There is an incredible amount of aromas one can detect in a wine. Some are fruity, some aren’t. As you can well imagine some smells are okay and other signify that something has gone awry.
Causes of Banana Odors in Wine
It is certainly possible to pick up banana flavors from the fermentation process. As Emmet points out, Beaujolais is known to have banana aroma and flavors as a result of carbonic maceration. Other wines may also exhibit these aromas if they contain Isomyl Acetate or Ethyl Octanoate, both of which are naturally occurring byproducts of the fermentation process (read more here).
As long as these chemicals are in small enough concentrations the wines will be fine to drink and the banana may even add a bit of a pleasing twist to the flavor profile. In higher concentrations, however, these wines may be undrinkable.
There are two main causes of banana odors in wine outside of what can occur naturally. The first cause is cool fermentation temperatures and the second is spoilage. Let’s look at each in a little more depth.
Cool Fermentation Temperatures
According to Master of Wine Debra Meilburg, “Banana-like aromas appear in wine when grapes are fermented at excessively cool temperatures.” So if your fermentation dipped below the recommended temperature range for your yeast, banana aromas may ensue.
As you know, when yeast digest the sugars and nutrients in a wine must they alter them. Chemical compounds in grapes called stereoisomers are taken in by the yeast where they are subjected to enzymes used to digest sugars. This interaction changes the stereoisomers and makes them take on flavor and aroma characteristics of other fruits.
When yeast is under stress, such as when temperatures get too hot or cold, the yeast will produce off flavors. Just like you and I can’t do our best work if we’re not comfortable, yeast will struggle with an extreme environment. Banana odors are one example of an odor that can be produced by stressed yeast.
Spoilage
The other possibility is that your wine is contaminated with a spoilage micro-organism. When acetobacter contaminates a wine it converts the alcohol into acetic acid (vinegar). In the presence of 1-pentatol (a rather unpleasant form of alcohol) the acetic acid will mix with it and produce amyl acetate which smells like artificial banana flavoring. Unfortunately for our friend Emmet, this was the case.
What started out as a mild banana odor gradually gave way to vinegar odors. Somehow his wine was exposed to acetobacter and, perhaps due to stress on the yeast, 1-pentanol was likely present. The two combined to give the banana odors. Over time, however, the acetobacter converted more and more of the alcohol to vinegar and it just overwhelmed the wine.
A wine with acetic acid may be treated though you have to catch it early and act swiftly. The treatment is not all that safe if you’re not very careful. So unless you have to I would avoid trying to treat a wine that has an acetic acid problem.
Avoiding Banana Odors
The way to avoid picking up these flavors is pretty obvious. First, make sure that you clean and sanitize everything thoroughly. In Winemaker’s Academy Podcast episode 7 I discussed the ins and outs of Cleaning and Sanitizing Wine Making Equipment in case you want a refresher on best practices.
Micro-organisms are all around us all the time. They float in the air, settle on us and our stuff. This is why we have to be so vigilant about or cleaning and sanitizing practices. If we’re not then it’s only a matter of time before one of these little organisms find their way into our wine.
Second, keep those fermentation temperatures within the recommended range for your yeast strain. It can be different for different yeast strains so be sure to look into what your yeast is most comfortable with. You may want to consider putting a thermometer in the area you’d like to make wine in for a day or two and check it at different times of the day. This way you’ll have an idea of the ambient temperature and the range of temperature swings you can expect once your wine is fermenting.
Photograph of bananas by: Steve Hopson (license)