The Aroma Files #005: Banana
Banana, the signature weissbier aroma, and a secondary player in a handful of other beer styles is really rather simple. Unlike most beer aromas that are a cocktail of molecules, the banana aroma and flavour is typified by the ester isoamyl acetate. A few other esters, namely ethyl butyrate, and ethyl hexanoate, both more closely associated with pineapple, can add some tropical fruitiness, enhancing the character somewhat. Phenethyl acetate, which has the sort of floral, honey aroma, can also serve to enhance the overall picture. Beers fermented with brettanomyces often contain ethyl caproate, ethyl octanoate or ethyl decanoate, again mostly associated with pineapple but can manifest banana qualities at certain levels.
But standing alone, isoamyl acetate is banana. In fact it is used in artificial banana flavouring, and is also produced by actual bananas, forming one of the strongest crosslinks we’ve explored yet. Isoamyl acetate is an ester, formed by the marriage of isoamyl alcohol and acetic acid. Specifically acetyl-CoA in the case of beer. Sensory threshold for detection is around 1-2mg/L, and over 2mg/L the aroma becomes dominant.
Yeast produces this ester, and in order to do so, requires the inputs isoamyl alcohol and excess acetyl-CoA. It also requires the coding for specific enzymes to carry out the reactions. So first we need yeast with the ATF1 gene turned on, which codes for the alcohol acetyltransferases that are responsible for the esterification reaction. Next we need wort high enough in leucine, something the higher proteins in wheat help to accomplish. Warmer fermentation temperatures of 18-22℃ will enhance the expression of banana, 22-24℃ will push it even higher. The increased temperature boosts fatty acid synthesis which means more acetyl-CoA and more isoamyl alcohol production. Underpitching the yeast can cause stress leading to higher demands for fatty acid synthesis, also generating more acetyl-CoA. Poor oxygenation can again lead to an accumulation of excess acetyl-CoA. Finally, higher gravity/sugar wort can lead to metabolic stress and result in higher levels of esters.
The road to banana looks like this. Leucine from the wort goes through transamination, a process whereby an amino group is transferred from an amino acid to an ⍺-keto acid, forming a new amino acid and a new keto acid. Kind of a switcharoo. This reaction is catalyzed by branched-chain amino acid transaminase, and results in ⍺-ketoisocaproate. Pyruvate decarboxylase and phenylpyruvate decarboxylase then remove the carboxyl group to make isoamyl aldehyde. Isoamyl aldehyde is then reduced to form isoamyl alcohol via the alcohol dehydrogenase enzyme. Alcohol transferase then esterifies the isoamyl alcohol with acetyl-CoA, and presto! Isoamyl acetate…… banana!
To further hone your aroma identification skills, Master Cicerone® Jen Blair’ ‘s Aroma: Explore The Wheel offers 100 common beer aromas! https://underthejenfluence.beer/
Five down, five-hundred to go!