of drying were very similar to each other and different from the rest.
o Samples taken on the last day were similar to the sample day taken right before it.
§ Results indicate that the drying process tends to homogenize the must composition.
- At the beginning of the dehydration process, herbaceous aromas were most prominent.
o As the drying process continued, these herbaceous aromas diminished and aromas of floral and milky became more prominent.
o Samples displaying primarily floral and milky aromas increased when the grape dehydration was at 18.8%.
o Samples displaying primarily toasty aromas reached a maximum 6 days into drying.
- Historically, sugar concentrations have been measured to determine the end of the drying process.
o This may be problematic, as while the sugars are still increasing by the last stage of dehydration, aromatic compounds are decreasing significantly by this time.
o Optimum drying time for volatile aroma compounds maximized when grape dehydration was around 26%.
§ According to the authors, sensors could help the winemaker determine this optimum drying point for each wine, depending on the grapes and other conditions that act to alter the chemical and aromatic composition of the finished wine.
- Based on discriminant analysis of readings taken by the E-nose, two separate phases were observed during the grape drying process.
o All grapes under 19% dehydrated fell into one group, while grapes from the later stages of the dehydration process fell into a second group.
§ Results indicate the E-nose is able to discriminate between grapes at different stages of the dehydration process.
- By combining both E-nose data and results obtained from the chemical analysis and running a multiple regression, the model was able to explain 99.796% of the variability in the associated with the dehydration process.
o Multiple regression analysis showed that there is a relationship between the dehydration process of grapes and the value that is associated with dried fruit, herbaceous, floral, and milky aromas (which are highly correlated with stage in dehydration process).
Based on the results of this short study, the authors concluded that there is a strong relationship between data reported by the E-nose and the aromatic volatile components of the finished Pedro Ximénez wines. Therefore, the E-nose could be used as a way to quickly and accurately measure the optimum drying time for grapes in order to produce these sweet wines with high aromatic quality.
There wasn’t really a lot done in this study, as it was simply a validation of methodologies, however, its results could have important applications for the wine industry, particularly when it comes to determining aromatic quality of wines created from dehydrated grapes. What other types of wines do you think this type of aromatic quality testing could be employed? What other tests would you have liked to see performed during this study?
I’d love to hear your thoughts on this topic! Please feel free to leave your comments below (reminder: all unauthorized html tags will be treated as solicitations and promptly removed).
Source: Lopez de Lerma, N., Bellincontro, A., Mencarelli, F., Moreno, J., and Peinado, R.A. 2012. Use of electronic nose, validated by GC-MS, to establish the optimum off-vine dehydration time of wine grapes. Food Chemistry 130: 447-452.
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