If you've ever had a Hefeweizen-style beer - and I'm guessing you have if you're reading this blog (and if you haven't - what are you doing with your life?) - then you are familiar with what is arguably the most distinguishing trait of a Hefeweizen: banana flavor, but not just any banana flavor - artificial banana flavor.
That banana flavor and smell is called isoamyl acetate and is what is known as an ester, which is the largest group of flavor compounds in alcoholic beverages. Esters are formed by the reactions of organic acids and alcohols created during fermentation. In fact, the naming convention for esters is alcohol for the first word and acid for the second word, hence isoamyl is the alcohol part of the reaction during fermentation and acetate is the acid.
The yeast strain used for fermentation is the most important factor in isoamyl acetate production, although isoamyl acetate is present in all beers. Hefeweizen and Belgian yeast strains will produce very high levels of isoamyl acetate, which is why Hefeweizen beers have such a pronounced banana flavor and aroma, but American-style wheat beers do not.
Fun fact: Brettanomyces is very good at breaking up isoamyl acetate into its individual components, so you won't find Brett and isoamyl acetate together in a beer. Bad news for any of you who have been daydreaming about a funky, farmhouse-y, banana beer. Good news for the rest of us because that mashup sounds disgusting.
Isoamyl acetate produces flavor compounds reminiscent of not just artificial banana flavoring, but also pear drops, circus peanuts, and bubblegum. If you've ever had banana flavored Runts or Laffy Taffy, you've had isoamyl acetate since it is very inexpensive to produce and very versatile.
But why doesn't isoamyl acetate taste like an actual banana? Actually, it does taste very similar to what used to be the most popular strain of banana in the Western world, known as the Gros Michel, which is also known as Big Mike.
Yes, the most popular banana used to be known as Big Mike. Big Mike the Banana sounds like the official fruit of the Blue Collar Comedy Tour, doesn't it? Although since my pedantry knows no bounds, I also have to tell you that well, actually, bananas are berries.
During the 20th century, Panama disease (a type of fungus called Fusarium oxysporum) wiped out almost all the Big Mikes. As it turns out, another strain of banana, known as the Cavendish banana. was resistant to Panama disease and thus became the most popular banana strain.
The Gros Michel banana is still grown by banana enthusiasts (you could say they're... bananimated when discussing their hobby) and is described as having a more monotonous flavor, which is to say they have a very high concentration of isoamyl acetate, much higher than the more familiar Cavendish bananas.
I guess it's fair to say that the Cavendish banana ain't no hollaback girl when it comes to Panama disease.
Because even I'm not lame enough to end a blog post with that reference, here's a link to the post I did last year on the difference between esters and phenols.