Ingeniero Quimico

Biotechnol. Prog. 1999, 15, 105−113

105

Immobilized Yeast Bioreactor Systems for Continuous Beer Fermentation
Murthy Tata,* Patricia Bower, Susan Bromberg, Dick Duncombe, Jeff Fehring, Vera Lau, David Ryder, and Paul Stassi
Miller Brewing Company, 3939 West Highland Boulevard, Milwaukee, Wisconsin 53201

Two different types of immobilized yeast bioreactors were examined for continuousfermentation of high-gravity worts. One of these is a fluidized bed reactor (FBR) that employs porous glass beads for yeast immobilization. The second system is a loop reactor containing a porous silicon carbide cartridge (SCCR) for immobilizing the yeast cells. Although there was some residual fermentable sugar in the SCCR system product, nearly complete attenuation of the wort sugars was achievedin either of the systems when operated as a two-stage process. Fermentation could be completed in these systems in only half the time required for a conventional batch process. Both the systems showed similar kinetics of extract consumption, and therefore similar volumetric productivity. As compared to the batch fermentation, total fusel alcohols were lower; total esters, while variable, weregenerally higher. The yeast biomass production was similar to that in a conventional fermentation process. As would be expected in an accelerated fermentation system, the levels of vicinal diketones (VDKs) were higher. To remove the VDKs, the young beer was heat-treated to convert the VDK precursors and processed through a packed bed immobilized yeast bioreactor for VDK assimilation. The finishedproduct from the FBR system was found to be quite acceptable from a flavor perspective, albeit different from the product from a conventional batch process. Significantly shortened fermentation times demonstrate the feasibility of this technology for beer production.

Introduction
Traditionally, beer fermentations have been conducted as batch processes. Although the advantages of a continuousfermentation process are well recognized, there are relatively few continuous brewing processes in operation, the most notable and perhaps the most successful being some breweries in New Zealand (1). Several different schemes of continuous free cell fermentation processes have been tried at various scales of operation as early as the 1960s (2), but most of these did not remain commercially viable.These systems fell into disfavor for several reasons such as the increased complexity of operations required relative to the traditional “fill and shut” batch process, the dangers of culture contamination, a general “instability” of the system from an operations standpoint, unrealized cost and productivity advantages in the process plant, and the fear of genetic drift of the yeast strain. Continuoussystems based on immobilized yeast technology seem to be more advantageous from a commercial operations standpoint because of the potential for high fermentation rates afforded by the high cell concentrations of yeast in the bioreactor. There is already a vast amount of literature discussing immobilized cell technology for other fermentation processes that have shown significant process economicand quality advantages over the batch process. The matrixes used for immobilizing the yeast cells include gel type materials such as alginate,
* To whom correspondence should be addressed. Telephone: (414)931-2199.Fax: (414)931-2506.E-mail: Tata.Murthy@MBCo.com.
10.1021/bp980109z CCC: $18.00

as well as porous structures that allow the matrix to shed viable cells as the immobilized biomassgrows. Among the latter are porous matrixes such as porous glass beads, ceramic beads (3-5), chitosan (6, 7), polymeric foam (8, 9), and even loofa sponge (10). While the relative instability of the gel matrixes has been a challenge to overcome, it appears that at least one brewer in North America is actively developing this technology (11). Immobilization of the yeast cells can impact the cell...