| Summary: | 碩士 === 大葉大學 === 食品工程研究所 === 90 === PHAs are thermoplastic and biodegradable polymers that are synthesized with hydroxyalkanoates (HA) monomers, and can be produced by various bacteria in some specific conditions. Within these PHAs, PHB (polyhydroxybutyrate) is most widely studied.
In this study, the influence of media with different limiting substrates (nitrogen or phosphorus) on the microbial growth of Ralstonia eutropha ATCC 17699 and the production of PHB in a batch fermenter at various temperatures (26, 30 and 35℃) is explored. Carbon source and limiting substrate (nitrogen or phosphorus) consumption rates are also examined. When limiting substrates depleted, bacteria can get into the pathway of PHB synthesis. The yields of PHB in bacteria intracellular for different limiting substrates are compared. The culture condition having a highest maximum specific growth rate (μm) has been selected for later study in a semi- continuous fermentation. A flow rate of 50 mL/h of medium with extra sodium propionate was fed into the fermenter in order to prolong the PHBV accumulating period.
Regardless of limiting nitrogen or phosphorus medium, higher temperatures have significantly shortened the generation time of R. eutropha. When cultivated at 35℃, μm was 0.2476 h-1, td was 2.80 h. Similarly, μm (0.2769 h-1) and td (2.50 h) was also the highest at 35℃ when cultivated in a limiting phosphorus medium.
In a limiting nitrogen medium for a batch culture at 35℃, PHB has reached its highest accumulation, the PHB/biomass ratio being 75.6%, at 64 h. For a batch culture in a limiting phosphorus medium at 35℃, PHB has reached its highest accumulation, the PHB/biomass ratio being 43.1%, at 26 h.
The yield of PHB for the limiting nitrogen medium, at 30℃ YP/Glucose = 0.41 g PHB/g glucose. For the limiting phosphorus medium at 35℃, YP/Glucose = 0.19 g PHB/g glucose.
Using a flow rate of 50 mL/h to feed the medium with no phosphorus into a semi-continuous fermenter, because of lacking phosphorus, the microbial growth rate tends to decline. The HV/PHBV ratio was about 11.3% (w/w) and the HB/PHBV ratio 88.7% (w/w) for the entire process. If phosphorus was added in the medium, microbial growth can be significantly prolonged. However, the HV/PHBV ratio was only 6.4% (HB/PHBV ratio being 93.6%), and these ratios may be affected by the concentrations of glucose and sodium propionate in the medium.
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