Päivi Ylitervo

Ethanol production at elevated temperatures using encapsulation of yeast

The ability of macroencapsulated Saccharomyces cerevisiae CBS 8066 to produce ethanol at elevated temperatures was investigated in consecutive batch and continuous cultures. Prior to cultivation yeast was confined inside alginate-chitosan capsules composed of an outer semi-permeable membrane and an inner liquid core. The encapsulated yeast could successfully ferment 30 g/L glucose and produce ethanol at a high yield in five consecutive batches of 12 h duration at 42°C, while freely suspended yeast was completely inactive already in the third batch. A high ethanol production was observed also through the first 48 h at 40°C during continuous cultivation at D=0.2 h(-1) when using encapsulated cells. The ethanol production slowly decreased in the following days at 40°C. The ethanol production was also measured in a continuous cultivation in which the temperature was periodically increased to 42-45°C and lowered to 37°C again in periods of 12h. Our investigation shows that a non-thermotolerant yeast strain improved its heat tolerance upon encapsulation, and could produce ethanol at temperatures as high as 45°C for a short time. The possibility of performing fermentations at higher temperatures would greatly improve the enzymatic hydrolysis in simultaneous saccharification and fermentation (SSF) processes and thereby make the bioethanol production process more economically feasible.

Effects of encapsulation of microorganisms on product formation during microbial fermentations

This paper reviews the latest developments in microbial products by encapsulated microorganisms in a liquid core surrounded by natural or synthetic membranes. Cells can be encapsulated in one or several steps using liquid droplet formation, pregel dissolving, coacervation, and interfacial polymerization. The use of encapsulated yeast and bacteria for fermentative production of ethanol, lactic acid, biogas, l-phenylacetylcarbinol, 1,3-propanediol, and riboflavin has been investigated. Encapsulated cells have furthermore been used for the biocatalytic conversion of chemicals. Fermentation, using encapsulated cells, offers various advantages compared to traditional cultivations, e.g., higher cell density, faster fermentation, improved tolerance of the cells to toxic media and high temperatures, and selective exclusion of toxic hydrophobic substances. However, mass transfer through the capsule membrane as well as the robustness of the capsules still challenge the utilization of encapsulated cells. The history and the current state of applying microbial encapsulation for production processes, along with the benefits and drawbacks concerning productivity and general physiology of the encapsulated cells, are discussed.

Membrane bioreactors’ potential for ethanol and biogas production: a review

Companies developing and producing membranes for different separation purposes, as well as the market for these, have markedly increased in numbers over the last decade. Membrane and separation technology might well contribute to making fuel ethanol and biogas production from lignocellulosic materials more economically viable and productive. Combining biological processes with membrane separation techniques in a membrane bioreactor (MBR) increases cell concentrations extensively in the bioreactor. Such a combination furthermore reduces product inhibition during the biological process, increases product concentration and productivity, and simplifies the separation of product and/or cells. Various MBRs have been studied over the years, where the membrane is either submerged inside the liquid to be filtered, or placed in an external loop outside the bioreactor. All configurations have advantages and drawbacks, as reviewed in this paper. The current review presents an account of the membrane separation technologies, and the research performed on MBRs, focusing on ethanol and biogas production. The advantages and potentials of the technology are elucidated.

Reverse membrane bioreactor : Introduction to a new technology for biofuel production

The novel concept of reverse membrane bioreactors (rMBR) introduced in this review is a new membrane-assisted cell retention technique benefiting from the advantageous properties of both conventional MBRs and cell encapsulation techniques to tackle issues in bioconversion and fermentation of complex feeds. The rMBR applies high local cell density and membrane separation of cell/feed to the conventional immersed membrane bioreactor (iMBR) set up. Moreover, this new membrane configuration functions on basis of concentration-driven diffusion rather than pressure-driven convection previously used in conventional MBRs. These new features bring along the exceptional ability of rMBRs in aiding complex bioconversion and fermentation feeds containing high concentrations of inhibitory compounds, a variety of sugar sources and high suspended solid content. In the current review, the similarities and differences between the rMBR and conventional MBRs and cell encapsulation regarding advantages, disadvantages, principles and applications for biofuel production are presented and compared. Moreover, the potential of rMBRs in bioconversion of specific complex substrates of interest such as lignocellulosic hydrolysate is thoroughly studied.

