In eukaryotic green microalgae, manipulation of metabolic pathways by altering the culture medium and/or culture conditions represents a powerful tool for physiological control and is usually more ...practicable than metabolic or genetic engineering. Strategies for nutrient-induced shifts in biomass composition are generally cost-efficient, environmentally friendly, applicable on a large scale and flexible for various industrially attractive microalgae species. In addition, processes, such as nutrient limitation/deprivation, can be readily scheduled and optimised to achieve high levels of productivity for the desired target compound(s). These strategies are currently used in microalgae to achieve overproduction of metabolites such as lipids, polysaccharides and pigments. This paper presents an overview of the species and strain-specific responses of eukaryotic, green microalgal cells that are triggered by variations in selected macronutrient and micronutrient availability. Individual and mutually associated physiological responses to nutrient supply status are described at the molecular level as well as discussed from the perspective of potential biotechnological applications.
Immobilization of brewing yeast onto a cellulose-based carrier obtained from spent grains, a brewing byproduct, by acid/base treatment has been studied in a continuously operating bubble-column ...reactor. The aim of this work was to study the mechanisms of brewing yeast immobilization onto spent grain particles through the information on physicochemical surface properties of brewing yeast and spent grain particles. Three mechanisms of brewing yeast immobilization onto spent grains carrier were proposed: cell-carrier adhesion, cell-cell attachment, and cell adsorption (accumulation) inside natural shelters (carrier's surface roughness). The possibility of stable cell-carrier adhesion regarding the free energy of interaction was proved and the relative importance of long-range forces (Derjaguin-Landau-Verwey-Overbeek theory) and interfacial free energies was discussed. As for the cell-cell attachment leading to a multilayer yeast immobilization, a physicochemical interaction through localized hydrophobic regions on cell surface was hypothesized. However, neither flocculation nor chain formation mechanism can be excluded so far. The adsorption of brewing yeast inside sufficiently large crevices (pores) was documented with photomicrographs. A positive effect of higher dilution rate and increased hydrophobicity of base-treated spent grains on the yeast immobilization rate has also been found.
The aim of this study is to demonstrate the influence of production strains (bottom-fermenting Saccharomyces pastorianus and S. cerevisiae with disruption in the KGD2 gene), carrier materials (spent ...grains and corncobs), reactor arrangements (packed-bed and gas-lift reactors), and mixing regimes (ideally mixed and plug flow) on the formation of flavor-active compounds during alcohol-free beer production. In addition, the composition of alcohol-free beer produced on a laboratory scale was compared with those of commercial products. The results confirmed the influence of each component of the production system (yeast strain, reactor, and carrier) on flavor formation, but their individual importance was case specific. The results indicate that the interplay between the appropriate production strain, carrier material, and bioreactor design is very important in continuous immobilized cell reactors and that suitable combinations could be used to improve both system performance and product quality.
The biotechnological potential of
Microcystis aeruginosa
brings requirements for efficient cultivation and harvesting of biomass. Flocculation of
M. aeruginosa
at alkaline pH induced by calcium or ...magnesium precipitates was studied under model conditions, in culture medium with/without cellular organic matter (COM). The effect of independent variables (Ca
2+
, Mg
2+
, PO
4
3−
, and pH) on the zeta potential and turbidity of cells and inorganic precipitates was quantified by response surface methodology. The experimentally obtained flocculation efficiencies (FEs) were compared with predictions of physicochemical interaction (DLVO) models. The results presented here delimited the concentration ranges of Ca
2+
, Mg
2+
, PO
4
3−
, and pH, resulting in FE > 85%. The DLVO prediction model suggested that for high FE, positively charged precipitates and sufficient precipitate turbidity were required. At pH 10, alkaline flocculation was more advantageous using magnesium precipitates, since it required less phosphate. High FE with COM was achieved at pH 12 when precipitate formation was induced at a low phosphate concentration by the addition of magnesium hydroxide.
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.
Among the most important factors influencing beer quality is the presence of well-adjusted amounts of higher alcohols and esters. Thus, a heavy body of literature focuses on these substances and on ...the parameters influencing their production by the brewing yeast. Additionally, the complex metabolic pathways involved in their synthesis require special attention. More than a century of data, mainly in genetic and proteomic fields, has built up enough information to describe in detail each step in the pathway for the synthesis of higher alcohols and their esters, but there is still place for more. Higher alcohols are formed either by anabolism or catabolism (Ehrlich pathway) of amino acids. Esters are formed by enzymatic condensation of organic acids and alcohols. The current paper reviews the up-to-date knowledge in the pathways involving the synthesis of higher alcohols and esters by brewing yeasts. Fermentation parameters affecting yeast response during biosynthesis of these aromatic substances are also fully reviewed.
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