Mousy Off-Flavor of Wines

Mousy taint is a microbiological defect of wine due to Brettanomyces/ Dekkera yeasts, as well as certain lactic acid bacteria, such as Leuconostoc oenos (Oenococcus oeni) and Lactobacillus. Brettanomyces yeasts are frequently found in wooden casks. Mousy taint can occur particularly 

2-Acetyl-l-pyrroline was reported to be the major contributor to mousy off-flavor (Herderich, et al., 1995), with an aroma impact of one order of magnitude greater than ATHP (Buttery et al., 1982), but it is a relatively unstable compound and was found in wine in trace quantities up to 7.8pg/L (Grbin et al., 1996). At the pH of wine these compounds are not volatile and as a consequence they have a low sensory impact. However, when mixed with the neutral pH of saliva they can become very apparent on the palate as mouse cage or mouse urine (Snowdon et al.,2006). 

After addition of 4-acetylpyridine as an internal standard, 250 mL of wine is adjusted to pH 2.5 and extracted with Freon 11 (3 x 100mL) in order to remove acidic and neutral compounds to reduce the GC/MS background interference. Continuous liquid-liquid extraction of basic mousy /V-heterocycles is then performed overnight with Freon 11 at pH 8.0 (Herderich et al., 1995). After the extract is dried over Na2S04, 3-acetylpyridine can be added as a second internal standard, and the organic phase is concentrated by distillation at 37 °C, replacing Freon 11 with 0.5 mL of dichloromethane. Prior to GC/MS, the extract is further concentrated to 10 pL of isooctane. 

Analysis is performed with a PEG capillary column and an oven temperature program from 60 to 220 °C at 3°C/min. Analytes are detected by recording the signals at m/z 111 and 110 for FTTTP, m/z 111 and 83 for APY, m/z 125 and 82 for ATHP. Quantification of ATHP, ETHP, and APY is performed recording the signals at m/z 111,111 and 125, respectively, and the signal at m/z 121 for 4-acetylpyridine. 

For the chromatographic conditions used, in addition to the two tautomeric forms of ATHP, two isomers of both ETHP and APY can be yielded as well (Costello and Henschke, 2002). 

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Costello, R, Henschke, R. (2002). Mousy off-flavor of wine Rrecursors and biosynthesis of the causative N-Heterocycles 2-ethyltetrahydropyridine, 2-acetyltetrahydropyridine, and 2-acetyl-1-pyrrohne by Lactobacillus hilgardii DSM 20176.7. Agric. Food Chem., 50, 7079-7087. 

Figure 8.17. Chemical structures of A-heterocycles responsible for mousy off-flavor of wines (8) 2-acetyl-l-pyrroline (APY), (9) 2-acetyltetrahydropyridine (ATHP), (10) 2-ethyltetrahydropyridine (ETHP).

Methoxypyrazines 97,106,109,110 Micro- and Nano-LC/ESI/MS 25 MIKE spectrum 78 Mousy N-Heterocycles 269 Mousy off-flavor of wines 268 MS/MS in space 80 MS/MS in time 83... 

Besides volatile phenols, Brettanomyces synthesizes a number of odor-active compounds, many of which have yet to be identihed (Licker et al., 1999). For instance it is known that Brettanomyces produces isovaleric acid, an odoriferous compound described as rancid (Wang, 1985 Licker et al., 1999). In addition, some strains can impart mousy off-flavors to wine through synthesis of various nitrogen-containing compounds (Section... 

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