Bioethanol production from sweet sorghum bagasse by Mucor hiemalis
Graphical abstract
Highlights
► Ethanol can be successfully produced from sweet sorghum bagasse. ► A pretreatment is necessary for efficient ethanol production from bagasse. ► Sodium hydroxide and phosphoric acid treatments are suitable pretreatment processes. ► Bagasse hydrolyzates are efficiently fermented to ethanol by Mucor hiemalis. ► M. hiemalis is among the best alternatives for ethanol production from hydrolyzates.
Introduction
Sweet sorghum is a C4 crop growing in geographical areas with a temperate climate and requires limited fertilizer rates and water. It also has a shorter growing season than sugarcane, higher sugar yield per hectare than sugar beet, and high resistant to drought and salinity (Barbanti et al., 2006, Dolciotti et al., 1998, Zhao et al., 2009). The juice extracted from the fresh stems of this plant contains approximately 16–18% fermentable sugars predominately sucrose, glucose, and fructose. Furthermore, sweet sorghum has a high yield of green biomass (20–30 t/ha), and huge amount of lignocellulosic residue are produced as byproduct of sweet sorghum (Maiorella, 1985, Sipos et al., 2009, Teetor et al., in press, Wu et al., 2010). This lignocellulosic residue, so-called bagasse, has only non-food applications e.g., cattle feed roughage and soil fertilizer after composting (Sipos et al., 2009). Sweet sorghum bagasse, like other lignocellulosic materials, contains of considerable amount of carbohydrate polymers (cellulose and hemicellulose) and lignin. The carbohydrates can be hydrolyzed to fermentable sugars by acids or enzymes and then fermented to ethanol. Ethanol production is suggested as a promising future utilization of sweet sorghum bagasse due to high biomass yield per hectare and lower production costs than many other plants (Li et al., 2010). However, since the cellulose is chemically stable and well protected by lignin in the natural structure, it requires a pretreatment prior to the hydrolysis (Cheng et al., 2008, Macesic and Darko, 2008, Maiorella, 1985, Taherzadeh and Karimi, 2008a, Taherzadeh and Karimi, 2008b).
In order to enhance the yield of ethanol production from lignocelluloses, extensive pretreatments by physical, chemical, or biological means were suggested (Maiorella, 1985, Taherzadeh and Karimi, 2007). The pretreatment process should alter or remove lignin, increase the accessibility of enzymes to the cellulose, and decrease the cellulose crystallinity (Jeihanipour and Taherzadeh, 2009, Sun and Cheng, 2002). Phosphoric acid is a non-corrosive, nontoxic, safe, and inexpensive chemical compared to other mineral acids. Furthermore, concentrated phosphoric acid is an ideal solvent, which can dissolve cellulose in presence of water without inhibitory effect in the subsequent hydrolysis and fermentation processes. The remaining acid may also be used as nutrient for the microorganism. Compared to other pretreatment technologies, this process utilizes lower temperatures and pressures and can be carried out at ambient conditions (Margeot et al., 2009, Zhang et al., 2007, Zhang et al., 2010). Similarly, pretreatment by sodium hydroxide is among the pretreatment methods that received high attention over the years. It is a low energy intensive and relatively inexpensive technique, which has high impact on many feedstocks such as agricultural residues. Sodium hydroxide can simultaneously disrupt the structural linkages, affect the lignin barriers, reduce the cellulose crystallinity, and increase the cellulose accessibility, which results in a sharp increase in saccharification yield (Mosier et al., 2005, Taherzadeh and Karimi, 2008b, Xu et al., 2010).
Saccharomyces cerevisiae is the most common organism for ethanol production from hexoses. However, wide ranges of sugars, e.g., pentoses, are available in lignocellulosic substrates, which cannot assimilate to ethanol by S. cerevisiae. In this direction, Mucor hiemalis is a relatively unexplored filamentous fungus that has some advantages compared to other ethanol-producing microorganisms. It is able to grow at higher temperature compared to baker's yeast and utilize a wide range of monosaccharides including pentoses to ethanol (Hjorth, 2005, Millati et al., 2005). High chitosan content of these types of fungi is another advantage, which makes the biomass suitable for various applications in water purification, drug, and food industries (Karimi et al., 2006, Tauk-Tornisielo et al., 2007). However, to our knowledge, no previous work on fermentation of enzymatic hydrolyzates of lignocellulosic materials has been studied by M. hiemalis.
The overall objective of this study was to investigate ethanol production from sweet sorghum bagasse by filamentous fungus M. hiemalis using separated enzymatic hydrolysis and fermentation. In order to increase the yield of ethanol production, sodium hydroxide and phosphoric acid treatments with/without ultrasonication were used as pretreatment processes before enzymatic hydrolysis of the bagasse.
Section snippets
Microorganism strain and media
The strain M. hiemalis CCUG 16148 (Culture Collection University of Gothenburg, Sweden) was used for ethanol production. The fungus was cultivated on agar slants with medium composition of (g/L): glucose, 40, agar, 20, and peptone, 10 at pH 5.5 ± 0.1 and 32 ± 0.5 °C for 5 days, where the strain grows to form a cotton-like mycelium and spores (Abedinifar et al., 2009, Sharifia et al., 2008). The agar slants were stored at 4 °C until use.
Raw materials
Sweet sorghum (Sofra, Italy) was planted in May 2010 and
Characterization and pretreatment
The sweet sorghum bagasse used in this study mainly contained glucan (41.3%), xylan (17.9%), and lignin (18.2%) (Table 1). The native bagasse was pretreated with NaOH solution at 0 °C for 3 h and concentrated phosphoric acid for 30 min at 50 °C. The effects of ultrasonication in combination with each pretreatment method were also investigated (Table 1).
The compositions of the bagasse after the pretreatments were analyzed and results are presented in Table 1. Glucan was the dominant component in all
Discussion
Sweet sorghum considered as a potential energy crop in nearly all temperate, subtropical, and tropical climates. It produces sugar juice, grains with high starch content, and lignocellulosic bagasse. The bagasse is usually used for energy production by incineration (Kundiyana et al., 2010, Sipos et al., 2009, Wu et al., 2010). The current work dealt with the possibility of ethanol production from the bagasse by fermentation with fungus M. hiemalis.
The selected filamentous fungus, M. hiemalis,
Conclusions
Ethanol can be successfully produced from sweet sorghum bagasse with M. hiemalis. The fungus can be considered as a reliable microorganism and good alternative to baker's yeast in fermentation of lignocellulosic hydrolyzates to ethanol in the terms of the yield and productivity. In order to have a high yield of ethanol from the bagasse, a pretreatment is necessary. NaOH treatment with ultrasonication could be an efficient alternative for pretreatment of bagasse before enzymatic hydrolysis. The
Acknowledgments
The authors are grateful to the culture collection of Gothenburg University (Sweden) for providing the filamentous fungi. Furthermore, the authors would like to thank Dr. Almodares (Department of Biological Science, University of Isfahan, Iran) for providing the sweet sorghum bagasse.
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