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Innovative Applications of Microbial EPS in Food
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Gellan Gum
Gellan gum is developed as the result of a systematic search for a polysaccharide of the required properties followed by the identification of the organism. It is obtained from cultures of Pseudomonas elodea, found growing on the elodea plant. It has a linear structure with a repeat unit of a tetrasaccharide each with one carboxyl group and in the native state one acetyl group. It is therefore sensitive to calcium levels but has rheological properties similar to those of xanthan that has a similar charge density. It was clearly intended to be a xanthan competitor, though before permission for food use was obtained it was promoted as an agar substitute, particularly for use in growth media.
Curdlan Gum
Curdlan is one of two polysaccharides produced by Alcaligines faecalis with a repeat unit of three β(1→3) linked D-glucose residues, and unlike the other microbial exopolysaccharides is not charged. Production is now from Agrobacterium mutants that make only curdlan in high yield, and is mostly confined to Japan. Curdlan suspensions gel on heating by auto-association, apparently irreversibly. After curdlan is added, the volume of the product is not reduced by cooking.
Bacterial Alginates
Although commercial alginates derive from algal sources, there is a large potential for producing 'tailor-made' alginates from bacterial sources, especially if advantages is taken of the genetic tools for controlling the production of the enzymes that are responsible for the synthesis and epimerization (conversion of D-mannuronic, 'M', to L-guluronic residues, 'G') of the polymeric alginate chain. There appears to be greater structural diversity (poly-M, poly-G, and poly MG residues) and our understanding of the genes and the enzyme gene produces is much greater for bacterial alginate production compared with the case for seaweed.
The main alginate-producing bacteria that have been studied are Pseudomonas aeruginosa and Azotobacter vinelandii. P. aeruginosa has been the subject of particular attention because of its association with respiratory disease and is found in patients suffering from cystic fibrosis. A. vinelandii appears to be the most promising in terms of industrial production because of its stable output of alginate. P. aeruginosa alginate has no poly-G residues (and hence has a low G-content) whereas A. vinelandii can, like seaweed alginate, possess all three block sequences (poly-M, poly-G, and poly-MG residues).
Dairy-Associated EPS
EPS produced by dairy-related bacteria influences the rheology of milk-based food products. Many ropy strains isolated from fermented dairy products had been tested for EPS production, including Lactobacillus, Lactococcus, and Streptococcus. Bifodobacterium longum was found capable of producing an EPS (molecular weight > 200 kDa) by utilizing lactose more efficiently than other carbon sources such as glucose or fructose. It has been reported that, when lactose was used as the primary carbon source, 0.466 g/L (by weight) or 0.168/L (in glucose equivalents) of EPS was produced.
Production of EPS is relatively new to bifidobacteria. Bifidobacteria are anaerobic, gram-positive rods that are one of the predominant groups in the gut of humans. The potential probiotic and therapeutic benefits of bifidobacteria to human health have been reported since the beginning of 20th century. Evidences have shown that EPS produced by dairy-related bacteria could influence the rheological properties of milk-based food products. Thus, their findings suggested that B. longum BB-79 has potential for directly modifying the texture of fermented dairy products if the efficiency of lactose utilization could be further increased and the rheological properties, e.g. shear-thinning or shear-thickening, of the EPS could be characterized.
Other Exopolysaccharides
Knowledge on both EPS-producing cyanobacterial strains and chemical characteristics of the cyanobacterial EPS is still relatively limited. On the whole, cyanobacterial RPSs are characterised by a great variety in both number (from 2 to 10) and type of constitutive monosaccharides (various arrangements of acidic and neutral sugars). Most polymers show an anionic nature due to the presence of uronic acids and/or other charged groups such as pyruvyl or sulfate. Polypeptide moieties as well as acetyl substituents have also sometimes been found, causing additional structural complexity.
All the cyanobacterial EPS so far tested showed a pseudoplastic behavior, but with marked differences in both viscosity values and shear thinning. In terms of EPS production, the responses of cyanobacteria to changes of culture conditions appear strain-dependent. EPS productivities shown by some cyanobacteria are well comparable with those reported for other photosynthetic microorganisms proposed for polysaccharide production, but very low in comparison with those of heterotrophic microorganisms. Nevertheless, cyanobacteria may be regarded as a very abundant source of structurally diverse polysaccharides, some of which may possess unique properties for special applications, not fulfilled by the polymers currently available.
However, much work has still to be done to bridge the wide gap existing between data on the biology of the EPS-producing strains and information concerning technological and other useful properties of the EPS. Volcaniella eurihalina, a moderately halophilic eubacterium, produces an extracellular polysaccharide whose physical and chemical properties could also be of interest for various industrial applications.
Innovative applications of EPS
The development of nutraceutical products has offered another great opportunity for the food industry. The most critical issue in nutraceutical products is the retention of effectiveness in addition to explicit control of the active ingredients in the product. For instance, xanthan has recently been added to the list of hydrophilic matrix carriers, along with chitosan, cellulose ethers, modified starches, and scleroglucan. Tablets containing 5% xanthan gum were shown under low shear conditions to give successful controlled release of acetaminophen into stomach fluid, and tablets containing 20% xanthan successfully carried a high loading (50%) of the drug theophylline.
The high suspension stability is made use of in pharmaceutical cream formulations and in barium sulphate preparations. This high cream stability is taken advantage of also by the cosmetic industry, including toothpaste technology where the toothpaste will hold its ingredients (high viscosity) and then easily brush onto and off the teeth (high shear thinning). Uniform pigment dispersal, along with other ingredients and long-term stability, make xanthan a good base for shampoos.
There are a number of potential applications for curdlan in the food industry, mostly as a replacer for existing gums, and it has be used in Japan in some products. Novel products based on its heat-set ability may also appear. The insolubility of curdlan in water makes it an ideal candidate to serve as a carrying agent for other organic materials (e.g. orange oil or other flavor compound) that are not soluble in water. The ability of curdlan to form either reversible or irreversible gels by controlling the processing temperature and the salt concentration in solution is also of value for the development of unique controlled release mechanisms.
The fibrous sheets formed within the curdlan gel could also be taken advantage of when the formation of a multi-layer structure is desired. This unique property might have application in nutraceutical products, especially when higher concentrations of active ingredients are needed, since it provides a protective yet ready-to-release 'sandwich' structure that might also have special effects on product texture.
It is of significant advantage for the EPS approved by FDA as food grade because it could then be used not only as a food ingredient but also in packaging material in contact with food and in pharmaceutical and biomedical applications that involve ingestion. For example, in other industries xanthan are used in a number of aspects of well-bore technology such as:
- A proppant into fissures for rock fraction,
- The suspension and removal of debris from the bore area even in the presence of harsh environments (seawater included) and pipelines, and
- Enhancing oil recovery using the technique of 'polymer flooding'. The high stability of suspensions makes xanthan an ideal base for suspending adhesive agents for wallpapers.
Like alginates, the suspension stabilizing property of xanthan makes it ideal for producing sharp prints from dyes with a minimum risk of running, for application to textiles and (in conjunction with guar) carpets. Specifically, by controlling the solubility of EPS in hot and cold water, the high viscosity of EPS suspensions could be interwoven with its pourability to be incorporated into the development of edible packages.
Y. Martin Lo, Ph.D., is an Associate Professor of Food Bioprocess Engineering and an Extension Specialist of Food Processing and Safety in the Department of Nutrition & Food Science, University of Maryland. For article feedback, contact Martin at ymlo@umd.edu
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