Factors Impeding Enzymatic Wheat Gluten Hydrolysis at High Solid Concentrations
Hardt, N.A. ; Janssen, A.E.M. ; Boom, R.M. ; Goot, A.J. van der - \ 2014
Biotechnology and Bioengineering 111 (2014)7. - ISSN 0006-3592 - p. 1304 - 1312.
functional-properties - water activity - plastein synthesis - biomass - lignocellulose - inhibition - proteins - softwood
Enzymatic wheat gluten hydrolysis at high solid concentrations is advantageous from an environmental and economic point of view. However, increased wheat gluten concentrations result in a concentration effect with a decreased hydrolysis rate at constant enzyme-to-substrate ratios and a decreased maximum attainable degree of hydrolysis (DH%). We here identified the underlying factors causing the concentration effect. Wheat gluten was hydrolyzed at solid concentrations from 4.4% to 70%. The decreased hydrolysis rate was present at all solid concentrations and at any time of the reaction. Mass transfer limitations, enzyme inhibition and water activity were shown to not cause this hydrolysis rate limitation up to 50% solids. However, the hydrolysis rate limitation can be, at least partly, explained by a second-order enzyme inactivation process. Furthermore, mass transfer impeded the hydrolysis above 60% solids. Addition of enzyme after 24 h at high solid concentrations scarcely increased the DH%, suggesting that the maximum attainable DH% decreases at high solid concentrations. Reduced enzyme activities caused by low water activities can explain this DH% limitation. Finally, a possible influence of the plastein reaction on the DH% limitation is discussed.
Modelling ethanol production from cellulose: separate hydrolysis and fermentation versus simultaneous saccharification and fermentation
Drissen, R.E.T. ; Maas, R.H.W. ; Tramper, J. ; Beeftink, H.H. - \ 2009
Biocatalysis and Biotransformation 27 (2009)1. - ISSN 1024-2422 - p. 27 - 35.
enzymatic-hydrolysis - lignocellulose - biomass - conversion - softwood - technology - inhibition - substrate - economics
In ethanol production from cellulose, enzymatic hydrolysis, and fermentative conversion may be performed sequentially (separate hydrolysis and fermentation, SHF) or in a single reaction vessel (simultaneous saccharification and fermentation, SSF). Opting for either is essentially a trade-off between optimal temperatures and inhibitory glucose concentrations on the one hand (SHF) vs. sub-optimal temperatures and ethanol-inhibited cellulolysis on the other (SSF). Although the impact of ethanol on cellobiose hydrolysis was found to be negligible, formation of glucose and cellobiose from cellulose were found to be significantly inhibited by ethanol. A previous model for the kinetics of enzymatic cellulose hydrolysis was, therefore, extended with enzyme inhibition by ethanol, thus allowing a rational evaluation of SSF and SHF. The model predicted SSF processing to be superior. The superiority of SSF over SHF (separate hydrolysis and fermentation) was confirmed experimentally, both with respect to ethanol yield on glucose (0.41 g g-1 for SSF vs. 0.35 g g-1 for SHF) and ethanol production rate, being 30% higher for an SSF type process. High conversion rates were found to be difficult to achieve since at a conversion rate of 52% in a SSF process the reaction rate dropped to 5% of its initial value. The model, extended with the impact of ethanol on the cellulase complex proved to predict reaction progress accurately.
Pilot-scale conversion of lime-treated wheat straw into bioethanol: quality assessment of bioethanol and valorization of side streams by anaerobic digestion and combustion
Maas, R.H.W. ; Bakker, R.R.C. ; Boersma, A.R. ; Bisschops, I. ; Pels, J.R. ; Jong, E. de; Weusthuis, R.A. ; Reith, H. - \ 2008
Biotechnology for Biofuels 1 (2008). - ISSN 1754-6834
ethanol-production - lignocellulosic materials - saccharomyces-cerevisiae - enzymatic-hydrolysis - biomass - pretreatment - fermentation - acid - saccharification - softwood
The limited availability of fossil fuel sources, worldwide rising energy demands and anticipated climate changes attributed to an increase of greenhouse gasses are important driving forces for finding alternative energy sources. One approach to meeting the increasing energy demands and reduction of greenhouse gas emissions is by large-scale substitution of petrochemically derived transport fuels by the use of carbon dioxide-neutral biofuels, such as ethanol derived from lignocellulosic material. Results This paper describes an integrated pilot-scale process where lime-treated wheat straw with a high dry-matter content (around 35% by weight) is converted to ethanol via simultaneous saccharification and fermentation by commercial hydrolytic enzymes and bakers' yeast (Saccharomyces cerevisiae). After 53 hours of incubation, an ethanol concentration of 21.4 g/liter was detected, corresponding to a 48% glucan-to-ethanol conversion of the theoretical maximum. The xylan fraction remained mostly in the soluble oligomeric form (52%) in the fermentation broth, probably due to the inability of this yeast to convert pentoses. A preliminary assessment of the distilled ethanol quality showed that it meets transportation ethanol fuel specifications. The distillation residue, which contained non-hydrolysable and non-fermentable (in)organic compounds, was divided into a liquid and solid fraction. The liquid fraction served as substrate for the production of biogas (methane), whereas the solid fraction functioned as fuel for thermal conversion (combustion), yielding thermal energy, which can be used for heat and power generation. Conclusion Based on the achieved experimental values, 16.7 kg of pretreated wheat straw could be converted to 1.7 kg of ethanol, 1.1 kg of methane, 4.1 kg of carbon dioxide, around 3.4 kg of compost and 6.6 kg of lignin-rich residue. The higher heating value of the lignin-rich residue was 13.4 MJ thermal energy per kilogram (dry basis).