PRODUCTION OF BEER USING IMMOBILIZED YEAST ENCODING ALPHA-ACETOLACTATE DECARBOXYLASE

Genetically modified brewer's yeast encoding alpha-acetolactate decarboxylase (alpha-ALDC) was tested in immobilized yeast bioreactors for main fermentation of beer. The alpha-ALDC enzyme produced by the transformant catalyzes the direct conversion of alpha-acetolactate to acetoin without formation of diacetyl. The long lagering period required for beer maturation in conventional brewing can thus be shortened or even omitted. Three different packed bed bioreactors were employed, with volumes of 1.6 dm3, 5 dm3 and 25 dm3. The 5 dm3 column had a slightly conical geometry in contrast to the others which had cylindrical shapes. Sintered glass beads were chosen as the carrier material on the basis of experiments with the parent strain. The brewing performance of the transformant compared well with that of the parent strain in the immobilized system. Fermentation, utilization of amino acids (including isoleucine, valine and leucine) and flavour formation were practically identical with both strains, the only difference being a marked decrease in the formation of diacetyl by the transformant. Small differences were, however, observed in the long-term biochemical stability. By using yeast encoding alpha-ALDC in the immobilized yeast system the total (primary and secondary) fermentation time could be reduced to approximately 2-6 days, compared with 3-6 weeks in a conventional batch process.