engleski

Production of Plastics from Waste Derived from Agrofood Industry.

The literature contains a number of articles about microbially mediated production of polyhydroxyalkanoates (PHAs). These polyesters constitute biodegradable polymers possessing physical properties of thermoplastics or elastomers. Therefore, they can be applied as alternatives to common plastic materials originating from mineral oils. The most common representative of these biopolymers is poly-3-hydroxybutyrate (PHB). PHB constitutes a rather stiff and brittle material with a low extension to break. These mechanical properties can be enormously enhanced by incorporation of different co-monomers into the PHB matrix. This results in co- and terpolyesters possessing mechanical properties similar to high-tech materials. PHAs are synthesized in the cytoplasm of various prokaryotic strains, usually from carbohydrates, but also from other renewable resources. Under unfavorable growth conditions (surplus of carbon source plus limitation of an essential substrate, i.e. nitrogen, phosphate, or oxygen), some strains are able to divert the usual carbon flux (acetyl-CoA synthesized in the central metabolic pathways of the microorganism) from biosynthesis of protein (biomass) constituents to the formation of compounds acting as precursors for the production of PHA, mainly PHB. This typical feature is valid for most PHA producers, e.g., Wautersia eutropha, the production strain of BIOPOL^® (Metabolix Inc., Cambridge, MA) that was commercialized in the 1990s by Zeneca Bio Products and later by Monsanto. Some organisms exist that accumulate PHA during balanced growth at substantial rates. This was first found for strains of Alcaligenes latus. The outstanding properties of these growth-associated PHA producers enabled the development of an industrial polyester production process by the Austrian company Chemie Linz in the late 1980s. Due to a high degree of polymerization, the average molecular weights of PHAs can reach several millions. These polymers are organized in granules and serve as intracellular reserve carbon and energy sources that normally are degraded when external carbon sources are depleted. Because of their outstanding property of complete degradability to water and CO_2, PHAs are embedded in nature’ s closed cycle of carbon. If PHAs are applied instead of fossil-oil-originated polymers, the mass balance of carbon from biomass will be closed, but its durability will be prolonged if compared with the usual biological life cycle of PHA carbon. The fossil fuels energy demand during the span of life of PHAs will not exceed the amount necessary for the industrial production and processing of PHAs itself. The negative effects of CO_2 accumulation in the atmosphere, i.e., the greenhouse effect, will be related merely to the amount originated from energy production and not to the carbon in the biopolymer itself. Therefore, substituting common plastic materials by PHAs significantly lowers the amount of fossil-oil-originated carbon in the cycle of nature. Hence, it can be concluded that there are ecologically sound reasons for using PHAs instead of mineral-oil-originated plastics.

polyhydroxyalkanoates, poly-3-hydroxybutyrate, production, whey lactose, glycerol (waste of biodiesel production), waste lipids, hydrolyzed meat and bone meal

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