Two stage biodiesel and hydrogen production from molasses by oleaginous fungi and Clostridium acetobutylicum ATCC 824
Introduction
Fossil fuels are not renewable and will be exhausted sooner or later [1]. The increase in prices of petroleum based fuels, future depletion of worldwide petroleum reserves and environmental polices to reduce CO2 emissions have stimulated research into the development of biotechnology to produce clean energy from renewable resources [2]. Thus, it is necessary to find alternative energy sources that are renewable and environmentally friendly [3], [4], [5], [6], [7], [8]. Hydrogen and biodiesel are very promising alternative energy sources and have received more attention all over the world in recent years.
Biodiesel fuel is fatty acid mono alkyl (methyl, propyl or ethyl) esters produced from natural, renewable sources such as vegetable oils or animal fats by transesterification of such triglycerides [9], [10]. Biodiesel is rather an attractive alternative to the conventional petroleum diesel fuels because of its biodegradable, nontoxic and clean renewable characteristics [10], [11]. One way to increase the world bio oil production that would cause low ecosystem impact is to use lipids from single-cell oil microorganisms [12] for biodiesel production. Oleaginous yeasts and fungi have also been considered as potential oil sources for biodiesel production because they accumulate large amounts of lipids. Among these microorganisms, particular attention has been dedicated to Epicoccum purpurascens [13] and Mortierella isabelina [14].
Hydrogen is one of the most abundant elements in the universe. It is an odorless, colorless, tasteless and non-poisonous gas. Hydrogen gas has been proposed as the ultimate transport fuel for vehicles and vessels because of its non-polluting characteristics. When hydrogen is used as a fuel, it generates no pollutants but produces water. Hydrogen also enables the use of highly efficient fuel cells to convert chemical energy to electricity [15]. Another major use of hydrogen is in reduction reactions to produce industrial chemicals.
Hydrogen is produced mainly from natural gas, a finite resource, through steam reforming, a process that generates large quantities of carbon dioxide (CO2) which is a principal cause of global warming. Hydrogen production through dark or photofermentative conversion of organic substrates is of great interest due to its dual function of waste reduction and clean energy production, thereby acting as a promising option for biohydrogen production [16], [17], [18]. Anaerobic bacteria capable of hydrogen production can use various renewable biomass and numerous agriculture, municipal and food processing waste and wastewater sources [19], [20].
Molasses is the by-product from sugar industry and is considered as one of the most promising feedstocks in the production of biofuel. Black strap molasses is another byproduct of the sugar industry, obtained during sugarcane refining and, like beet molasses, has a high sugar content, around 50% (36% sucrose, 6% fructose, 3% glucose), making it a promising substrate for biohydrogen production [21]. Sugarcane molasses is also an important organic waste due to its high sugar content (55%) and high volume of production. It is even more viscous than beet molasses and the total sugar content is higher. The availability and cost of sugarcane molasses make it an attractive feedstock for use in many countries.
The generation of lipid by fermentative oleaginous fungi accompanies the formation of organic acids as metabolic products [22]. The accumulation of these acid results in a sharp drop in culture pH and subsequent inhibition of fungal growth. But, it is somewhat difficult to achieve the complete utilization of molasses sugar to lipid through fungal fermentation. This appears to be one of the major bottlenecks in the molasses fermentation process for lipid production. So, the process outlined in the present paper takes into consideration the non-utilization of sugar and the acids produced therein (spent media). Combining fermentative oleaginous fungi with hydrogen producing Clostridium could provide an integrated system for maximizing the biofuel yield from sugarcane molasses. In such a system, the fermentation of spent medium containing residual sugar and organic acids generated by fungi, which are then converted into hydrogen by Clostridium in the second step in a bioreactor. In the present study, sugarcane molasses was considered as a preliminary substrate for biodiesel production by various fungal species isolated from Egypt and the spent medium from this process was used as a substrate for hydrogen production by the strictly anaerobic Clostridium acetobutylicum ATCC 824 that also produces an acetone–butanol–ethanol mixture as well as hydrogen.
Section snippets
Fungal inoculums
Forty-five fungal isolates recovered from various sources in Upper Egypt and maintained in potato dextrose agar (PDA) medium at 4 °C were used in this study. Identification of fungal isolates was carried out based on the morphological features using the keys of Pitt [23], Raper and Fennell [24], Ellis [25], Kendrick [26] and Domsch et al. [27]. Fungi were grown on PDA plates at 25 ± 1 °C. Seven day old culture PDA plates of fungi grown at 25 ± 1 °C were used for inoculum preparation. Fungal
Lipid accumulation by fungi
Sugarcane molasses is an agro-industrial by-product often used in fermentation due to the presence of fermentative sugars, being an optimal for the microbial metabolism. In this study, our rationale is to investigate the potency of forty-five fungal isolates to produce oils in a remarkable manner for biodiesel production from low cost waste molasses. Furthermore, the fatty acid methyl ester composition of biodiesel from the detected oleaginous fungi was investigated. In this study 6 fungal
Conclusion
The current study indicated that the six fungal species were found to be oleaginous; containing more than 20% lipids per dry mass. A. alternata was the highest lipid producer from sugarcane molasses. Methyl esters of palmitic, stearic, linoleic and elaidic acids represented the major components of biodiesel produced from tested oleaginous fungi. The spent medium from first stage of sugarcane molasses fermentation by oleaginous fungal species is found to have the capacity for the dark-production
Transparency declaration
The authors declare no conflicts of interest.
Acknowledgments
The authors would like to thank Prof. Dr. George Bennett, Rice University, USA, for providing Clostridium acetobutylicum ATCC 824, precious comments and critical reading of the manuscript. The authors are very grateful to the precious suggestions, constructive comments and careful corrections made by three anonymous reviewers for further improvements of this paper. This research was supported by Assiut University Fund.
References (72)
- et al.
Sustainable fermentative hydrogen production: challenges for process optimization
Int J Hydrogen Energy
(2002) - et al.
Production of acetone–butanol–ethanol from spoilage date palm (Phoenix dactylifera L.) fruit by a mixed culture of Clostridium acetobutylicum and Bacillus subtilis
Biomass Bioenergy
(2012) - et al.
Hydrogen production from rotten dates by sequential three stages fermentation
Int J Hydrogen Energy
(2011) Hydrogen production from acid hydrolyzed molasses by the hydrogen overproducing Escherichia coli strain HD701 and subsequent use of the waste bacterial biomass for biosorption of Cd(II) and Zn(II)
Int J Hydrogen Energy
(2011)- et al.
Hydrogen production by biological processes: a survey of literature
Int J Hydrogen Energy
(2001) - et al.
Advances in biological hydrogen production processes
Int J Hydrogen Energy
(2008) - et al.
Biodiesel production: a review
Bioresour Technol
(1999) Diesel fuel from vegetable oils: status and opportunities
Biomass Bioenergy
(1993)- et al.
A potent lipid producing isolate of Epicoccum purpurascens AUMC5615 and its promising use for biodiesel production
Biomass Bioenergy
(2011) - et al.
Microbial lipid production from xylose by Mortierella isabellina
Bioresour Technol
(2013)