Influence of the growth conditions on the properties of bacterial cellulose produced in a rotating-bioreactor.
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The aim of this thesis was to evaluate the impact of the growth conditions on the physiomechanical properties of bacterial cellulose (BC) produced in a rotating biological contactor, referred here as the rotating-bioreactor. This fermentor was selected because it facilitated the manipulation of the growth conditions during BC production. BC was also produced using the alternative method known as static-culture. This method was used to establish a baseline for the strain of bacteria used. A morphological investigation of the BC produced in both static-culture and rotating-bioreactor revealed both the macroscopic and microscopic properties of the BC structure, produced by both the methods, were different. BC produced in the static-culure was made up of layers, wider cellulose ribbons and greater extent of scarring (melted BC) as compared to BC produced in the rotatingbioreactor. Their crystallinity index too was different at 89% and 83% for BC produced in static-culture and rotating-bioreactor, respectively as determined by X-ray diffraction. The initial glucose concentration (2-100 g/L) was varied in both the static-culture and the rotating-bioreactor. Although no morphological changes were observed in the BC produced at varying initial glucose concentrations the yield of BC, was influenced by the same. It was found that the increase in glucose concentration (8-77 g/L) led to an increase in acetic acid production (19 to 255 mM). The gluconic acid production too increased from 56 to 209 mM with an increase in glucose concentration (8 to 46 g/L). The enhanced acid production impacted the yield especially at higher glucose concentrations. No BC was produced in both static-culture and rotating-bioreactor at initial glucose concentration of 100 g/L. The growth conditions in the medium were altered by varying, the pH (3.0-6.0) at the innoculum stage in the static-culture. Due to the inadequacy of the experimental set-up, the pH was allowed to fall naturally after adjusting the same during inoculation. There was no influence of varying pH on the morphological structure, bioprocessing efficiency (yield and rate of production) and the properties of BC produced in static-clture. Similar studies were performed using the rotating-bioreactor. Since the rotating-bioreactor had better set up for controlling the pH, the BC produced at different pH values (3.0-6.0) was controlled for the entire duration of the fermentation. It was found that increase in pH (3.0-6.0) reduced the compactness of the BC network when produced in the rotatingbioreactor. The most interesting observations were made in the BC produced in the rotating-bioreactor when the pH was not controlled. The BC was made up of macrolayers (macroscopically visible layers, also observed in wet BC) that were not observed when BC was produced at any controlled pH. The maximum yield of 0.66 g BC/g glu was also obtained when the pH was not controlled. Another noteworthy observation was the change in the orientation of the micro-layers (layers observed in freeze dried and dry BC under SEM) of the BC produced in the rotating-bioreactor with a slight variation in pH from 3.0 to 4.0 and vice versa during the BC production. The tangential velocity of the cylinders was varied by varying the RPM (2-22 RPM) to test the impact on the yield and rate of production of the BC. It was found that the rate of production increased from 1.75 to 4.0 g BC /m2·day with the increase in tangential velocity (0.013 to 0.16 m/s). Additionally, there was an increase in the average water holding capacity (WHC) from 92 to 176 g water/g BC and a marginal reduction in the mechanical strength (0.03 to 0.023 MPa) of the BC produced with the increase in tangential velocity from 0.013 to 0.16 m/s. Testing the mechanical strength of the BC in its wet form was very challenging. Different instruments and methodologies were tried without much success. Finally, a testing devise was custom built to test the wet samples using compression under submerged conditions. The BC produced in the rotating-bioreactor without pH control had the highest mechanical strength (compared to BC produced at different pH and tangential velocity) with modulus of elasticity (MOE) ≈ 0.08 MPa. Although, this was very less in comparison to the average MOE ≈ 0.46 MPa of BC produced in the static-culture under similar conditions. Different miscible polymers/chemicals (gelatin, chondroitin sulphate and chitosan) were tested for their ability to associate with BC when dissolved in the medium to produce a modified BC (composite). This was first tested in the static-reactor in order to determine the right concentration of the additives. A novel composite was produced using chondroitin sulphate. Its properties and bioprocessing efficiency was also determined. Additionally, the rewetting potential of both wet and dry BC produced in the rotatingbioreactor was also determined.