Anaerobic fermentation of glycerol: a path to economic viability for the biofuels industry

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Although biofuels such as biodiesel and bioethanol represent a secure, renewable and environmentally safe alternative to fossil fuels, their economic viability is a major concern. The implementation of biorefineries that co-produce higher value products along with biofuels has been proposed as a solution to this problem. The biorefinery model would be especially advantageous if the conversion of byproducts or waste streams generated during biofuel production were considered. Glycerol-rich streams generated in large amounts by the biofuels industry, especially during the production of biodiesel, present an excellent opportunity to establish biorefineries. Once considered a valuable ‘co-product’, crude glycerol is rapidly becoming a ‘waste product’ with a disposal cost attributed to it. Given the highly reduced nature of carbon in glycerol and the cost advantage of anaerobic processes, fermentative metabolism of glycerol is of special interest. This review covers the anaerobic fermentation of glycerol in microbes and the harnessing of this metabolic process to convert abundant and low-priced glycerol streams into higher value products, thus creating a path to viability for the biofuels industry. Special attention is given to products whose synthesis from glycerol would be advantageous when compared with their production from common sugars.

Introduction

Our economy and lifestyle rely on the use of fossil resources for the generation of transportation fuels and materials; however, there has been rising concern over their cost, sustained availability, and impact on global warming and pollution [1]. This has led to a search for technologies that generate fuels and materials from renewable carbon sources, such as plant biomass. Depending on the component of the biomass used as feedstock (e.g. sugars, oils/fats) and the technology employed to transform this component into the desired product, at least three general platforms have been envisioned: the sugar [2], syngas (synthesis gas) [3], and oil [4] platforms. The sugar and oil platforms are the most well-established today, with bioethanol and biodiesel being examples of their commercial products, respectively. Bioethanol is produced through the microbial fermentation of sugars derived from corn, sugarcane or sugar beet [5]. Biodiesel is produced by the transesterification of vegetable oils or animal fats with an alcohol to produce esters [4].

Given the increasing demand for biofuels [6], there is an urgent need to investigate new and more efficient alternatives for their production. For example, the conversion of lignocellulosic biomass to ethanol and the use of oil-accumulating algae in the production of biodiesel are being investigated [7, 8]. These approaches are very promising and will provide abundant nonfood feedstocks for the production of biofuels with environmental benefits and large net energy gains. However, an outstanding issue in both current and future biofuel production platforms is economic viability. For example, the feedstock and operating costs for the production of biodiesel from soybean amount to $2.94 per gallon of biodiesel, which almost matches the market price of $3.02 per gallon of biodiesel (04/11/2007: www.thejacobsen.com). The implementation of biorefineries has been proposed as a means to increase the economic viability of the biofuels industry [9]. In its ‘conventional’ form, a biorefinery would make use of a fraction of the feedstock (e.g. a portion of sugars or oils) to co-produce a higher value, small-market chemical along with the biofuel(s). The higher revenue from the co-product, which benefits itself from the economies of scale available in a large biofuels plant, would improve the economics of biofuel production. A more economically viable model for a biorefinery, however, should consider the use of by-products or waste streams generated during the production of the biofuel. Glycerol-rich streams generated by the biofuels industry (Figure 1a) have the potential to be used in this context. This review focuses on the anaerobic conversion of crude glycerol into higher value products as a means to improve the economic viability of the biofuels industry.

Section snippets

Conversion of glycerol into higher value products: a need and an opportunity

Glycerol is generated in large amounts during the production of both bioethanol [10] and biodiesel [11]. With every 100 lbs of biodiesel produced by the transesterification of vegetable oils or animal fats, 10 lbs of crude glycerol are generated. The tremendous growth of the biodiesel industry created a glycerol surplus that has resulted in a dramatic 10-fold decrease in crude glycerol prices over the past two years (Figure 1b). Glycerol-producing/refining operations by companies such as Dow

Fermentative metabolism of glycerol by microbes

Although many microorganisms are able to metabolize glycerol in the presence of external electron acceptors (respiratory metabolism) [15, 16], few are able to do so fermentatively (i.e. in the absence of electron acceptors). Fermentative metabolism of glycerol has been studied in great detail in several species of the Enterobacteriaceae family, such as Citrobacter freundii and Klebsiella pneumoniae. Dissimilation of glycerol in these organisms is strictly linked to their capacity to synthesize

Use of Klebsiella, Citrobacter, Enterobacter and Clostridium species

Several attempts have been made to achieve efficient production of chemicals and fuels in microorganisms that ferment glycerol in a 1,3-PDO-dependent manner. The largest body of work has focused on optimizing the conversion of glycerol into 1,3-PDO, a major product of glycerol fermentation in these species. Strain-based improvements include the inactivation of the aldehyde dehydrogenase gene [21] and overexpression of genes of the dha regulon [22], the latter coding for the enzymes in the

Conclusions

The use of anaerobic fermentation to convert abundant and low-priced glycerol streams generated in the production of biodiesel into higher value products represents a promising route to achieve economic viability in the biofuels industry. Several organisms are able to ferment glycerol and synthesize products with a wide-range of functionalities. If fuels and reduced chemicals are targeted, there are many advantages for the use of glycerol over sugars, which together translate into higher yields

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This work was supported by grants from the National Research Initiative of the US Department of Agriculture Cooperative State Research, Education and Extension Service (2005-35504-16698) and the US National Science Foundation (CBET-0645188).

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