New analytical methods for the study of wine aroma. Applications to the characterization of the headspace evolution and ultra-trace analysis

Resumen

One of the main challenges in wine chemistry is to answer the question that whether flavor perception can be explained or even predicted by the determination of the chemical composition or not. Although knowing the general volatile composition in the liquid phase of wine is essential, it is not enough for explaining the aroma nuances that are more related to the headspace above the wine. It is clear that interpreting the role played by aroma stimuli in the overall perception needs to involve other aspects, such as the integration of the perceptual processes, the changes associated to the release of volatiles and their temporal modifications. Another important aspect is that some impact odorants can dominate the aroma profile of a particular variety of wine even at ultra-trace levels, requiring more sensitive quantitative strategies. The present thesis has investigated the characterization of the release of volatile compounds and their evolution in the headspace above wines and also has researched the quantitative analysis of ultra-trace alkylmethoxypyrazines in wines by developing new analytical methodologies. With the consideration to study the headspace composition of wine under non-equilibrium conditions that are more closely to the headspace of real wine tasting, an automated dynamic headspace (DHS) method combined with thermal desorption (TD) and gas chromatography-mass spectrometry (GC-MS) has been developed in Chapter 1. Thanks to this fast dynamic sampling strategy, the method could provide a snapshot of the contents in the wine vapors of up to 40 aroma compounds, including methanethiol, sulfur dioxide, aldehydes, fusel alcohols or volatile phenols. In Chapter 2, the validated DHS method has been applied to assess the changes in the wine headspaces with time, monitoring the levels of 34 odorants emitted to the headspace by 4 different wines during five consecutive time points. Three patterns of behavior within the evolution of the aromas were found. These patterns corresponded to the physicochemical characteristics of volatile compounds and the potential interactions with wine matrix components, which suggests that prediction of the aroma impact in these cases should include an estimation of the odorant-matrix interactions in wine. Chapter 3 presents the application of the validated DHS method to study the potential influences of physical parameters on the release of volatile compounds from wine, which occur during tasting period, including agitation, evaporation, oxidation and degasification that could occur during wine drinking. Furthermore, the trends of the headspace evolution of a red and a white wine were studied during a 30 min period. The physicochemical characteristics of the volatile compounds were crucial to explain their release behavior. Chapter 4 and Chapter 5 mainly studied the impact of the potential volatile-matrix interactions on the release behavior of volatile compounds, using the DHS method to detect the contents of given volatile compounds in the headspace within different evaporation times. Chapter 4 reconstituted 7 model wines by adding a standard aroma solution to deodorized wines with distinct matrices. Chapter 5 applied the same strategy but to the study of the headspace changes relating to macromolecules that were oenological additives, such as polysaccharides and polyphenols. Chapter 6 was devoted to the comprehensive study of two premium category wines. In this case, both the analytical data of the volatile components in the liquid phase and those of the headspace at different time points were combined with a complete simultaneous sensory study carried out by a panel of tasters. The sensory data at different moments of wine tasting were correlated with the corresponding concentration data in the headspace of volatile compounds that were relevant to each aroma descriptor. Alkylmethoxypyrazines are potent odorants in many food products, as well as in wines. Usually, they are presented at extremely low concentration levels. In Chapter 7, a new method for the identification and quantitation of 3-isobutyl-2-methoxypyrazine, 3-isopropyl-2-methoxypyrazine and 3-sec-butyl-2-methoxypyrazine has been developed and applied to wine. According to the validated method, the analytes were extracted from 5 mL of wine using stir-bar sorptive extraction followed by thermal desorption and multidimensional gas chromatography-mass spectrometry analysis in a single oven. The method is not only automatable and environmentally friendly but also provides the best limits of detection for these compounds published to date. The method has been applied to the analysis of 111 Spanish and French wine samples produced with minor and rare grape varieties.