**2. Cellulose structure and sources**

#### **2.1 Structure**

The simplest and well-known definition of cellulose considers the polymer as D-glucopyranose subunits linked by β-1,4-glycosidic bond (C6H10O5)n. The degree of polymerization (n) of native cellulose expresses the number of glucose units that range between 1000 and 30,000 units depending on the source. For example, native wood cellulose of 10,000 glucose units is smaller than cotton cellulose of 15,000 units. Cellulose structure is typically in a linear form, where adjacent glucose units rotate by 180o to create subunits of a repeating cellobiose disaccharide. Larger organization of these subunits forms fibrous structures, commonly referred to as microfibrils or lignocellulosic fibers. Microfibril subunits are organized to generate cellulose fiber structure. The micro-scale interactions of cellulose and associated

#### *Advances in Cellulose-Based Packaging Films for Food Products DOI: http://dx.doi.org/10.5772/intechopen.110817*

bonds have significant effect on its characteristics and functionality. For example, hydrogen bonds between three hydroxyl groups of the anhydroglucose unit and D-glucopyranose oxygen atom provide rigidity and generate the three-dimensional arrangement of cellulose in their polymer network. Besides, strong hydrogen bonding of the hydroxyl groups is responsible for the reduced solubility, crystallinity, and reactivity of the polymer.

The microfibril regions are composed of crystalline and amorphous regions. The crystalline structure formed by hydrogen bonding between adjacent microfibrils. Crystalline units are found in two different phases, namely, α-cellulose and β-cellulose. The predominance of crystalline phase is determined by its source (i.e., plant- and animal-based celluloses are predominantly in the β-cellulose form, and the bacteria-based cellulose is in α-form). The crystalline cellulose appears as rod-shaped particles, which can be extracted using enzymatic, chemical, or mechanical methods. The amorphous cellulose lack a certain shape and order, and formed by randomly ordered microfibrils surrounded by hemicelluloses and. In the amorphous region, the cellulose microfibrils are more available for hydrogen bond and subsequent interactions with water, protein or enzymes. Overall, the composition of crystalline and amorphous regions determine the physicochemical properties (e.g., rigidity, strength, extractability, etc.) of cellulose. For example, crystalline cellulose may display deuteration whereas amorphous cellulose swells in hydrophilic solvent and can penetrate inside the amorphous matrix by disrupting the intermolecular hydrogen bonds [9]. The high crystallinity can increase the rigidity and reduce the elasticity of cellulose-based films. Furthermore, these structures can be modified to obtain derivatives with desired functionalities as explained in Section 3.

#### **2.2 Sources**

*Plants* are the most common source of cellulose and cellulose derivatives. A schematic diagram of cellulose sources and modification methods is given **Figure 1**. The literature and industry have various examples for the use of various cellulose sources in packaging applications. Following extraction, cellulose is typically purified to eliminate lignin and hemicellulose for the manufacturing of cellulose derivatives. Among plant sources, cotton fibers have the advantage of having relatively low non-cellulosic components with around 90 wt% purity with long chains of crystalline regions (60%). Cellulose fibers from cotton displays twisted morphology with an internal structure of cell wall layers. The strength of cotton fiber increases with moisture. Therefore, cotton-based cellulose are suitable for film applications, where mechanical integrity is required. The second most abundant source of cellulose is wood. It displays not only good mechanical properties but also potential antibacterial characteristics due to the presence of natural phytochemicals [6]. Cellulose extracted from wood includes lignin and other polysaccharide compounds with ca. 50 wt% cellulose content. The presence of hemicellulose aids extraction of wood fibers easily during the pulping process.

*Bacteria-based cellulose or cellobiose* is produced by Gram (−) aerobic bacteria such as *Acetobacter xylinum*, *Gluconacetobacter xylinus*, *Komagataeibacter xylinus*, *Agrobacterium*, *Achromobacter*, *Pseudomonas*, and *Rhizobium*, and Gram (+) bacteria such as *Sarcina spp*. The bacteria-based cellulose is typically of high purity with ribbon-like shaped microfibril structures. Isolation and purification of bacterial cellulose do not require additional chemical treatment unlike plant-based cellulose. The bacteria-based cellulose microfibrils may agglomerate into a gel-like network (i.e.,

**Figure 1.**

*A diagram of common cellulose sources and modification techniques.*

microfibril gels) via interfibrillar hydrogen bonds and Van der Waals interactions, ideal for obtaining natural cellulose packaging films [10]. Another related structure is bacterial nanofibrils, which are thinner than microfibrils with higher surface area/ volume ratio. This structure is desirable for film manufacturing since it can impart higher elasticity and strength. The microfibril structure can be controlled by altering the bacterial growth conditions. For example, using different carbon sources (e.g., replacing glucose with fructose, galactose, mannitol or xylose) can modify the surface area, porosity and crystallinity index of cellulose fibrils, and subsequently enhance resistance of their cellulose films against the oxygen and water vapor permeabilities [11]. Therefore, bacterial cellulose serves as a suitable source of cellulose with tunable characteristics for film applications.

*Algae* are known as renewable cellulose sources due to their ability to get nutrients from waste streams. Several species of green, gray, red, yellow, or brown algae, such as *Caldophora*, *Micrasterias denticulata*, *Gelidium elegans*, *Micrasterias rotate*, *Valonia*, *Boergesenia* and *Posidonia Oceanica*, were used to obtain cellulose microfibrils from their cell walls. Red algae contain the largest amount of cellulose. In comparison, green algae cellulose shows higher crystallinity (above 70%) than others, and therefore lower moisture adsorption capacity and limited the swelling. The purity of algae cellulose is typically lower than bacterial and plant cellulose. This is related to the presence of cellular proteins in the microfibril isolates. Algae based cellulose was used for biopolymer film applications, but they exhibited poor water vapor permeability and low mechanical strength [12]. Moreover, crystalline cellulose extracted from seaweed species of *Alaria esculenta*, *Saccharina latissima* have higher pure cellulose content as compared to cellulose isolated from *Ascophyllum nodosum* with heterogeneous content with minerals and proteins. Impurities reduces the presence of free hydroxyl group to limit their applications. The researchers showed that films produced from high purity cellulose exhibit better visual appearance and less moisture permeability [13].

*Tunicates* are invertebrate marine animals that can serve as a unique animal source of cellulose. Tunicate has three subclasses: *Ascidiacea*, *Thaliacea* and *Appendicularia.* Tunicate epidermal cells contain many enzyme complexes in epidermis membrane, which are responsible for cellulose production. The tunicate cellulose can be isolated nanofibril form, which are bundled in the outer tissues of tunicates. Tunicate cellulose exhibits higher crystallinity (ca. 95%) and surface area compared to other cellulose types. This provides excellent mechanical and thermal properties for cellulose-based films [14].
