In describing the characteristics of a beer, sometimes happens that we come across aromas and flavors that at first glance can be surprising. Indeed, in one of the most historic beer styles in the world, we find a characteristic fruity aroma that is immediately recognizable.
We are of course talking about the Bavarian weissbier and their typical banana aroma.
The style is rich in tradition, rooted at least in the 1500s, and its evolution has followed that of German dukes and rulers. It is a top-fermented style of beer characterized by the use of unmalted wheat, with high carbonation, and distinct notes of clove and banana. Although neither of these ingredients is used in the brewing process. So where do these aromas come from?
The answer lies in the yeast used and its fermentation process. To better understand how these aromas develop, however, it is necessary to lay the groundwork on what fermentation is in beer and how this process works. Not a walk in the park, but I will try to make it easy
A brief review of fermentation processes
The key elements of fermentation are two: sugars and yeast. The former is supplied by the grains used (barley, wheat, oats, rye, etc.), in various forms of more or less complex sugars depending on their length; the latter is inoculated (with a few exceptions) by the brewer. Yeast, being a living organism, aims to grow and reproduce, and to do this it needs energy. This energy is provided by sugars. Hence the name Saccharomyces (from the Greek σάκχαρον – sugar and μύκης – fungus) means sugar fungus. There are many types of Saccharomyces and some of them are also used in bread and wine production.
The strains of Saccharomyces used in beer production are Saccharomyces cerevisiae, also known as top-fermenting yeast, and Saccharomyces pastorianus, also known as bottom-fermenting yeast, each with its own particular characteristics in terms of performance and development. As mentioned above, the mission of yeast is to grow and develop, in doing so it absorbs the energy provided by sugar and develops alcohol and carbon dioxide as waste products.
Thus, as the fermentation process develops, yeast produces “waste” products that make our beverage unique and unmistakable, in fact without the alcohol component and carbonation, our drink would not be the same. Having clarified this basic concept, we can proceed.
In addition to the already mentioned alcohol and carbon dioxide produced by all yeast strains, there are numerous chemical substances also produced as waste by our Saccharomyces during its life cycle. These substances, called by-products, are dependent on the yeast itself – bottom-fermenting yeasts tend to produce fewer of these while top-fermenting more – but also on the specifical strain used.
Different strains produce different substances leading mainly to spicy (phenols) or fruity (esters) aromas in various combination of quality and intensity. Everything of this it’s a question of chemistry, choosing a yeast strain despite another, will lead to different aroma and different taste in the beer, and in the same way act the way in which you manage it during its fermentation process.
As you may have already guessed, we are approaching the answer to our initial question. Why does my beer taste like a banana?
A few more steps into the world of chemistry and you will get the picture.
Esters: the fruity component
Compared to other yeast by-products (also called metabolites), esters are only trace elements. Nevertheless, despite being “a drop in the ocean” of beer constituents, esters are the most important aroma components produced by yeast. That is because esters have a very low odor threshold in beer (5-15 µg/l) and yet to a large extent may define its final aroma.
However, as everything in beer, it’s a question of balance, and if overproduced they can negatively affect the beer, leading to an unpleasant solvent aroma. Thus, it is crucial for the brewer to keep the optimum fermentation conditions to obtain a balanced beer in terms of ester profile.
From a simplified chemical point of view, an ester is a compound formed by the “union” of an organic acid with an alcohol. In beer, esters are formed mainly by the esterification of fatty acids by ethanol, and the various fatty acids available can lead to the formation of different esters. There are about 60 different esters found in beer, which type, perception threshold and character depend on the exact chemical reaction.
The aromatic spectrum that these substances offer is very broad, ranging from pear, apple, pineapple, mango, apricot, peach and… banana!
Choose the right yeast
With all the elements now available, we can finally answer our question: The typical banana aroma of weissbier is given by an ester, the name of which is isoamyl acetate. The characteristic of this substance is precisely the distinct fruity banana aroma, the origin of which is found in the fermentation by-products normally produced by certain specific yeast strains, and the yeast used in the production of weissbier produces a lot of this substance.
Obviously, not all yeasts produce esters in perceptible quantities and not all yeasts produce the same types of esters, so it is the yeast strain itself that is the core ingredient of the fruity banana aroma in weissbier.
This fruity aroma then combines in the olfactory pathway with the aromatic component of the other elements (malt, hops) and can offer many different nuances, ranging from unripe to ripe banana, thus offering the brewer the possibility of creating the desired flavour profile based on its recipe.
In addition, several weissbier yeast strains are available on the market, each with its own characteristics in terms of intensity, complexity and cleanliness of the final aroma. Beware though, not all beers require this kind of flavour profile or intensity, and in some beers the presence of fruity esters is regarded as a defect or in any case as a symptom of a poorly brewed beer. Imagine approaching your nose to a glass of Pils and perceiving the aroma of banana… that would be very bad!