Production of Bacterial Cellulose with Enhanced Mechanical Properties Using Pullulan Additive and Co-Culturing Methods
Open Access
- Author:
- Hu, Hetian
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 18, 2020
- Committee Members:
- Jeffrey M Catchmark, Thesis Advisor/Co-Advisor
Ali Demirci, Thesis Advisor/Co-Advisor
Edward G Dudley, Committee Member
Paul Heinz Heinemann, Program Head/Chair - Keywords:
- Bacterial Cellulose
Co-culture Fermentation
Bionanocomposite
Exopolysaccharides
Aureobasidium pullulans
Gluconacetobacter hansenii
Pullulan
Ribbon Assembly
Crystallinity
Crystal Size
Young's Modulus
Tensile Strength - Abstract:
- Bacterial cellulose (BC), due to its high porosity, high tensile strength, biocompatibility, and crystal structure, is a value-added product that can be used in many different applications including biomedical, pharmaceutical, cosmetic, fiber composite, and filtration. Studies had been conducted aimed at enhancing the BC production and mechanical properties, and polysaccharide addition is one of the most effective ways to increase BC production. Agitated fermentation, compared to conventional static cultivation method, is also reported to improve the BC production, but there is a lack of research regarding combining agitated fermentation and polysaccharide additive. Moreover, it might not be practical to use pure polysaccharides directly in industrial scales due to high cost. Therefore, this research is undertaken to study the effect of adding pullulan, an α (1-6) linked maltotriose polymer produced by the fungus Aureobasidium pullulans on enhancing the production and mechanical behaviors of BC cultivated by agitated fermentation. The research also studied an agitated co-culturing fermentation system in which Gluconacetobacter hansenii, the BC-producing microorganism, grows together with A. pullulans. The research included three phases. Phase 1 of the study included the addition of different concentrations of purified pullulan produced by A. pullulans directly into the fermentation broth while cultivating G. hansenii (Chapter 4). The BC nanocomposites produced in Phase 1 was evaluated based on the production, crystallinity, crystal size, d-spacing, Young’s modulus, stress at the point of break, and elongation capability. The result had shown that pure pullulan additive could enhance the bundling behavior of BC and disrupt co-crystallization at different concentrations, altering both the production and the mechanical properties of BC. The production of BC increased significantly at a relatively high pullulan concentration (> 15 g/L). However, the mechanical behaviors were compromised significantly as the pullulan concentration level increased. On the other hand, at a low concentration of pullulan addition, the mechanical properties of BC nanocomposite increased, but no change in the BC production was observed. The result of Phase 1 had led to the second phase, co-culture fermentation, since co-culturing could lower the BC production cost compared to the addition of pure pullulan. For Phase 2, co-culture fermentations were conducted in shake-flasks, and the BC nanocomposite produced was also analyzed based on the same parameters in Phase 1 (Chapter 4). The co-culture fermentation had produced BC with improved mechanical properties, but no significant changes in the production of BC had been observed. Phase 3 was, therefore, related to improving the production of BC. For phase 3, the co-culture fermentation was optimized using response surface methodology (RSM), in which the BC production was modeled in response to three nutrients in the medium that could alter the BC production significantly: glucose, yeast extract, and peptone. The RSM model had generated combination of the three nutrients to determine the optimum BC production medium. In Phase 3, the optimum condition was validated both in shake-flasks (Chapter 4) and in benchtop bioreactors (Chapter 5). The results had shown an increase in BC production for one of the optimized media in shake-flasks (from 0.249 to 0.3048 g/L), but BC production in the bioreactor was reduced significantly (0.12-0.18 g/L) for co-culture fermentation. The enhanced mechanical behaviors of BC were maintained for the optimum conditions in the shake-flasks and the bioreactors. In conclusion, this research had shown that pullulan additive could improve both the BC production and the BC mechanical properties by varying co-crystallization of BC protofibrils and the bundling behavior of BC microfibrils. The result of this research also showed that co-culture fermentation could be an effective way to produce more BC pellicles with better mechanical properties by lowering the production costs.