Page 78 - Biodegradable Polyesters

56 3 Microbial Synthesis of Biodegradable Polyesters: Processes, Products, Applications

3.7
Industrial Production of Bacterial Polyhydroxyalkanoates: PHAs via Fermentation

From the 1980s onward, there have been many companies trying to produce var-
ious PHAs on pilot or industrial scales based on the assumption that petroleum
prices would increase because of exhaustion and people will be open to use
environmentally friendly nonpetrochemical-based plastics, termed biodegrad-
able plastics, green plastics, bioplastics,or ecoplastics. Scientific breakthroughs
allowed the successful large-scale production of PHB by Chemie Linz AG Aus-
tria, copolymer PHBV (poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate) of
(R)-3HB, and (R)-3-hydroxyvalerate (3HV) by ICI, United Kingdom, and TianAn,
China, and copolymer PHBHHx of (R)-3HB and (R)-3-hydroxyhexanoate (3HHx)
by the joint efforts of Tsinghua University, KAIST, and P&G. Several applications
have been developed on the basis of availability of the above PHAs. Thanks to
the new molecular biology technology, more and more of these industrial break-
throughs have become possible. In 2009, Metabolix (USA) and Tianjin Green
Bioscience (China) established the capacity to produce 50 000 and 10 000 tons
per year of PHA, respectively. At this point, global polymer companies should
have enough PHA materials to research with. This should hopefully mean that
a new wave of PHA development with a focus on new applications is on its
way.
In the past years PHAs, as polymeric materials, have been considered for the
development into applications such as e.g. medical implants, drug delivery car-
riers, printing and photographic materials, nutritional supplements, drugs, and
fine chemicals. Recently, PHAs have been found useful as a potential new type of
biofuel. In addition, PHA-related proteins and genes have been used to regulate
metabolisms and to enhance the robustness of industrial microorganisms, even
for specific drug targeting and protein purifications. The applications of PHAs are
rapidly expanding.
As of now, poly-(R)-3HB, poly((R)-3-hydroxybutyrate-co-4-hydroxybutyrate)
(P3HB4HB), and PHBV are produced in a large scale.
PHA production uses strain development, shake flask optimization, lab and
pilot fermentor studies, and industrial scale up (Figure 3.6). Effective microbial
production of PHAs is dependent on a variety of factors, which include the final
cell density, bacterial growth rate, percentage of PHA in cell dry weight, time taken
to reach high final cell density, substrate to product transformation efficiency,
price of substrates and a convenient and cheap method to extract and purify the
PHAs (Figure 3.6).
A range of factors need to be considered in the different stages of development.
Wild-type and recombinant bacteria were used for large-scale production of var-
ious PHAs. For large-scale application, PHA production costs should be as low
as possible. Thus energy saving microaerobic processes and increasingly, the use
of wastewater or activated sludges for PHA production requires attention. This
needs the development of industrial strains or mixed cultures that are capable