Flocculation onset in Saccharomyces cerevisiae: effect of ethanol, heat and osmotic stress

Aims: To examine the effect of different stress conditions on the onset of flocculation in an ale-brewing strain, Saccharomyces cerevisiae NCYC 1195. Methods and Results: Flocculation was evaluated using the method of Soares, E.V. and Vroman, A. [Journal of Applied Microbiology (2003) 95, 325]; plasma membrane integrity was accessed using propidium iodide and the staining of the yeast cell wall was performed using calcofluor white M2R. Cells in exponential phase of growth were subjected to different stress conditions. The addition of 1%, 3% and 5% (v/v) ethanol, 1% and 3% (v/v) isopropanol or a brief heat shock (52ºC, 5 min), did not induce an early flocculation phenotype when compared with control cells. The addition of 10% (v/v) ethanol, a continuous mild heat-stress (37ºC) or an osmotic stress (0.5 or 1 mol l-1 of NaCl) did not induce a flocculent phenotype. Conclusions: Flocculation seems not to be induced as a response to different chemical (ethanol and isopropanol) and physical (heat and osmotic) stress conditions. Conversely, osmotic and ethanol [10% (v/v)] stress, as well as a continuous mild heat shock (37ºC), have a negative impact on the phenotype expression of flocculation. Significance and Impact of the Study: The findings reported here contribute to the elucidation of the control of yeast flocculation. This information might be useful to the brewing industry, as the time when the onset of flocculation occurs can determine the fermentation performance and the beer quality, as well as in other biotechnological industries where flocculation can be used as a cell separation process.ERASMUS; ISEP (Portugal)

Carbohydrate carbon sources induce loss of flocculation of an ale-brewing yeast strain

Aims: To identify the nutrients that can trigger the loss of flocculation under growth conditions in an ale-brewing strain, Saccharomyces cerevisiae NCYC 1195. Methods and Results: Flocculation was evaluated using the method of Soares, EX. and Vroman, A. [Journal of Applied Microbiology (2003) 95, 325]. Yeast growth with metabolizable carbon sources (glucose, fructose, galactose, maltose or sucrose) at 2% (w/v), induced the loss of flocculation in yeast that had previously been allowed to flocculate. The yeast remained flocculent when transferred to a medium containing the required nutrients for yeast growth and a sole nonmetabolizable carbon source (lactose). Transfer of flocculent yeast into a growth medium with ethanol (4% v/v), as the sole carbon source did not induce the loss of flocculation. Even the addition of glucose (2% w/v) or glucose and antimycin A (0.1 mg lˉ¹) to this culture did not bring about loss of flocculation. Cycloheximide addition (15 mglˉ¹) to glucose-growing cells stopped flocculation loss. Conclusions: Carbohydrates were the nutrients responsible for stimulating the loss of flocculation in flocculent yeast cells transferred to growing conditions. The glucose-induced loss of flocculation required de novo protein synthesis. Ethanol prevented glucose-induced loss of flocculation. This protective effect of ethanol was independent of the respiratory function of the yeast. Significance and Impact of the Study: This work contributes to the elucidation of the role of nutrients in the control of the flocculation cycle in NewFlo phenotype yeast strains.Instituto Politécnico do Porto (IPP) - Fundo de Apoio à Investigação - Project P24/96 , P24/97.Programa Plurianual de Unidades de I&D-CIEA/ ISEP

Physicochemical surface properties of brewing yeast influencing their immobilization onto spent grains in a continuous reactor

Immobilization of brewing yeast onto a cellulosebased 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 longrange 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.Fundação para Ciência e Tecnologia (FCT

An optimal dissipative encoder for the toric code

We consider the problem of preparing specific encoded resource states for the toric code by local, time-independent interactions with a memoryless environment. We propose the construction of such a dissipative encoder which converts product states to topologically ordered ones while preserving logical information. The corresponding Liouvillian is made up of four local Lindblad operators. For a qubit lattice of size L × L, we show that this process prepares encoded states in time O(L), which is optimal. This scaling compares favorably with known local unitary encoders for the toric code which take time of order Ω(L^2) and require active time-dependent control

Carrier-free, continuous primary beer fermentation

Developing a sustainable continuous fermentation reactor is one of the most ambitious tasks in brewing science, but it could bring great benefits regarding volumetric productivity to modern breweries. Immobilized cell technology is often applied to reach the large densities of yeast needed in a continuous fermentation process. However, the financial cost associated with the use of carriers for yeast immobilization is one of the major drawbacks in the technology. This work suggests that yeast flocculation could address biomass immobilization in a gas-lift reactor for the continuous primary fermentation of beer. Nearly 25 g dry wt L−1 of yeast was flocculated in the reactor before interruption of the fermentation. Stable sugar consumption and ethanol production (4.5% alcohol by volume) from an 11°P wort was evidenced. The key esters and higher alcohols measured in the young beer met the standards of a finished primary beer fermentation.Eduardo Pires gratefully acknowledges the Fundacao para a Ciencia e a Tecnologia (FCT, Portugal) for the PhD fellowship support (SFRH/BD/61777/2009) and Marcel Karabin from the Department of Biotechnology of the Institute of Chemical Technology (Prague) for technical support. The financial contributions of the EU FP7 project EcoBioCAP - Ecoefficient Biodegradable Composite Advanced Packaging, grant agreement no. 265669 as well as of the Grant Agency of the Czech Republic (project GACR P503/12/1424) are also gratefully acknowledged