Rapid maturation of beer using an immobilized yeast bioreactor. 2. Balance of total diacetyl reduction and regeneration

Operational conditions for the maturation of beer using an immobilized yeast reactor were investigated. α-Acetolactate generated during alcohol formation could be reduced to 0.06 mg l −1 , below the threshold level, by a combination of heat treatment and bioreactor treatment. Operating temperatures above 4°C resulted in regeneration of α-acetolactate (precursor of diacetyl) and it was preferable to run the reactor between −1.0 to 1.0°C. For an initial diacetyl concentration below 1.0 mg l −1 , a residence time of 3 to 20 h was required to control regeneration of α-acetolactate below threshold. A pressure of 0.6 to 1.0 kg cm −2 was sufficient to prevent gas holdup if the extract consumption in the reactor was less than 0.5%. It was not necessary to maintain tight control over dissolved oxygen. Under the above conditions, almost no amino acids were consumed and the diacetyl (or total diacetyl) level was reduced to below threshold, where approx. 0.03% of extract was consumed per h of residence time. Because of the low operational temperature, diacetyl reducing performance could be maintained for more than 6 months.