Primary beer fermentation by immobilised yeast - a review on flavour formation and control strategies

High volumetric cell densities of yeasts can be obtained by packing the microorganisms in a small defined volume by entrapment or adsorption on the surface of a carrier matrix. This leads to higher volumetric productivities and, in consequence, to smaller bioreactor sizes, with resulting decreased capital costs and shorter residence times. Using immobilised cells, the production of beer in a continuous fermentation process becomes very financially and technically feasible. Additional benefits associated with continuous fermentation are ease of biomass separation and recovery (the flocculation properties of brewing yeast strains used become inconsequential), simplification of process design, lower risk of microbial contamination of the pitching yeast population and potential savings. On the other hand, a number of technical challenges require resolution:[1] removal of excess yeast, removal of carbon dioxide, sustaining yeast viability, optimisation of oxygen (air) feed, prevention of microbial contamination and prevention of clogging or channelling of the reactors. Regeneration of large amounts of carrier may be rendered unnecessary by the use of a cheap, replaceable carrier, such as wood chips.

Immobilised cell technology (ICT) processes have been designed for different stages in the beer fermentation process: wort acidification, bioflavouring during the secondary fermentation, primary fermentation and fermentations for the production of alcohol-free or low-alcohol beers (for recent reviews, see Brányik et al.[2] and Nedovic et al.[3]). The most challenging and complex application is to the combined main and secondary fermentation.

Traditional beer fermentation technology uses freely suspended yeast cells to ferment wort in an unstirred batch reactor, a time-consuming process. The traditional primary fermentation for lager beer takes approximately 7 days with a subsequent secondary fermentation (maturation) of several weeks. However, the resulting beer has a well-balanced flavour profile, which is well accepted by the consumer. Nowadays, large breweries use an accelerated fermentation scheme, which is based on using a higher fermentation temperature and a selected specific yeast strain. This allows the production of finished lager beer in 12-15 days.

ICT is able to produce lager beer in a much shorter time period (usually 1-3 days). A major difficulty is to achieve the correct balance of sensory compounds to create an acceptable flavour profile in such a short time frame. ICT for beer production can only be introduced successfully on an industrial scale if the flavour profile can be controlled and fine-tuned.