Microbial Production of Extracellular Polysaccharides from Biomass

Ebru Toksoy Oner

Abstract The interest in polysaccharides has increased considerably in recent years, as they are candidates for many commercial applications in different industrial sec­tors like food, petroleum, and pharmaceuticals. Because of their costly production processes, industrial microbial polysaccharides like xanthan, dextran, curdlan, gel — lan, and pullulan constitute only a minor fraction of the current polymer market. Therefore, much effort has been devoted to the development of cost-effective and environmentally friendly production processes by switching to cheaper fermentation substrates. In this chapter, various microbial polysaccharide production processes utilizing cheap biomass resources like syrups and molasses, olive mill wastewater, cheese whey, various vegetable and fruit pomace, pulp and kernels as well as carbon dioxide and lignocellulosic biomass like rice hull and bran, sawdust, and fibers are discussed with a special focus on the employed pretreatment methods.

Keywords EPS ■ Microbial exopolysaccharides ■ Polysaccharides ■ Biomass ■ Fermentation

2.1 Introduction

Since the beginning of twentieth century, technologies related to microbial pro­duction of biomolecules like enzymes, antibiotics, metabolites, and polymers have matured to a great extend. Currently, microbes are used for commercial production of a wide variety of products such as pesticides, fertilizers, and feed additives in agrochemical sector, biopharmaceuticals and therapeutics in the healthcare sector, biopolymers and biofuels in the energy and environment sectors. According to re­cent market reports, growing environmental concerns and increasing demands from end-use sectors are expected to increase the global market for microbial products to about 250 billion US dollars by 2016 [1].

E. T. Oner (H)

IBSB—Industrial Biotechnology and Systems Biology Research Group, Bioengineering Department, Marmara University,

Goztepe Campus, 34722 Istanbul, Turkey e-mail: ebru. toksoy@marmara. edu. tr

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_2, © Springer-Verlag Berlin Heidelberg 2013

Polysaccharides are natural, non-toxic, and biodegradable polymers that cover the surface of most cells and play important roles in various biological mechanisms such as immune response, adhesion, infection, and signal transduction [2, 3]. In­vestigations on the alternative treatments applied by different cultures throughout the history revealed the fact that the utilized plants and fungi were rich in bioac­tive polysaccharides with proven immunomodulatory activity and health promoting effects in the treatment of inflammatory diseases and cancer. Hence considerable research has been directed on elucidating the biological activity mechanism of these polysaccharides by structure-function analysis [4].

Besides the interest on their applications in the health and bionanotechnology sectors, polysaccharides are also used as thickeners, bioadhesives, stabilizers, pro­biotic, and gelling agents in food and cosmetic industries [5-7] and as emulsifier, biosorbent, and bioflocculant in the environmental sector [8].

Polysaccharides are either extracted from biomass resources like algae andhigher — order plants or recovered from the fermentation broth of bacterial or fungal cultures. For sustainable and economical production of bioactive polysaccharides at industrial scale, rather than plants and algae, microbial sources are preferred since they en­able fast and high yielding production processes under fully controlled fermentation conditions. Microbial production is achieved within days and weeks as opposed to plants where production takes 3-6 months and highly suffers from geographical or seasonal variations and ever increasing concerns about the sustainable use of agricul­tural lands. Moreover, production is not only independent of solar energy which is indispensible for production from microalgae but also suitable for utilizing different organic resources as fermentation substrates [5].

According to recent reports, the global hydrocolloid market dominated by algal and plant polysaccharides like starch, galactomannans, pectin, carrageenan, and al­ginate is expected to reach 3.9 billion US dollars by 2012 [9]. Superseding these traditionally used plant and algal gums by their microbial counterparts requires in­novative approaches and considerable progress has been made in discovering and developing new microbial extracellular polysaccharides (exopolysaccharides, EPSs) that possess novel industrial significance [6, 7]. A recent review pointed out to four EPSs, namely, xanthan, pullulan, curdlan, and levan, as biopolymers with outstand­ing potential for various industrial sectors [5]. However, when compared with the synthetic polymers, natural origin polymers still represent only a small fraction of the current polymer market, mostly due to their costly production processes. Therefore, much effort has been devoted to the development of cost-effective and environmen­tally friendly production processes such as investigating the potential use of cheaper fermentation substrates.

In this chapter, after a brief description of microbial polysaccharides, various microbial production processes utilizing cheap biomass resources as fermenta­tion substrates are discussed with a special focus on the employed pretreatment methods.