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. alpha-Acetolactate generated during alcohol formation could be reduced to 0.06 mg l(-1), below the thresho ld level, by a combination of heat treatment and bioreactor treatment. Operating temperatures above 4 degrees C resulted in regeneration of alpha-acetolactate (precursor of diacetyl) and it was preferable to ru n the reactor between -1.0 to 1.0 degrees C. For an initial diacetyl c oncentration below 1.0 mg l(-1), a residence time of 3 to 20 h was req uired to control regeneration of alpha-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 t he above conditions, almost no amino acids were consumed and the diace tyl (or total diacetyl) level was reduced to below threshold, where ap prox. 0.03% of extract was consumed per h of residence time. Because o f the low operational temperature, diacetyl reducing performance could be maintained for more than 6 months.