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Somellier, A Mouse Must Have Wee'd in My
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Ask winemakers' what they consider to be the most revolting of wine faults, and I'd bet a large number would put "mousiness" top of their list.
Mousey taint is a microbiological fault caused by the spoilage yeasts belonging to the genera Brettanomyces and Dekkera, and by the lactic acid bacteria, Lactobacillus hilgardii and Lactobacillus brevis. The compound primarily responsible for the taint is 2-acetyl-3,4,5,6-tetrahydropyridine which is commonly described as 'mouse cage', 'mouse urine', 'popcorn' or 'corn-chip'.
As alcohol is a necessary ingredient for the production of this compound, the taint is formed in finished wine rather than prior to fermentation. For the same reason, the taint is most commonly encountered in fortified wine which has a high alcohol content. Mousiness is an unusual taint in that it is difficult to detect on the nose, but becomes very pronounced as a flavour following tasting. The reason for this, is that at the acidity level found in wine, the compound responsible for mousiness is in a form that is not volatile and therefore cannot be detected by smell. However upon tasting, the residual wine left in the mouth after swallowing mixes with saliva which causes the residual wine acidity to fall as it is mixed with the more neutral saliva. At this lower acidity, the compound is in a very potent volatile form and is exceedingly persistent. To make matters worse, the more saliva that mixes with the residual wine that is left in the mouth, the stronger the mousey flavour becomes.
Often wine tasters report an intense metallic bitterness associated with mousiness in red wines, due to the production of the compound acrolein by the same micro-organisms. Mousey taint can be avoided by ensuring that adequate sulfur dioxide levels are maintained during the winemaking process and particularly during wood maturation.
Grbin et al. (1995) provides an excellent description of the sensory, chemical and microbiological aspects of mousey taint. Costello et al. (2001) gives a more recent account of the organisms that are the likely cause of mousey taint.
Grbin, P.R., Costello, P.J., Herderich, M., Markides, A.J., Henschke, P.A. and Lee, T.H. (1995) Developments in the sensory, chemical and microbiological basis of mousy taint in wine. Proceedings of the 9th Australian Wine Industry Technical Conference, Winetitles, Adelaide, pp57-61.
Costello, P.J., Lee, T.H., and Henschke, P.A. (2001) Ability of lactic acid bacteria to produce N-heterocycles causing mousy off-flavours in wine. Australian Journal of Grape and Wine Research, 7, 160-161