The Oxford Companion to Beer definition of
Isoamyl Acetate, a key ester (combination of an acid and an alcohol) present in all beers. See esters. At its flavor threshold (around 0.6 to 1.2 parts per million) it provides pronounced fruity-fresh, banana, or pear drop–like aromas. It is widely used to reproduce banana-like aromatics in artificial flavorings. As with other esters, it is produced by yeast during fermentation and has a major flavor impact in certain beer styles, particularly Bavarian-style wheat beers. In general it contributes to the fruity qualities of beer. See fruity. The aroma of isoamyl acetate, which is created by traditional weissbier yeast strains, combines with the phenolic clove-like notes of 4-vinyl guaiacol to form the basis of the typical Bavarian weissbier aroma. As such, the concentration of isoamyl acetate is one of the major separators between the flavors of Bavarian-style weissbier and the so-called American hefeweizen (American-style wheat beer), which generally shows little or no banana-like aromatics. German research has shown that high concentrations of isoamyl acetate in weissbier are partially dependent on a high glucose level in the original wort, which, if desired, can be achieved by a targeted decoction mashing regime.
See also american wheat beer and weissbier.
Eder, MichaelJosef.Brewing a wheat beer with intensive banana aroma: A European perspective. The New Brewer, November/December 2009.