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Innovative Applications of Microbial EPS in Food
By Y. Martin Lo, Associate Professor, University of Maryland
Classified as hydrocolloids, microbial exopolysaccharides (EPS) have enjoyed their traditional roles in the food industry as thickening, gelling and suspending agents for many years mainly due to their high dispersibility and viscosity in water solutions. Xanthan, approved by the USA Food and Drug Administration (FDA) since 1965 as food grade, is the most widely used EPS as a thickening agent in food products because of its stability over a wide range of temperature. Besides high viscosity, thickening and suspending ability, xanthan suspensions have high acid stability. This makes them highly popular in sauces, syrups, toppings and salad dressings.
In drinks, the addition of xanthan together with carboxymethylcellulose adds 'body' to the liquid and assists with uniform distribution of fruit pulp etc. It is also used to add body to dairy products. The high freeze-thaw stability of xanthan suspensions makes them particularly attractive for the frozen food industry. The high suspending and stability properties are also taken advantage of by the animal feed industry for transporting liquid feeds with added vitamins and other supplements that would otherwise sediment out with transport or storage time. With a unique shear-thinning property, xanthan also found its application as an excellent flow-controlling agent not only in the food industry but also in the chemical, pharmaceutical and petroleum industries.
Other EPS', e.g. dextrans, gellan and curdlan, also received considerable attentions from the food industry primarily because of their ability to contribute to the texture of food. Increasing interest has also been given to dairy-associated acidifying bacteria that are capable of producing EPS by direct fermentation of lactose within dairy-based products. Some of the bacteria have been found to be candidates in providing probiotic benefits to human being, indicating a great potential of extending their applications to non-dairy products. In addition, many cyanobacteria are known to be able to synthesize outermost slimy investments and to release polysaccharidic material into the culture medium during cell growth.
Cyanobacteria are photoautotrophic prokaryotes that include a large variety of species of widespread occurrence and with diverse morphological, physiological and biochemical properties. These released EPS, being easily recoverable from the culture medium, are attracting much interest in view of their possible uses in several industrial applications. Therefore, it is the objective of this article to provide an overview on the development of commercially important EPS with emphasis on their potential of offering innovative applications to the food industry beyond their traditional roles.
What is Microbial Exopolysaccharide?
Microbial Exopolysaccharides (EPS), different in function from those of higher plants, are the polysaccharides secreted from bacteria to form a layer over the surface of the organism. To distinguish them from any polysaccharides that might be found within the cell, they are characterized as exo-polysaccharides. In nature, their functions are thought to be mainly protective, either as a general physical barrier preventing dehydration and access of harmful substances, or as a way of binding and neutralizing bacteriophage. Under appropriate environments they may also prevent phagocytosis by other microorganisms or the cells of the immune system. The capsular polysaccharides are often highly immunogenic, and may have evolved their unusual diversity as a way of avoiding antibody responses.
The secreted EPS also have a role in adhesion and penetration of the host, hence might be involved in pathogenicity. Pseudomonas aeruginosa, commonly found in respiratory tract infections, produces alginate that contributes to blockage in the respiratory tract and leads to further infection. On the other hand, for example, the very high viscosity of xanthan at low concentrations (e.g. at 5 mg ml-1 a viscosity of ~1000 cP has been observed at room temperature) makes it ideal as a thickening and suspending agent. This valuable glycan with a high molecular mass is now commercially produced by the aerobic fermentation of Xanthomonas campestris as a culture on glucose. The molecular weight average of around 6 million Daltons for xanthan gum means that it is one of the largest of the aqueous soluble polysaccharides, making it possible to create extremely viscous solutions.
Development of EPS Research
The advancement of biotechnology has explored many useful tools in understanding the metabolic pathways associated with EPS production. Examples in literature have shown that, by manipulating their pathways it is now possible to perform specific controls over the charge and/or functional groups on the EPS side chains, which are crucial to the functionality and stability of EPS in food systems. Equally important is the advancement of techniques available in measuring the conformational changes of EPS under different environmental conditions. It is recognized that the conformation of EPS in a solution will greatly affect its rheological properties.
Xanthan
The structure of xanthan has now been identified. The principal chain of xanthan is made up of D-glucose units bound in β(1→4) and carrying side chains on every other residue on the C3, consisting of a triholoside formed by a β-D-glucuronic acid surrounded by D-mannose units. On approximately half of the terminal mannose units a pyruvic acid is bound to the C4 and C6; non-terminal mannose carries an acetyl group on the C6. It is thus a charged polymer. Some of the mannose residues may also carry acetyl groups. It is useful because it forms relatively rigid-rod-like structures in solution at ambient temperatures, though they convert to the random configuration on heating.
These rods are able to align themselves - like agarose and the carrageenans - with the unsubstituted regions of galactomannans, such as guar and its derivatives and locust bean gum, to produce fairly rigid mixed gels with applications in food manufacture. By itself xanthan, however, only forms transient weak gels since the junction zones are weaker than in those used for networking in carrageenan and agarose. There has been considerable interest in improving the weak gelation characteristics of xanthan by inclusion of galactomannans such as locust bean gum into mixtures. The use of synergistic interactions between galactomannans with xanthan give stronger gels with an optimum mixing ratio ~50:50 by weight. It has been shown that deacetylating the xanthan side chains seems to enhance these synergistic interactions.
Dextrans
Bacterial dextrans are produced in substantial quantities by Leuconostoc mesenteroides and are familiar to laboratory workers as the basis for cross-linked dextran beads used in gel filtration columns. The product with an average molecular weight of about 60,000 Da is used in medicine as a blood extender, while fractions of defined molecular weights (e.g. the Pharmacia 'T-' series, where 'T500 Dextran' would stand for dextran of weight average molecular weight 500 kDa) are familiar in laboratories and to some extent serve as polysaccharide standards in molecular weight calibrations. They are also used as part of incompatible phase separation systems, usually with polyethylene glycol.
'Blue dextran' is a well-known marker for the void volume for gel filtration studies. Dextrans have found very wide application in laboratory work because they are particularly free from positive interactions with proteins. The interaction can be almost entirely characterized as coexclusion. This has found application in gel filtration media, but cross-linked dextran gels show other effects. For example, they swell and shrink in a way related to the osmotic pressure of the solvent system, and can be used to make miniature osmometers.
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