Continuous Bioethanol Fermentation from Wheat Straw Hydrolysate with High Suspended Solid Content Using an Immersed Flat Sheet Membrane Bioreactor

Finding a technological approach that eases the production of lignocellulosic bioethanol has long been considered as a great industrial challenge. In the current study a membrane bioreactor (MBR) set-up using integrated permeate channel (IPC) membrane panels was used to simultaneously ferment pentose and hexose sugars to ethanol in continuous fermentation of high suspended solid wheat straw hydrolysate. The MBR was optimized to flawlessly operated at high SS concentrations of up to 20% without any significant changes in the permeate flux and transmembrane pressure. By the help of the retained high cell concentration, the yeast cells were capable of tolerating and detoxifying the inhibitory medium and succeeded to co-consume all glucose and up to 83% of xylose in a continuous fermentation mode leading to up to 83% of the theoretical ethanol yield.

Co-Utilization of Glucose and Xylose for Enhanced Lignocellulosic Ethanol Production with Reverse Membrane Bioreactors.

Integrated permeate channel (IPC) flat sheet membranes were examined for use as a reverse membrane bioreactor (rMBR) for lignocellulosic ethanol production. The fermenting organism, Saccharomyces cerevisiae (T0936), a genetically-modified strain with the ability to ferment xylose, was used inside the rMBR. The rMBR was evaluated for simultaneous glucose and xylose utilization as well as in situ detoxification of furfural and hydroxylmethyl furfural (HMF). The synthetic medium was investigated, after which the pretreated wheat straw was used as a xylose-rich lignocellulosic substrate. The IPC membrane panels were successfully used as the rMBR during the batch fermentations, which lasted for up to eight days without fouling. With the rMBR, complete glucose and xylose utilization, resulting in 86% of the theoretical ethanol yield, was observed with the synthetic medium. Its application with the pretreated wheat straw resulted in complete glucose consumption and 87% xylose utilization; a final ethanol concentration of 30.3 g/L was obtained, which corresponds to 83% of the theoretical yield. Moreover, complete in situ detoxification of furfural and HMF was obtained within 36 h and 60 h, respectively, with the rMBR. The use of the rMBR is a promising technology for large-scale lignocellulosic ethanol production, since it facilitates the co-utilization of glucose and xylose; moreover, the technology would also allow the reuse of the yeast for several batches.

Continuous Ethanol Production with a Membrane Bioreactor at High Acetic Acid Concentrations

The release of inhibitory concentrations of acetic acid from lignocellulosic raw materials during hydrolysis is one of the main concerns for 2nd generation ethanol production. The undissociated form of acetic acid can enter the cell by diffusion through the plasma membrane and trigger several toxic effects, such as uncoupling and lowered intracellular pH. The effect of acetic acid on the ethanol production was investigated in continuous cultivations by adding medium containing 2.5 to 20.0 g·L−1 acetic acid at pH 5.0, at a dilution rate of 0.5 h−1. The cultivations were performed at both high (~25 g·L−1) and very high (100–200 g·L−1) yeast concentration by retaining the yeast cells inside the reactor by a cross-flow membrane in a membrane bioreactor. The yeast was able to steadily produce ethanol from 25 g·L−1 sucrose, at volumetric rates of 5–6 g·L−1·h−1 at acetic acid concentrations up to 15.0 g·L−1. However, the yeast continued to produce ethanol also at a concentration of 20 g·L−1 acetic acid but at a declining rate. The study thereby demonstrates the great potential of the membrane bioreactor for improving the robustness of the ethanol production based on lignocellulosic raw materials.

Fermentation of lignocellulosic hydrolyzate using a submerged membrane bioreactor at high dilution rates.

A submerged membrane bioreactor (sMBR) was developed to ferment toxic lignocellulosic hydrolyzate to ethanol. The sMBR achieved high cell density of Saccharomyces cerevisiae during continuous cultivation of the hydrolyzate by completely retaining all yeast cells inside the sMBR. The performance of the sMBR was evaluated based on the ethanol yield and productivity at the dilution rates 0.2, 0.4, 0.6, and 0.8h(-1) with the increase of dilution rate. Results show that the yeast in the sMBR was able to ferment the wood hydrolyzate even at high dilution rates, attaining a maximum volumetric ethanol productivity of 7.94 ± 0.10 g L(-1)h(-1) at a dilution rate of 0.8h(-1). Ethanol yields were stable at 0.44 ± 0.02 g g(-1) during all the tested dilution rates, and the ethanol productivity increased from 2.16 ± 0.15 to 7.94 ± 0.10 g L(-1)h(-1). The developed sMBR systems running at high yeast density demonstrate a potential for a rapid and productive ethanol production from wood hydrolyzate.