Polymers Obtained by Microbial Production

Polymers Obtained by Microbial Production 5.5.3.1 Polyhydroxyalkanoates (bacterial polyesters)... 

Some of the potential uses of the fats and oils found in plants have been reviewed and some uses of carbohydrate-based polymers briefly discussed. Plants contain a whole variety of other chemicals including amino acids, terpenes, flavonoids, alkaloids, etc. When the potential for these naturally occurring materials are combined with the secondary products that can be obtained by fermentation or other microbial processes or by traditional chemical transformations, the array of chemicals that can readily be created from renewable resources is huge. In this section a few of the more interesting examples are considered. 

In order to improve microbial production, serious attention was given to determine the effect of fermentation conditions on the production yield and on the polymers physicochemical properties. Generally, the yield and composition of the polysaccharide depend on the microbial species used, age of the producing microbial cells and growth, cultivation medium, and conditions. The chitinous compounds content also depends on the type of fermentation and extraction method. An increase in chitinous material can be obtained either by increased biomass yield or by an increase in the cell wall content of chitin/chitosan. A summary of the advantages and disadvantages of several biotechnological possibilities to produce chitin/chitosan can be found in Table 3.1. 

Photosynthetic products are synthesized by plants from water and carbon dioxide, and therefore renewable monomers obtained from plants are very attractive. Though natural polymers from plants are also very attractive for the same reason, monomers are preferentially focused on in this chapter due to limitations of space (monomers obtained by digestion of natural polymers are included here). Types of renewable monomers obtained from plants without any chemical, enzymatic, and microbial conversion are limited, since plant cells are mainly composed of polymers. The principal monomer compounds that can be directly extracted from plants are oils triglycerides of fatty acids and essential oils. 

There are three basic routes to produce polymers from renewable resources feedstock. Direct extraction yields polymer materials such as cellulose, starch, fibres, oils and proteins from which plastic materials can be developed. The second pathway is to convert raw materials first into biomonomers by hydrolysis, and then to polymers by chemical synthesis. A good example is PLA, the most commercialised so far. The third route is to obtain polymeric materials directly by microbial way from carbon sources through biosynthesis (fermentation). A typical example is the production of PHAs by bacteria. 

Though there are several reports on production of PHAs from cheap earbon substrates by wild type producers, the polymer concentration and the eontent obtained were relatively low than those obtained using purified carbon somces. Therefore there is a need for the development of more efficient fermentation strategies for production of PHA from cheap carbon sources using different microbial strains. 

Microbial polymers (e.g. poly(3)-hydroxybutyrate-hydroxyvalerate) are excreted or stored by micro organisms cultivated on starch hydrolysates or lipidic mediums. Isolation and purification costs could be high for those products that are obtained Ifom complex mixtures. Monsanto stopped the commercialisation of its product Biopol in 1999. Since then, production has been low but some new producers are entering the market (e.g. Coopeazucar in Brazil which has built new facilities for a pilot plant production of these polyhydroxyalkanoates). 

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