RAPID FERMENTATION OF BEER USING AN IMMOBILIZED YEAST MULTISTAGE BIOREACTOR SYSTEM - BALANCE CONTROL OF EXTRACT AND AMINO-ACID-UPTAKE

A multistage bioreactor system for rapid beer fermentation was develop ed. The main fermentation process, which conventionally requires 7 d, could be shortened to 2 d by this system. The concentration of esters and higher alcohols are major factors in brewery fermentation, their p roduction being closely related to the yeast growth phase. Yeast metab olism was successfully subdivided into a growth and a restricted phase through a combination of a continuous stirred-tank reactor (CSTR) and an immobilized yeast packed-bed reactor (PBR). Production of higher a lcohols was high in the CSTR because of its association with the level of biosynthetic activity de novo. A small amount was also produced in the PBR, however, possibly a result of an overflow in carbohydrate me tabolism. Ester formation mainly occurred in the PBR, a linear increas e in the level of ester being observed with flow through the PBR. The reactor system control strategy was to maximize the level of both high er alcohol and ester formation. The CSTR/PBR control range, based on e xtract consumption, was varied between 1:1 and 1:2. A ratio of 1:1 ten ded to create a flat beer, whereas a ratio 1:2 gave a beer of richer q uality. Amino acid uptake by the yeast directly contributed to a reduc tion in the wort pH, whereas no relation could be observed between the level of organic acid production and pH.