Concepts for improving ethanol productivity from lignocellulosic materials: Encapsulated yeast and membrane bioreactors
Doctoral thesis, 2014

Lignocellulosic biomass is a potential feedstock for production of sugars, which can be fermented into ethanol. The work presented in this thesis proposes some solutions to overcome problems with suboptimal process performance due to elevated cultivation temperatures and inhibitors present during ethanol production from lignocellulosic materials. In particular, continuous processes operated at high dilution rates with high sugar utilisation are attractive for ethanol fermentation, as this can result in higher ethanol productivity. Both encapsulation and membrane bioreactors were studied and developed to achieve rapid fermentation at high yeast cell density. My studies showed that encapsulated yeast is more thermotolerant than suspended yeast. The encapsulated yeast could successfully ferment all glucose during five consecutive batches, 12 h each at 42 °C. In contrast, freely suspended yeast was inactivated already in the second or third batch. One problem with encapsulation is, however, the mechanical robustness of the capsule membrane. If the capsules are exposed to e.g. high shear forces, the capsule membrane may break. Therefore, a method was developed to produce more robust capsules by treating alginate-chitosan-alginate (ACA) capsules with 3-aminopropyltriethoxysilane (APTES) to get polysiloxane-ACA capsules. Of the ACA-capsules treated with 1.5% APTES, only 0–2% of the capsules broke, while 25% of the untreated capsules ruptured within 6 h in a shear test. In this thesis membrane bioreactors (MBR), using either a cross-flow or a submerged membrane, could successfully be applied to retain the yeast inside the reactor. The cross-flow membrane was operated at a dilution rate of 0.5 h-1 whereas the submerged membrane was tested at several dilution rates, from 0.2 up to 0.8 h-1. Cultivations at high cell densities demonstrated an efficient in situ detoxification of very high furfural levels of up to 17 g L-1 in the feed medium when using a MBR. The maximum yeast density achieved in the MBR was more than 200 g L-1. Additionally, ethanol fermentation of nondetoxified spruce hydrolysate was possible at a high feeding rate of 0.8 h-1 by applying a submerged membrane bioreactor, resulting in ethanol productivities of up to 8 g L-1 h-1. In conclusion, this study suggests methods for rapid continuous ethanol production even at stressful elevated cultivation temperatures or inhibitory conditions by using encapsulation or membrane bioreactors and high cell density cultivations.

Encapsulated yeast

Acetic acid


S. cerevisiae



Membrane bioreactors

KE-salen, Kemigården 4, Göteborg
Opponent: Prof. José Teixeira, University of Minho, Portugal


Päivi Ylitervo

Chalmers, Chemical and Biological Engineering, Industrial biotechnology

Mechanically robust polysiloxane – ACA capsules for prolonged ethanol production

Journal of Chemical Technology and Biotechnology,; Vol. 88(2013)p. 1080-1088

Journal article

Impact of Furfural on Rapid Ethanol Production Using a Membrane Bioreactor

Energies,; Vol. 6(2013)p. 1604-1617

Journal article

Ethanol production at elevated temperatures using encapsulation of yeast

Journal of Biotechnology,; Vol. 156(2011)p. 22-29

Journal article

Driving Forces

Sustainable development

Subject Categories

Industrial Biotechnology

Biological Sciences

Bioprocess Technology


Other Industrial Biotechnology

Areas of Advance


Life Science Engineering (2010-2018)



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 3669

KE-salen, Kemigården 4, Göteborg

Opponent: Prof. José Teixeira, University of Minho, Portugal

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