3-Alquil-2-metoxipirazines en mostos i vins: determinació analítica i estudi de la influència d'alguns factors vitivinícoles
3-alkyl-2-methoxypirazines are found in many plants, Vitis vinifera among them. The most important ones regarding theire presence and potential influence on the flavor of wine are: 3-isobutyl-, 3-sec-butyl, and 3-isopropyl-2-methoxypyrazine. Those compounds can be found amongh the varietal aromas of Cabernet sauvignon, but also in Merlot noir, Sauvignon blanc and Cabernet franc, among other.
The vegetative character that the 3-alkyl-2-methoxypirazines provide to wines is generally considered a low quality factor, since it is associated to the lack of maturity of the grapes at harvest. The possibility of monitoring the contents of those compounds in grapes, must and wines during ripening and winemaking is interesting to producers, since it could allow them to optimize the parameters of pruning, sunlight exposure, press, fermentation and even coupage of wines in order to obtain a final product with the maximum quality.
The objective of this work is the development of a method based on gas chromatography with nitrogen-phosphorous detection, in order to quantify the 3-alkyl-2-methoxpyrazines with oenology interest, in musts and wines. The levels of those compounds have been monitored during grape ripening and wine making of experimental samples. Finally, the possible influence of certain ripening conditions has been studied: sunlight exposure, irrigation, vine training and plantation density.
3-alkyl-2-methoxypyrazines are found at the ultra-trace level (ng/L) in musts and wines, in a complex matrix. Despite those low levels, they can have an influence on wine quality because their sensory percepcion thresholds are at the same level. Therefore, their analysis demands the maximum selectivity and sensitivity.
In optimum conditions, the detection limits of the nitrogen-phosphorous detector by direct injection are 0.1-0.2 ng/mL. Since the detection limits required for the analysis of those compounds in grapes, musts and wines are lower than 1 ng/L, the analysis by gas chromatography with this detector requires a concentration factor of 10,000 or higher. The head-space solid phase microextraction technique (HS-SPME) is suitable, since it provides this level of sensitivity and an excellent clean up. The methods developed, based on this technique, allow the quantification of the 3-alkyl-2-methoxypirazines in musts and wines at the ng/L level, providing percentages of recovery of 80- 94%, and detection limits of 0.1-0.3 ng/L.
3-isobutyl-2-methoxypirazine is the most interesting pyrazine from the oenologic point of view, since it has been generally found at higher contents than the other and it is probably the one with the highest sensory impact in wine. The contents of this compound in grapes are of 2.6 42.5 ng/L, whereas the levels in the final wines are of 4.6 15.5 ng/L. The levels of this pyrazine decrease with the increasing maturity of the grape, mainly at the begining of ripening. However, these contents increase a lot after one day of maceration, probably due to the extraction of the solid parts of the grapes, and increase slightly along the alcoholic fermentation, but not after racking.
The average contents of 3-isobutyl-2-methoxypyrazine found in the wines from the trellised-trained vines are significantly higher than the ones of the goblet-trained vines. Sunlight protection can result on a significantly lower level of this compound in final wines. Irrigated vines can produce grapes and wines with a significantly higher level of this pyrazine. Finally, vines with a higher plantation density, of 4,000 plants per hectarea, can produce wines with significantly higher contents of 3-isobutyl-2-methoxypirazine than vines with lower plantation densities of 3,000 or 2,000 plants per hectarea.
Advisor:Guasch Torres, Josep; Busto Busto, Olga
School:Universitat Rovira i Virgili
Source Type:Master's Thesis
Keywords:departament de bioquímica i biotecnologia
Date of Publication:11/25/2004