A one-stage continuous primary beer fermentation with immobilized brewing yeast was studied. The objective of the work was to optimize the operational conditions (aeration and temperature) in terms of volumetric productivity and organoleptic quality of green beer. The system consisted of an internal-loop airlift reactor and a carrier material prepared from spent grains (a brewing by-product). An industrial wort and yeast strain were used. The immobilized biomass (in amounts from two to sevenfold greater than free biomass) contributed 45-75% to the total fermentation. The volumetric productivity of the continuous system was as much as five times higher than that of the batch fermentation. An optimum higheralcohols-to-esters ratio in green beer was found at approximately 2 mg/L of oxygen dissolved in wort, mixing induced by pure CO 2 , and temperatures at 13-16°C. At high total biomass concentration, the diacetyl formation was low, and the volumetric productivity of the system was high. Therefore, the amount of immobilized biomass in the reactor has to be kept at high concentration by regular replacement of the carrier losses.

This work demonstrated the technological feasibility of the three-phase airlift bioreactor (ALR) with brewing yeast immobilized on spent grains (a brewing by-product) for continuous beer production. The optimum fermentation performance of the one stage immobilized cell bioreactor was achieved at residence times between 18-25 h (dilution rate 0.04-0.055 h -1 ) and was characterized by an apparent degree of attenuation in the range of 70-80%. The productivity of the system in terms of ethanol concentration in green beer (ca. 4.2%) was satisfactory. Although the diacetyl concentration in the young beer was high (0.32 mg L -1 at D = 0.04 h -1 ) it is speculated that the level could be reduced by cell growth control, aeration and temperature optimisation. The immobilized yeast fermentation in the ALR was shown to be robust in recovery after process upsets.

A novel carrier obtained from spent grains, a brewing by-product, was used for brewing yeast immobilisation in a continuous bubble-column reactor. The multiple-layer cell adhesion to the carrier particles resulted in a maximum cell load of 430 mg dry cell g −1 dry carrier (d.c.). After 120 h of reactor operation, the cell load of DEAEmodified carrier was below 40 mg dry cell g −1 d.c. while the values for non-modified carrier reached at least 100 mg dry cell g −1 d.c. The changes in substrate composition on the rate of yeast attachment and on its stability were also studied.