Aroma formation by immobilized yeast cells in fermentation processes.

Immobilized cell technology has shown a significant promotional effect on the fermentation of alcoholic beverages such as beer, wine and cider. However, genetic, morphological and physiological alterations occurring in immobilized yeast cells impact on aroma formation during fermentation processes. The focus of this review is exploitation of existing knowledge on the biochemistry and the biological role of flavour production in yeast for the biotechnological production of aroma compounds of industrial importance, by means of immobilized yeast. Various types of carrier materials and immobilization methods proposed for application in beer, wine, fruit wine, cider and mead production are presented. Engineering aspects with special emphasis on immobilized cell bioreactor design, operation and scale-up potential are also discussed. Ultimately, examples of products with improved quality properties within the alcoholic beverages are addressed, together with identification and description of the future perspectives and scope for cell immobilization in fermentation processes.

Modeling of saccharide utilization in primary beer fermentation with yeasts immobilized in calcium alginate.

Immobilized beer fermentation was studied using an industrial bottom-fermenting yeast strain Saccharomyces cerevisiae. The yeast cells were immobilized in 2.5% calcium alginate gel and used for brewing in a five-vessel cascade reactor. The fermentation was performed at 15 degrees C at various flow rates. A nonstructured mathematical model was developed to simulate the performance of continuous primary fermentation of lager beer. The model was based on the following variables: maltose, maltotriose, glucose, fructose, ethanol, and cell concentration. Experimental values of these variables were determined in samples taken at regular intervals. For experimental data fitting a nonlinear regression was used. Substrate consumption was characterized by specific substrate consumption rate and saturation constant. The values of these two parameters were optimized for all four substrates. Inhibition effects of substrates and product were analyzed using various inhibition patterns. Only the inhibition effect of maltose on maltose consumption was clearly identified. A good-fitting relationship for maltose inhibition was found, and inhibition constants were calculated.

Use of bioluminometry for determination of active yeast biomass immobilized in ionotropic hydrogels.

The technique of bioluminometry was used to determine the biomass concentration of yeast cells immobilized in ionotropic hydrogel beads, including alginate, pectate, and kappa-carrageenan. The method uses determination of ATP extracted from viable cells, the concentration of which is then expressed as the active biomass concentration. Seven yeast strains divided into three categories (brewing, wine-making, and ethanol-producing yeasts) were tested, and different biomass concentrations were determined in all three immobilization materials. The described method is characterized by a good correlation (up to 99%) to classical dry biomass determination. The method is quicker, easier, and not so laborious, providing sufficient determination accuracy, and can be used for a rapid estimation of viable biomass in most biotechnological processes using immobilized living cells.

Changes in the yeast metabolism at very high-gravity wort fermentation.

The rate of ethanol production increased with increasing wort gravity up to the initial wort concentration of 24%, reaching the maximum ethanol concentration of 6.2%, but its attenuation reached only 49%. The intracellular trehalose accumulation was proportional to the initial wort gravity, at 24 or 30% wort fermentation increased 3 or 4.5 times, respectively, compared to 12% wort fermentation. Trehalose accumulation began after exhaustion of glucose, ceased after uptake of approximately 65% reducing saccharides, despite of increasing ethanol or remaining saccharide concentration in the environment.

Enhancement of yeast ethanol tolerance by calcium and magnesium.

Ethanol-induced changes of CO2 production were compared in three strains of Saccharomyces cerevisiae. CaCl2 and MgCl2 exerted protective effects against the action of ethanol. Optimal concentrations ensuring maximum of CO2 production at 10% (V/V) of ethanol under non-growing conditions were 3 mmol/L Ca2+ and 2 mmol/L Mg2+. Yeast growth with and without ethanol addition was stimulated by Mg2+ more than by Ca2+ during fermentation, whereas ethanol production was more efficient when both Ca2+ and Mg2+ were added.

Influence of ethanol on the lipid content and fatty acid composition of Saccharomyces cerevisiae.

The content of total lipid as well as of ergosterol, squalene, and major fatty acids were compared in the cells of a distillery strain of Saccharomyces cerevisiae incubated for 3, 48 and 120 h in the presence of 5, 10 and 15% ethanol. Ethanol induced lipid accumulation with preferential ergosterol biosynthesis. The relative contents of palmitic and stearic acid decreased whereas the amount of palmitoleic and oleic acid increased. The total content of all fatty acids rose as a consequence of the ethanol treatment.

Fermentation of starch to ethanol by a co-culture of Saccharomycopsis fibuligera and Saccharomyces cerevisiae.

Static fermentation of starch to ethanol by a co-culture of Saccharomycopsis fibuligera and Saccharomyces cerevisiae without addition of nutritional supplements was investigated with respect to initial starch concentration, pH of the media and initial dry weight ratio of Sps. fibuligera to Sacc. cerevisiae biomass (I R).Optimal conditions for ethanol production were: starch from 20 to 30 g/l; initial pH values from 5.8 to 6.0; and I R values of 2.0 or 3.0. The highest attained ethanol concentration, 13.7 g/l, represented 88% of the theoretical yield.