Producing bioethanol from pretreated-wood dust by simultaneous saccharification and co-fermentation process
Graphical abstract
Introduction
Continued industrialization and population growth have increased annual energy consumption [1]. Increasing fossil fuel prices and greenhouse gases have motivated the research and development of renewable resources in many countries. The principal substitute for petrol in road vehicles is bioethanol. One of the advantages of using bioethanol is that doing so reduces greenhouse gas emissions. Also, blending bioethanol with petrol, as in E85, helps to extend diminishing oil supplies, increase fuel security, and eliminate heavy reliance on oil producing nations. Bioethanol is seen as the most promising prospective renewable energy source, which can be produced from microbial fermentation by converting sugars from cellulosic materials such as wood dust [2]. Lignocellulose is an abundant natural carbohydrate that can be transformed into a substitute renewable energy resource by microbial conversion [3]. The advantages of using cellulosic materials for bioethanol production are their low cost, ready availability, lack of conflict with use for food, and the potential production of fuel from lignin [4].
A pretreatment step is necessary for modifying structural characteristics of lignocellulose and increasing the availability of glucan and xylan for enzymatic saccharification [5]. The pretreatment step can destroy the lignin structure to allow for easier break down of the cellulose by the saccharification enzyme [4]. Traditional pretreatments of biomass for producing bioethanol include physical methods and chemical methods, involving liquid hot water and alkali (or acid), for example [6]. However, several disadvantages for all the different pretreatment options exist, and it is necessary to adapt suitable pretreatments based on the properties of the raw material. Physical and chemical pretreatments inhibit the hydrolysis of cellulose and hemicellulose and lignin fraction owing to the close among the components of lignocellulosic biomass. Therefore, in this study, steam explosion (SE) and supercritical fluid extraction (SFE) were used in this work to pretreat wood dust to avoid the aforementioned problems.
Steam explosion (SE) technology has become one of the most common and widely employed physicochemical pretreatments for lignocellulosic biomass. The process of SE, developed by William H. Mason in 1925, is used for the pretreatment of agricultural wastes, i.e., wood dust in this study [5]. SE technology is a hydrothermal pretreatment that consists of three main steps: (1) the treatment step, (2) the explosion step, and (3) the impact step [5]. The first step is a process wherein lignocellulosic biomass is treated with pressurized steam for a certain period. The SFE rapid release of pressure causes the explosion of lignocellulosic biomass. Finally, the impact of the lignocellulosic biomass mixture is done to form a raw material used for enzyme hydrolysis.
Supercritical fluid extraction is also one of the pretreatment methods for disrupting the crystalline structure of lignocellulose under certain conditions such as temperature and pressure above its critical point [7]. SFE is becoming necessary to reduce the environmental hazards of common chemicals and solvents used in traditional methods. SFE shows excellent potential as a method for lignocellulosic biomass pretreatment [8]. Reports have shown that SFE has been successfully used for the pretreatment of commercial cellulosic materials, recycled paper mix, and sugarcane bagasse [7], [8]. In addition, the use of SFE, as a green solvent for biomass pretreatment in a biorefinery concept, is increasing and it is expected to continue growing in the future.
Saccharification enzymes can be produced by fungi such as Trichoderma reesei and Aspergillus niger, and agricultural waste can be converted into bioethanol by Zymomonas mobilis. Conversion of agricultural waste reduces its environmental impacts by efficiently using it to produce secondary energy. The conversion of cellulose into monosaccharides by microorganisms has been proved and does not cause secondary environmental pollution. A modified bioreactor that supports simultaneous saccharification and co-fermentation (SSCF) was used for the effective conversion of agricultural waste into bioethanol. Previous studies have reported the conversion of cellulose into monosaccharides using microorganisms [9], [10]. The SSCF has recently been used to produce bioethanol according to its properties, i.e., reduced inhibition of cellulase by fermented hydrolyzed sugars, higher product yield, lower cellulase requirements, lower requirements for sterile conditions, shorter process time, and reduced reactor volume. Additionally, SSCF has many advantages over bioethanol production such as a higher yield, shorter processing time, and lower reactor volume.
This work has two parts. First, wood dust is pretreated by SFE and SE to form substrates on which Z. mobilis can produce bioethanol. The respective amounts of bioethanol produced are compared. Second, the activities of saccharification enzymes were evaluated by cocultivating T. reesei and A. niger. Pre-treated wood dust is applied as a medium in the SSCF bioreactor to evaluate its potential for bioethanol production. The whole experimental procedures were shown into the Fig. 1A.
Section snippets
Microorganisms and maintenance media
T. reesei BCRC 31,863 and A. niger BCRC 31,130 were obtained from the Bioresource Collection and Research Centre (BCRC) of Taiwan. The stock culture was maintained aseptically on potato-dextrose-agar (PDA) Petri plates (BD, New Jersey, USA). The PDA plates were incubated at 30 °C for 7 days until good sporulation was reached and then stored at 4 °C. Z. mobilis BCRC 10,809 was obtained from the BCRC of Taiwan. Plate stock culture medium consisted of yeast extract (5.0 g/L, BD), glucose 20 (g/l,
Identification of the composition of wood dust and the byproduct effects
Three major parameters, i.e., cellulose, hemicellulose, and lignin were observed when analyzing the composition of wood dust by SFE and SE. The remainder of the composition of wood dust by SFE and SE was made up of acetate, water and ash. According to the experimental results presented in Fig. 2, the percentages of cellulose, hemicellulose, lignin, and the others of wood dust were 39.6%, 11.7%, 39.40%, and 9.3%, respectively. The experimental results revealed that the combined content of
Conclusions
Our experimental results suggest that the pretreatment methods used in this study are feasible by using either SFE or SE. In addition, the advantage of using SFE pretreatment is that it can effectively reduce byproduct production because there is no acid treatment step in the whole process. This study also showed the synergistic effect of the cocultivation of A. niger and T. reesei on saccharification enzyme activity; the results also suggest the optimal alginate conditions used for
Conflict of interest
The authors declare no competing interests.
Acknowledgements
The authors gratefully acknowledge the financial support by the Ministry of Science and Technology of the Republic of China under grant number MOST 104-2622-8-007-001- and MOST 105-2622-8-007-009- and MOST 105-2221-E-155-070-. Valuable discussions and suggestions from Dr. Yu-Kuo Liu are gratefully acknowledged.
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