Microbial Synthesis of Polymers: Alcaligenes Eutrophus and the Production of Poly - B - 3 - hydroxybutyric Acid (PHB)
PHB formation in bacteria is a means to store reserves of energy and carbon. The location and discovery of PHB has been known since 1926. In the last 30 years, much attention has been given to it, due to the polymer's characteristics. It is a thermoplastic polyester, the first truly biodegradable plastic. The formation of PHB can be brought about by nutrient limitation, as the polymer is essentially a non-growth associated product in most bacteria (another species, latus, can produce it to excess of 60 % dry weight, in exponential cells). Typically, nitrogen, phosphate and oxygen limitation are used to initiate storage. This research set out to find a suitable production system. This first required a good growth medium. The maximum specfic growth rates of the research strain, H/16 S3O1/C5, and production strain, H/16 S301/TRON, were 0.73 and 0.61h~ 1 . This is believed to be faster than previously recorded rates. The scale-up from shake-flask to batch reactor was problematical; a 40-60% reduction in growth rate being typical. This was characterised as being due to Fe2+ limitation, due to precipitation with pH control. During fed-batch experiments, of up to 50 hours, a maximum cell density of 12.4g/l was reached, and with up to 64% PHB formation. Continuous culture was not suitable for industrial production of PHB; biomass and PHB productivities were low, and at low dilution rates. However, it was suitable for producing PHB-free cells, to feed to a larger fed-batch production vessel. Two-stage batch and fed-batch production gave 15.65g/1 cells, and up to 70% PHB production. Industrially, with an ICI CASE award session, fed-batch led to the production of 51g/l cells, with 80% copolymer. In a production run, a 50M3 vessel was operated, to produce 4 tonnes of polymer in 72 hours. Copolymer production is promoted by feeding organic acids. This leads to better polymer properties. One of the five experiments was run for 48 hours, reducing the running time by 24 hours. Enzyme determination of copolymer content was validated, and the current polymer extraction procedure was analysed, and found to be very satisfactory. An industrial model was formulated, using a system where a chemostat would feed several large fed-batch production vessels. This gave a very good production, with an optimum figure of 2000-2250 tonnes p.a. If 316L stainless had been correctly stipulated in the model, then the optimum price for this level of production would be £11-12,000/tonne.
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