ESTER FORMATION IN BREWERY FERMENTATIONS

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Abstract
Esters are secondary products produced by brewing yeasts during the anaerobic metabolism of sugars and constitute one of the largest and most important groups of compounds affecting beer flavour. Many esters can be formed, the most important being ethyl acetate, isoamyl acetate, isobutyl acetate, 2-phenylethyl acetate and ethyl hexanoate. The odour threshold levels for ester detection are very low. Esters are especially important in high gravity brewing, where over production occurs causing unwanted solvent-like flavours. They are also important in low alcohol beer production due to the low levels produced, which can result in beers with little flavour. The factors influencing ester production are reviewed, together with the ways in which they can be used to control ester synthesis. It is believed that acetate esters are synthesized by an enzyme called alcohol acetyl transferase (AAT) which uses as substrates an alcohol and acetyl co-enzyme A; the latter plays a central role in many intracellular reactions. However, esters can also be synthesized by esterase enzymes working in reverse. Several attempts have been made to locate the AAT enzyme and recent work suggests that it is located in either the yeast cell plasma membrane or in intracellular organelles called “vacuomes”.