Banana flavor in homebrew beer comes from the Isoamyl acetate ester (a combination of an acid and alcohol molecule) that is produced during the fermentation of all beers to some degree.
In the case of banana-like flavors in your beer, it all comes down to the Isoamyl acetate compound which is found in naturally high levels inside the banana plant.
Esters often have a fruity taste and aroma with notes of bananas, pears, anise, rose, or even bubblegum. In high enough concentrations, these esters can take on a heavy, solvent-like flavor that is highly undesirable.
There are many beer styles that incorporate bright and yeasty flavor elements that include notes of banana.
To avoid banana flavors in your beer you should select a yeast strain with a clean flavor profile and high attenuation, ferment at the lower end of the ideal temperature range, and consider leaving your beer in the primary fermenter a little longer or add a secondary fermentation stage.
If you taste banana flavors while the beer is still in the primary fermenter then consider leaving it there longer or moving it into a secondary fermenter for a little more conditioning to lessen the flavor.
To minimize banana-flavored esters, choose a yeast strain with a cleaner flavor profile. It is also worth noting that ale yeasts will typically produce more esters than lager yeasts.
– Avoid repitching yeast from previous batches of beer too often. – Aerate your wort well before pitching yeast to enable faster reproduction. – Keep temperatures stable within two degrees during fermentation.
For reference, here are the temperature ranges for the three common categories of yeast: – Belgian – 65 to 80+ °F – Ales – 55 to 75 °F – Lagers – 35 to 55 °F