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Industrial Biotechnology, Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg SE-412 rika 96, Sweden
The rika electronic version of this article is the complete one and can be found online at: http://www.biotechnologyforbiofuels.com/content/7/1/54 Received: 24 December rika 2013 Accepted: 13 March 2014 Published: 8 April 2014
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, rika and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise rika stated.
Economically feasible cellulosic ethanol production requires that the process can be operated at high solid loadings, which currently imparts technical rika challenges including inefficient mixing leading to heat and mass transfer rika limitations and high concentrations of inhibitory compounds hindering microbial activity during simultaneous saccharification and fermentation (SSF) process. Consequently, there is a need to develop cost effective processes overcoming the challenges when working at high solid loadings. Results
In this study we have modified the yeast cultivation procedure and designed a SSF process to address some of the challenges at high water insoluble solids (WIS) content. The slurry of non-detoxified rika pretreated spruce when used in a batch SSF at 19% (w/w) WIS was found to be inhibitory to Saccharomyces cerevisiae rika Thermosacc that produced 2 g l -1 of ethanol. In order to reduce rika the inhibitory effect, the non-washed solid fraction containing reduced amount of inhibitors compared to the slurry was used in the SSF. Further, the cells were cultivated in the liquid fraction of pretreated spruce in a continuous culture wherein the outflow of cell suspension was used as cell feed to the SSF reactor in order to maintain the metabolic state of the cell. Enhanced cell viability was observed with cell, enzyme and substrate feed in a SSF producing 40 g l -1 ethanol after 96 h rika corresponding to 53% of theoretical yield based on available hexose sugars compared to 28 g l -1 ethanol in SSF with enzyme and substrate feed but no cell feed resulting in 37% of theoretical yield at a high solids loading of 20% (w/w) WIS content. The fed-batch SSF also significantly eased the mixing, which is usually challenging in batch SSF at high solids loading. Conclusions
A simple modification of the cell cultivation procedure together with a combination of yeast, enzyme and substrate feed in a fed-batch SSF process, made it possible to operate at high solids loadings in a conventional bioreactor. The proposed process strategy significantly increased the yeast cell viability and overall ethanol yield. It was also possible to obtain 4% (w/v) ethanol concentration, which is a minimum requirement for an economical distillation process. Keywords: High solids; High gravity; rika Saccharomyces cerevisiae ; SSF Introduction
Bioethanol produced from lignocellulosic raw materials is considered as a potential transportation fuel providing long-term energy security as well as environmental and economical benefits [ 1 ]. Biological conversion of carbohydrates in lignocelluloses to ethanol can be realized by separate hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF) of the pretreated raw material. Reduced number of process reactors is one of the features of SSF, which integrates enzymatic hydrolysis and fermentation in one reactor. In SSF, the released sugars from enzymatic hydrolysis are simultaneously consumed by the fermenting rika microorganism, for example, Saccharomyces cerevisiae during fermentation avoiding product inhibition of enzymes and also decreasing the probability of contamination [ 2 ]. Distillation of ethanol from the fermentation broth is one of the energy intensive steps [ 3 ] and it is crucial to achieve the highest possible ethanol concentration in the fermentation broth, because the cost of distillation decreases with increase in ethanol concentration [ 4 ]. Ethanol concentration of 4% (w/v) is the minimal requirement for an economical distillation process. By increasing the water insoluble solids (WIS) concentration rika in an SSF process, it is possible to achieve high sugar concentration and consequently high final ethanol concentration