Enhanced production of microbial extracellular polysaccharides and materials property analysis.

摘要

The purpose of this research is to improve the rate of cellulose and pullulan production and their material properties by exploring new culture methods including a new biofilm reactor design and the addition of different additives.;First, a novel biofilm bioreactor configuration was implemented, including a solid substrate (plastic composite support, PCS) to promote effective nutrient delivery for biofilm development. PCS degrades slowly with time offering a distinct advantage: disruption of biofilm formation preventing thick film formation, which results in an oxygen and substrate diffusion barrier. Second, several additives were used to make EPS composite and the mechanical properties of this new EPS composite were evaluated.;Enhancement of bacterial cellulose (BC) production by Acetobacter xylinum was explored through the addition of different additives into the fermentation medium in agitated culture including agar, carboxymethylcellulose (CMC), microcrystal line cellulose, and sodium alginate. Among the evaluated additives, CMC yielded highest BC production (8.2 g/L) compared to the control (1.3 g/L). The results also indicated that CMC-altered BC production increased with CMC addition and reached saturation around 1%. The variation between replicates for all analysis was less than 5%. From XRD analysis, however, the crystallinity and crystal size decreased as CMC addition increased. FESEM results showed CMC-altered BC produced from agitated culture retained its interweaving property. TGA results demonstrated that CMC-altered BC had about 98% water retention ability, which is higher than BC pellicle produced with static culture. CMC-altered BC also exhibited higher T max compared to control. Finally, DMA results showed that BC from agitated culture loses its tensile strength in both stress at break and Young's modulus when compared to BC pellicle since the former were in pellet form.;As for BC production in biofilm reactors, the type SFYR+ PCS was selected as solid support for BC production by A. xylinum in a batch biofilm reactor due to its high nitrogen content, moderate nitrogen leaching rate, and sufficient biomass attached on PCS. The PCS biofilm reactor yielded BC production (7.05 g/L) that was 2.5-fold greater than the control (2.82 g/L). The XRD results indicated that the PCS-grown BC exhibited higher crystallinity (93%) and similar crystal size (5.2 nm) to the control. FESEM results showed the attachment of A. xylinum on PCS, producing an interweaving BC product. TGA results demonstrated that PCS-grown BC had about 95% water retention ability, which was lower than BC produced within suspended-cell reactor. PCS-grown BC also exhibited higher Tmax compared to the control. Finally, DMA results showed that BC from the PCS biofilm reactor increased its mechanical property values, i.e., stress at break and Young's modulus when compared to the control BC.;Using the optimal medium, a biofilm reactor with plastic composite support (PCS) was then evaluated for pullulan production using A. pullulans. In test tube fermentations, PCS with soybean hulls, defatted soybean flour, yeast extract, dried bovine red blood cells, and mineral salts was selected for biofilm reactor fermentation (due to its high nitrogen content, moderate nitrogen leaching rate, and high biomass attachment). Three pH profiles were later applied to evaluate their effects on pullulan production in a PCS biofilm reactor. The results demonstrated that when a constant pH at 5.0 was applied, the time course of pullulan production was advanced and the concentration of pullulan reached 32.9 g/L after 7-day cultivation, which is 1.8-fold higher than its respective suspension culture.;In conclusion, these studies clearly demonstrated that the biofilm reactor can be employed to enhance microbial extracellular polysaccharide production. BC production and its material property are highly affected by additives during fermentation. BC produced from PCS biofilm reactor exhibited tensile strength comparable to that produced in pellicle form. For pullulan production, optimal cultivation parameters and solid support for biofilm formation were determined. The produced pullulan maintained its high purity around 95%. The mathematical models proposed in this study can pave the way for further studies, for example, an online recovery of BC and/or pullulan from existing biofilm reactor. (Abstract shortened by UMI.)