Extraction of Polyhydroxyalkanoates

Supercritical fluid extraction is another method used in PHA recovery. Supercritical carbon dioxide is widely used, due to its high solubility with other compounds. It is safe (low toxicity and reactivity) and convenient to use. Furthermore, supercritical fluids possess the unique properties of high densities and low viscosities, making them a suitable choice of solvent for extraction. The efficiency of this extraction method is highly dependent on the optimisation of the operating parameters, such as temperature, pressure, modifier as well as the exposure times [8]. 

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Noda (1999) Films and absorbent articles comprising a biodegradable polyhydroxyatkanoate comprising 3-hydroxybutyrate and 3-hydroxyhexanoate comonomer units. US Patent 5,990,271 Noda I (2005) Plastic articles digestible by hot alkaline treatment. US Patent 6,872,802 B2 Noda I, Schechtman LA (1999) Solvent extraction of polyhydroxyalkanoates from biomass. US Patent 5,942,597... 

K. Narasimhan, A. C. Cearley, M. S. Gibson and S. J. Welling, Process for the solvent-based extraction of polyhydroxyalkanoates from biomass. United States Patent, 2008, US 7378266. 

Polyhydroxyalkanoates (PHA) are polymers synthesised by bacteria as intracellular carbon and energy sources. PHA are industrially produced by pure cultures psing as main substrates glucose and propionic acid. The major expenses in the PHA production are determined by the cost of substrate and extraction of polymer from inside the cells. ... 

Figure 2.9 Purification strategy for the polyhydroxyalkanoates. The recovery of polyhydroxyalkanoates is composed of three steps pre-treatment, extraction and purification.

An additional advantage of the polyhydroxyalkanoates is that the polymers can be produced by fermentation. Certain types of bacteria produce PHAs for energy storage when they are grown in glucose solution in the absence of specific nutrients. The polymer forms as discrete granules within the bacterial cell, and it is then removed by extraction to give a white powder that can be melted and modified into a variety of different products. 

Polymers derived from renewable resources (biopolymers) are broadly classified according to the method of production (1) Polymers directly extracted/ removed from natural materials (mainly plants) (e.g. polysaccharides such as starch and cellulose and proteins such as casein and wheat gluten), (2) polymers produced by "classical" chemical synthesis from renewable bio-derived monomers [e.g. poly(lactic acid), poly(glycolic acid) and their biopolyesters polymerized from lactic/glycolic acid monomers, which are produced by fermentation of carbohydrate feedstock] and (3) polymers produced by microorganisms or genetically transformed bacteria [e.g. the polyhydroxyalkanoates, mainly poly(hydroxybutyrates) and copolymers of hydroxybutyrate (HB) and hydroxyvalerate (HV)] [4]. 

I. Noda, Process for recovering polyhydroxyalkanoates using air classification, solvent extraction ofpolyhydroxyalkanoates from biomass facilitated by the use of marginal nonsolvent, United States Patent, 1998, US 5849854. 

Polyhydroxyalkanoates (PHAs) are homo- or heteropolyesters synthesized and intracellularly stored by numerous prokaryotes. They can be produced in large quantities from renewable resources by means of well known fermentation processes and the imposition of particular culture conditions, and a number of physical or chemical methods are known to extract them from the producing biomass. Production processes such as batch, semi-batch and continuous fermentation are all known to work. PHAs have properties similar to those of some polyolefins. This, combined with the fact that they are fully and rapidly biodegraded under the appropriate conditions, has generated a high interest in them as substitutes to petroleum-based polymers in many applications [1]. 

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