Introduction In EU countries a beer is considered alcohol-free when its alcohol concentration does not exceed 0.5 % volumetric. There are two main strategies of its production. One is based on the removal of alcohol from regular beer (dialysis, reverse osmosis, vacuum distillation or evaporation). This approach requires special equipment for alcohol removal, which increases both investment and running costs. The other strategy comprises methods of suppressed alcohol formation. The production of alcohol-free beer using immobilised yeast cell systems rank among methods of controlled fermentation using short but intensive contact between immobilized yeasts and wort. The approaches taking advantage of the suppression of alcohol formation or its early interruption are economically more attractive. Alcohol-free beers are usually characterized by worty off-flavours and lack of pleasant fruity (estery) aroma found in regular beers. Such defects may stem from a fermentation procedure that fails to reduce the chemical compounds responsible for the worty flavour and to produce fusel alcohols and esters. In order to study the formation and conversion of the most important off-flavours, the real wort used in alcohol-free beer fermentation was simulated by a defined synthetic medium containing glucose, yeast extract and aldehydes (hexanal, pentanal, 3-methybutanal, furfural). Several carbonyl compounds present in wort have high flavor potency (3-methyl butanal, 2-methyl butanal, hexanal, heptanal etc.). They contribute largely to the worty off-flavors detected particularly in low-alcohol beer produced by limited fermentation. Beer aldehydes arise mainly during wort production (mashing, boiling) and partially are formed during fermentation from yeast oxoacid pools both via anabolic process from carbon source and catabolic pathway from exogenous amino acids. The reduction of wort aldehydes is crucial namely in alcohol free beer production 1, 2 . In fermentations of alcohol-free beer it is also important to control the level of VDKs (diacetyl and 2,3-pentanedione) because their relatively low taste threshold (0.25 mg/l) 3 . Diacetyl and 2,3-pentanedione are by-products of the pathways leading to the formation of the amino acids valine and isoleucine, respectively. The α-acetohydroxy acids are partially excreted into the fermenting wort where they undergo spontaneous oxidative decarboxylation, giving rise to vicinal diketones. Diacetyl is then taken up by the yeast and reduced to acetoin and then 2,3-butanediol. The flavor thresholds of the latter compounds are relatively high 4 . Several alcohols, other than ethanol are formed in beer during fermentation, among which n-propanol, iso-butanol and isoamyl alcohols (2-methyl and 3-methly butanol) contribute most significantly to beer flavour. Control of higher alcohol formation in continuous systems can be well balanced by the choice of an appropriate yeast strain 5,6 , wort composition, fermentation conditions, immobilization method and reactor design 7,8 . The synthesis of aroma-active esters by yeast is of great importance because they represent the largest group of flavour active compounds in beer. Fundamentally, two factors are important for the rate of ester formation: the availability of the two substrates (acetyl/acyl-CoA and fusel alcohols), and the activity of enzymes involved in the formation of esters.