By Honorine Gaston, œnologist and Dominique Fink, biologist
Mouse taint has been well documented since the nineteen seventies and has been around for a long time. While considered relatively rare, it has been observed more and more in the last 10 years not only in Europe but here in Quebec and Canada. Many factors are involved in its recrudescence, and we will explore them is this short article.
What is Mouse Taint?
Mouse taint is often described as a popcorn, cooked rice or bread aroma. It is also reminiscent of caged mouse urine hence its name. While bread or popcorn aromas may be pleasant, they become a fault when they overpower everything else.
Which Molecules are Responible for Mouse Taint?
As of today, three molecules have been identified as responsible for “mousiness”: APY (2-acetyl-1-pyrroline), ATHP (2-acetyltetrahydropyridine) and ETHP (2-ethyltetrahydropyridine). They are present in wine in two forms: volatile and non-volatile.
“Mousiness” compounds arise when certain types of microorganisms like Oenococcus oeni metabolize naturally present amino acids from grapes which result in APY, ATHP or ETHP. Excess nitrogen, which has not been consumed during fermentation may increase this risk.
However, mouse taint has also been detected in wines that are microbiologically stable. In this case, it may be caused by the combination of sugars to certain amino acids under high dissolved oxygen conditions.
Why are We Seeing More "Mousiness"?
Winemaking practices have been evolving in recent years. Many winemakers are choosing to decrease the quantity of sulphites used during production. Also, many producers prefer using indigenous yeasts for fermentation. Climate change is a contributing factor as pH values measured in wines and other beverages are increasing.
When all these elements come together, it creates a favorable environment for the development of select microorganisms such as Brettanomyces bruxellensis and lactic bacteria like Oenococcus oeni. Interactions between these microorganisms is believed to increase the probability of occurence of mouse taint. This fault may appear not only during aging but also during fermentations and after bottling.
How Does Mousiness Express Itself ?
Mouse taint is very unusual. A person can be very sensitive to this fault while another can barely detect it. Furthermore, “mousiness” is only perceived once the wine has been tasted either by swallowing or spitting the wine. It will appear at the back of the throat by a process called “retro-olfaction”.
This phenomenon results from the fact that molecules responsible for mouse taint are not volatile under an acidic pH. Wine usually has a pH value of 3.0 to 4.0 which makes it acidic and so, “mousiness” is not detected by smell under those conditions. However, once the wine passes through the mouth, the pH value increases to 7 which causes the molecules responsible for mouse taint to become perceptible as they become volatile.
Can Mouse Taint Be Treated?
The short answer is no. There isn’t any treatment to date. Adding sulphites, tannins or activated carbon may decrease perception but mouse taint may not be eliminated entirely.
The action mechanism of these inputs is not well understood. Sulphites and tannins may mask or complex molecules causing mouse taint but not eliminate it.
The best solution is to prevent its apparition by managing dissolved oxygen, adding sulphites after malo-lactic fermentation, ensuring that Brettanomyces yeasts are well controlled and do not develop.
The good news is that mouse taint can dissipate in time. Mousiness is more often than not observed in young light-bodied wines without sulphites and destined to be sold quickly.
How To Prevent Mouse Taint?
Monitor dissoved oxygen regurlarly
Oxygen management during winemaking and ageing: high levels will increase the "mousiness" perception.
Grape Maturity Management
Avoid high pH to limit microorganism growth.
Avoid sluggish fermentations
A careful monitoring of fermentation parameters will decrease the risk of sluggish fermentations as well as nitrogen analysis in musts.
Taste, Taste and Taste again!
Exposing the wine to air may help increase development of this fault.
Maintaining good cellar hygiene
Cellar cleanliness will prevent the development of spoilage microorganisms
To Conclude With Mousiness...
Mouse taint, most often than not, results from interactions between microorganisms, namely Brettanomyces type yeasts, Oenococcus oeni and Lactobacillus type bacteria. It is an ancient fault which is brought about by evolving oenology practices. Natural wine production, while marginal until a few years ago, has become very popular and the the decreased use of sulphites may be a contributing factor for the recurrence of this fault. Mousiness is perceived through a retro-nasal olfaction mechanism as molecules responsible are non-volatile at the wine’s pH. This fault can occur at anytime during winemaking and event after bottling. It can appear and dissipate with time, or not. Treatment options are limited and have low efficacy. The best protection against this fault is prevention through risk management. Oenologists at OENOSCIENCE can help you identify mouse taint through wine tasting and better yet, help you prevent it. Contact us at 514-564-2024, by email at info@oenoscience.com or through our contact form.
