Bacterial cellulose is primarily produced using various sugars as feedstock conven­tionally using static culture and also by using agitated culture. Typical production rates of bacterial cellulose that have been reported are 5-8 g/l using fructose and corn syrup and 7-9.2 g/l using stirred tank bioreactors. Instead of using single sugars as feedstocks, Dahman et al. investigated the ability to produce bacterial cellulose from multiple sugars. Sugar compositions similar to those found in acid hydrolysates of some agricultural residues were utilized with Gluconacetobacter xylinus as the bacteria [10Dah]. Under identical conditions, the bacterial production from single sugars varied from 1.1 to 5.7 g/l compared to 2.4 to 5.2 g/l for the mixed sugars. Table 61.2 lists the type of sugar feedstock used and the yield of bacterial cellulose obtained. It was concluded that agricultural residues could be potential feedstocks for biocellulose nanofiber production.

In a similar approach, elephant grass (Pennisetum purpureum) was used as a feedstock [13Yan2]. Acid hydrolyzed and detoxified elephant grass was inoculated with bacteria for 14 days under static fermentation. About 60 % of sugar was converted into bacterial cellulose and the yield was about 6.4 g/l. Morphologically, the bacterial cellulose transformed from an initial dense pellicle into microfibrils after 8 days of fermentation. Figure 61.7 shows scanning electron microscopy (SEM) images of the formation of the bacterial cellulose fibers after different stages of fermentation with fibrils obtained having diameters between 15 and 100 nm. X-ray diffraction patterns of the cellulose showed diffraction peaks typical of

cellulose I with % crystallinity of the fibers increasing from 23 to 54 % after 2 weeks of fermentation. After the final fermentation, cellulose crystals that had a size of about 87 A were found to have a crystallinity index of up to 99 %.

Similar to using elephant grass for bacterial cellulose production, wheat straw was pretreated with [AMIM]Cl and later enzymatically hydrolyzed to obtain sugars. After fermentation, the bacterial cellulose obtained had cellulose I crystal form with a yield of 8.3 g/l [13Che1]. Crystallinity of the straw used was 49 % which decreased to 35.9 % after pretreatment with ionic liquids which promoted growth of bacterial cellulose. In another report, bacterial cellulose was developed from canola straw for potential reinforcement for paper [13You]. Bacterial cellulose nanofibers obtained had an average diameter of 45 nm, crystallinity of 80 % with crystal size being 6.2 nm. Tensile strength of the BC nanofibers was 1.4 g/den and modulus was 133 g/den when measured using a tensile tester. Figure 61.8 shows images of the bacterial cellulose produced, and the average

diameter of the fibers can be deduced from the SEM image [13You]. When the bacterial cellulose was made into paper, the specific strength was 142 Nm/g, substantially higher than the paper made from ground cellulose or micropaper obtained from the same wheat straw. Similarly, a higher burst strength was also observed for the bacterial cellulose paper. To decrease the cost of producing bacterial cellulose, wheat straw was hydrolyzed using dilute acid hydrolysis and the hydrolysate obtained was used as feedstock. However, detoxification of the straw hydrolysate was necessary to obtain good yields [11Hon]. A considerably high yield of 15.4 mg/l was obtained from the wheat straw hydrolysate.

To overcome the lack of antimicrobial activity, bacterial cellulose nanofiber membranes were surface functionalized with aminoalkyl groups using 3-aminopropyltrimethoxysilane. Treated membranes were found to have excellent antimicrobial activity to Staphylococcus aureus and Escherichia coli and were also nontoxic to adipose derived mesenchymal stem cells and therefore considered to be useful for biomedical applications [13Fer]. A marginal increase in the strength and elongation of the cellulose nanofiber mats was observed with strength being about 6 MPa, elongation of 1.2 %, and modulus of 3.6 GPa [13Fer].

Bacterial cellulose filaments obtained from Gluconacetobacter xylinum and Hestrin-Schramm medium were tested to determine their Young’s modulus using Raman spectroscopy, and mechanical properties of the bacterial cellulose sheet were determined using an Instron tester [08Hsi]. Cellulose sheets had a modulus of

9.1 GPa, tensile strength of 240 MPa and elongation of 2.6 %. To determine the modulus of a single filament, the shift in the position of the peak at 1,095 cm-1 corresponding to the C-C or C-O bond stretching was observed. Changes in the position of the peaks were correlated to strain and the modulus of the single filament was determined to be 877 g/den. Other theoretical estimates have reported the modulus of the cellulose sheets to be between 1,046 and 1,192 g/den. Bacterial cellulose nanocrystals obtained from Gluconacetobacter xylinum that are typically

longer (100 nm to several micrometers) were added into silk fibroin solution and electrospun into fibers. The amount of cellulose crystals was varied from 0 to 7 % with corresponding increase in the diameter of the nanofibers from 230 to 430 nm. Table 61.3 shows the mechanical properties of the nanofibers containing various levels of bacterial cellulose nanocrystals. The stability and degradability of the nanofibers were not studied [12Par].

