Erklärung von Fehlaromen im Wein

Aroma is one of the main quality parameters of wine and off-odour taints can be highly detrimental to it, since they may overpower and obscure its natural bouquet, thereby diminishing consumer enjoyment of this highly valued product. As such, avoiding generation of off-odours is an issue of great relevance for winemakers worldwide. In order to deepen the knowledge about wine off-odours, four wines were investigated by gas chromatography-olfactometry (GC-O) with regard to the presence of earthy-smelling compounds. The samples had similar characteristics: made from Pinot Noir variety, cultivated in the Burgundy region (France) and from the 2006 vintage. However, it was found that they differed with regard to the intensity of an earthy odour defect, from clearly affected samples to a high quality wine. 2-Methylisoborneol (2-MIB) was detected in the two most affected wines, although the samples were seven years old. This finding contradicted previous studies that reported the instability of this substance in wine and must (La Guerche, Dauphin, Pons, Blancard, & Darriet, 2006). In order to investigate this seemingly paradoxical situation, 2-MIB stability was reinvestigated in wine and in model solution. Although the concentration of 2-MIB decreased in both media with time, rather than a complete degradation, only a decelerating degradation was observed, still yielding a final concentration at 90 days of storage that was well above its odour threshold. Based on this observation, it was concluded that 2-MIB can still be detected in wine and may be responsible for earthy off-odours, on condition that the start concentration is high enough. The second part of this thesis dealt with the investigation of the effects of Botrytis cinerea and Erysiphe necator infections on wine aroma. These are among the most relevant fungi in viticulture and are responsible for bunch rot and powdery mildew diseases, respectively. The study characterized the impact on both grape must and final wine. With this aim, samples were produced from different grape varieties. The sample set included White Riesling, Red Riesling and Gewürztraminer to study the effects of bunch rot and the hybrid Gm 8622-3 in the case of powdery mildew. For each variety, a healthy sample and a sample that showed a pronounced case of infection were collected the same day in the same vineyard and were later processed identically in order to avoid the influence of additional factors. In each case, the main aroma compounds were determined by GC-O and ranked according to their relative intensities by means of aroma extract dilution analysis (AEDA). In case of the wine samples, these investigations were further supported by sensory analyses and quantitative determinations. In general, it was observed that fungus infections produced clear differences in the overall aroma composition of the samples, and these were predominantly related to quantitative shifts rather than qualitative changes in the odorant composition. In the must samples, bunch rot had a positive impact on fruity and floral notes while several earthy smelling compounds such as geosmin and 2-MIB were developed as result of the infection. Unlike in previous studies, however, no clear differences in the quantities of earthy, mushroom-like smelling substances as result of the powdery mildew infection were observed. Wine analysis revealed that bunch rot infection induced a general increase in the Flavour Dilution (FD) factor of fruity smelling lactones, such as γ-decalactone, γ-undecalactone and (Z)- 6-dodeceno-γ-lactone, and the curry-like smelling lactone sotolone together with an increase in vanillin. However, bunch rot led to inconsistent effects on the aroma composition in the three investigated varieties, demonstrating that both the type of fungal infection and the grape variety greatly influence the final product. Regarding the sensory tests, bunch rot affected samples were generally rated as more peach-like/fruity, floral and liquor-like/toasty than their corresponding healthy controls. Furthermore, in the hedonic evaluation, bunch rot affected samples were described as being more pleasant in all three varieties studied, which might be linked to the enhancement of positive aroma attributes already mentioned. On the contrary, the sample affected by powdery mildew was consistently evaluated as being more negative with regard to hedonics. This negative rating was, however, not related to any specific off-odour but was rather due to a lack of positive aromatic notes. In this sense, powdery mildew led to a decrease of the vanilla-like note in the affected sample corresponding to a decrease in the FD factor of vanillin, while the remaining sensory attributes were rated very similarly. In agreement with this, the FD factors of the acidic compounds, lactones and esters remained only slightly altered. Interestingly, no earthy smelling substances were detected in any wine sample. These compounds were perceived only with low FD factors in the musts, with the exception of 2-MIB, which was, most likely, degraded during fermentation. Lastly, the effects of the named fungal diseases were further characterized by quantification of representative aroma compounds by means of stable isotope dilution analysis (SIDA). Analyses revealed that both fungal diseases caused significant changes in the concentration of most target compounds. The greatest effects were an increase in the concentration of phenylacetic acid, acetic acid and γ-decalactone for both fungi and all grape varieties