Influence of Propagation Conditions on the Start-up Phase of Immobilised Yeast Reactors

Main fermentation with yeast immobilised on a carrier material has been a subject of research for many years. Immobilisation technology permits continuous beer fermentation. The whole process has been carried out in a continuously operated bioreactor with a residence time of approximately one day. There is an intention to apply immobilisation in an industrial scale and substitute the traditional batch fermentation system. Until now, primary fermentation with immobilised cells was only applied on a pilot-scale. Although it brings several improvements on volumetric productivity, cell densities, hygiene and continuous operation, the final product has a different taste and aroma, which influences quality and consequently consumer acceptance. To achieve a constant beer quality more information is required about propagation conditions, immobilisation and continuous mode of reactor operation, in order to know how this factors affect yeast cell metabolism and the viability which influences flavour production. The aim of this research was to find an optimal condition of propagation with a high yeast growth rate in a reduced time and a short period of adaptation in the immo-reactor. Temperature and aeration-scheme were the main parameters studied during propagation. Four test series were performed, shifting these parameters in the pilot-brewery of Versuchs – und Lehranstalt für Brauerei (VLB) in Berlin. Yeast was immobilised by adsorption on the DEAE-cellulose (diethylaminoethyl-cellulose) carrier material in a fixed-bed reactor. Samples of beer were taken daily to evaluate the “start-up” phase of the reactor. Analyses showed that propagation with an interrupted aeration at 10ºC was the best method. The results were regular and an acceptable content of diacetyl and acetaldehyde (the main compounds responsible for an off-flavour) was reached, even though the extract reduction was quite slow and the desired pH of 4,5 was not obtained. Furthermore, investigation about yeast physiology is needed to accomplish a beer with an unaltered flavour profile; a resource to genetically modified brewers yeast is a possible way. Probably, the approach to implement this technology in brewing industry within a relatively short period of time is limit future research to support matrices with industrial potential and reactors with lower mass transfer restrictions.