Effect of freeze-drying behavior on the density and structure of bacterial cellulosic films by different acidic and alkaline treatments

Significant research has been conducted on cellulose producing bacteria, including the 'xylinum' family of aerobic bacteria, as an alternative source of high quality purified cellulose fiber for applications including paper, filtration, textiles, food, wound care, tissue engineering and specialty high performance nanomaterials. Microbial cellulose is often produced in static cultures where the culture and cellulose is formed at the air-liquid interface where both oxygen and nutrients are present. In this case, the thickness of cellulose films increases gradually with cultivation time and may attain a thickness of 4 cm in a fermenter with vertical sidewalls. The density of the cellulose film produced depends on the fermentation conditions including oxygen and nutrient concentrations in the vicinity of the forming cellulose film. Density of the produced cellulose film is an important parameter for envisioned applications as it relates to porosity and the mechanical properties of the film produced. In this work, we demonstrate that bacterial cellulose films treated by different solutions exhibit significant changes in density after the process of freeze-drying, a common material preservation process in the food industry. The density of cellulose films initially treated by sodium hydroxide and sulfuric acid increased dramatically up to 350% after freeze drying and exhibited a significantly different nanostructure as evaluated using scanning electron microscopy (SEM). We hypothesize that this phenomenon is related to a shift in the freezing point of the solutions resulting in reduced sublimation and subsequent fiber collapse during drying. This process may open the possibility of producing cellulose nanomaterials with engineered properties such as density, porosity and strength.