No abstract

Despite extensive research carried out in the last few decades, continuous beer fermentation has not yet managed to outperform the traditional batch technology. An industrial breakthrough in favour of continuous brewing using immobilized yeast could be expected only on achievement of the following process characteristics: simple design, low investment costs, flexible operation, effective process control and good product quality. The application of cheap carrier materials of by-product origin could significantly lower the investment costs of continuous fermentation systems. This work deals with a complete continuous beer fermentation system consisting of a main fermentation reactor (gas-lift) and a maturation reactor (packed-bed) containing yeast immobilized on spent grains and corncobs, respectively. The suitability of cheap carrier materials for long-term continuous brewing was proved. It was found that by fine tuning of process parameters (residence time, aeration) it was possible to adjust the flavour profile of the final product. Consumers considered the continuously fermented beer to be of a regular quality. Analytical and sensorial profiles of both continuously and batch fermented beers were compared.

An expeditious method of yeast age estimation was developed based on selective bud scar staining (Alexa Fluor 488-labelled wheat-germ agglutinin) and subsequent fluorescence intensity measurement by flow cytometry. The calibration curve resulting from the cytometric determination of average bud scar fluorescence intensities vs. microscopically counted average bud scar numbers of the same cell populations showed a good correlation and allowed routine cell age estimation by flow cytometry. The developed method was applied for yeast age control in traditional batch and continuous beer fermentations. At the pitching rates used in industrial beer fermentations, our results support former findings by locating a gradient of increasing yeast age from the top to the bottom zone of the fermenter cone. The results also indicate that in continuous beer fermentation, the increasing bud scar fluorescence of immobilized cells could help to schedule the replacement of aged biomass, prior to loss of viability or deterioration of process performance and product quality.

Lehnert R., Novák P., Macieira F., Kuřec M., Teixeira J.A., Branyik T. (2009): Optimisation of labscale continuous alcohol-free beer production. Czech J. Food Sci., 27: 267-275.In order to study the formation and conversion of the most important flavour compounds, the real wort used in alcohol-free beer fermentation was mimicked by a complex model medium containing glucose, yeast extract, and selected aldehydes. The fermentation experiments were carried out in a continuously operating gas-lift reactor with brewing yeast immobilised on spent grains (brewing by-product). During the continuous experiment, parameters such as oxygen supply, residence time (Rt), and temperature (T) were varied to find the optimal conditions for the alcohol-free beer production. The formation of ethanol, higher alcohols (HA), esters (ES), as well as the reduction of aldehydes and consumption of glucose were observed. The results suggest that the process parameters represent a powerful tool in controlling the degree of fermentation and flavour formation brought about by immobilised biocatalyst. Subsequently, the optimised process parameters were used to produce real alcohol-free beer during continuous fermentation. The final product was compared with batch fermented alcohol-free beers using the methods of instrumental and sensorial analysis.

The influence of oxygen supply on the formation and conversion of the most important flavor compounds during continuous, alcohol-free beer production was studied in a complex model medium. The medium contained inorganic salts, nutrients, and aldehydes (hexanal, 2-methyl propanal, 3-methyl butanal, and furfural) and mimicked real brewery wort, with the advantage of a constant composition. Fermentation experiments were carried out in a continuously operating gas-lift reactor, with brewing yeast immobilized on spent grains. The formation (ethanol, higher alcohols, esters, vicinal diketones, and acetaldehyde) and reduction (aldehydes) of flavor-active compounds at different aeration rates were observed. The results suggest that the oxygen supply represents an influential tool for controlling the degree of fermentation and flavor formation carried out by an immobilized biocatalyst. Under optimal oxygen supply conditions in the continuously operating gas-lift reactor, it was possible to obtain a fermented model medium with a composition approaching that of commercial alcohol-free beers produced by batch process.