Cotton-based waste textiles were used as feedstock to produce bacterial cellu­lose using Gluconacetobacter xylinum. Before culture, the textiles were dissolved in 1-allyl-3-methylimidazolium chloride and the hydrolysate was used for culture. Yield of bacterial cellulose was 10.8 g/l, much higher than that obtained using glucose-based medium [12Hon]. The bacterial cellulose obtained had a water holding capacity of 99 % and tensile strength was 0.07 MPa [12Hon]. In a similar study prior to the report by Hong et al., colored 100 % cotton and 40/60 polyester/ cotton waste t-shirts were dissolved (concentrated phosphoric acid was found to dissolve 100 % cotton at 50 °C) using various solvents and the fermentable sugars obtained were used to culture Gluconacetobacter xylinus [10Kuo]. Fermentation yields of up to 1.8 g/l were obtained after 7 days of static culture. Figure 61.9 shows image of the cellulose pellicle obtained after 7 days of culture in sugar solution,

hydrolysate obtained from the fabrics, and the discolored hydrolysate [10Kuo]. Waste fiber sludge, a residue obtained during the processing of cellulose by pulp mills and lignocellulosic biorefineries was also used as a source to generate bacterial cellulose [13Cav]. Sludges obtained from the sulfate (SAFS) and sulfite (SIFS) processes were enzymatically hydrolyzed and the resulting hydrolysates were used for BC production. Table 61.4 shows some of the properties of the bacterial cellulose obtained using the two types of sludge and the reference medium. As seen from Table 61.4, the fiber sludge produced bacterial cellulose with properties comparable or superior to that of the reference medium.

Agricultural wastes such as pineapple peel juice and sugarcane juice were also used as culture media to produce bacterial cellulose with a yield of about 2.8 g/l, higher than that produced from the regular medium. Cellulose fibrils obtained had width of 20-70 nm but the formation of the fibrils was hindered due to the presence of other carbohydrates in the juice [11Cas]. Other agricultural residues such as wine and pulp wastes when used as sources for production of bacterial cellulose pro­duced considerably low yields of cellulose in the range of 0.6-0.3 g/l after 96 h of incubation but the yields increased by about 100-200 % for crude glycerol and grape skins when organic and inorganic nitrogens were added [11Car].

In a similar approach, waste beer yeast (WBY) was used for production of bacterial cellulose from the strain Gluconacetobacter hansenii CGMXX 3917 after a two-step pretreatment. The WBY was hydrolyzed using four different approaches and the hydrolysate obtained was directly used to produce bacteria [14Lin]. A highest bacterial cellulose yield of 7 g/l, six times higher than that obtained from untreated WBY, was achieved using WBY treated by ultrasonication [14Lin]. A new bacterium (Gluconacetobacter sp. F6) was isolated from rotten fruit and the conditions required to obtain optimum cellulose from the fruit waste were studied [12Jah]. In addition to rotten fruit, soil, vegetables, and vinegar were also studied as potential sources for bacterium. A cellulose yield of

4.5 g/l was obtained under the optimum conditions of pH 6, temperature of 30 °C, and using glucose as the carbon source [12Jah]. A thick leathery pellicle formed during production of grape wine was studied for its structure and properties and identified as bacterial cellulose from the Gluconacetobacter sp. strain. Films of 25 qm thickness were found to have an unusually high tensile strength of 41 MPa and elongation of 0.98 mm. The films had low oxygen permeability but high water

permeability [11Ran]. Figure 61.10 shows an SEM image of the bacterial cellulose generated by the cells.

Atomic force microscopy (AFM) of bacterial cellulose revealed that Gluconace- tobacter xylinus was reported to synthesize bacteria in the form of fine ribbons, similar to a three dimensional knitted structure. To improve the surface properties of the bacterial cellulose fabric, bacteria was cultured on nine different types of fabrics. Bacterial cellulose showed higher affinity for cotton and viscose compared to wool, silk, or the common synthetic polymers. Among the different fabrics studied, viscose rayon was found to be coated on both sides and such composite fabrics were expected to be suitable for medical applications [12Miz].