Continuous primary beer fermentation with brewing yeast immobilized on spent grains

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.