**4. Cellulose in symbiotic culture of bacteria and yeast (SCOBY)**

Symbiotic Culture of Bacteria and Yeast (SCOBY) is a cellulose biopolymer composed of the interaction of acetic acid bacteria and yeast. SCOBY can be formed through the kombucha fermentation process [13]. The cellulose formed in SCOBY has different characteristics with cellulose in plants. Cellulose that is synthesized through bacteria is considered more efficient and effective in the production process because it does not require a long time and a large amount of substrate [14]. Some of the advantages of producing cellulose from bacteria compared to plants include high purity, better mechanical strength, a higher polymerization rate and crystallinity index [15], a higher tensile strength based on a tensile test and a better hydrophobicity ability to water [16].

Bacterial cellulose has a basic structure of microfibrils with a glucan chain arrangement that is bound by hydrogen bonds to form a crystalline domain. Microfibrils in cellulose synthesized by bacteria are known to have a size 100 times smaller than plant cellulose fibers [17]. Electron microscopy observations show that the cellulose produced by bacteria will be synthesized in the form of cellulose fibers. Acetic acid bacteria produce two forms of cellulose, namely cellulose I in the form of a ribbon-like polymer and cellulose II in the form of an amorphous polymer which is more stable. The difference in the synthesis of cellulose I and II is in the process of forming cellulose outside the cytoplasmic membrane (**Figure 5**).

#### **Figure 5.**

*Synthesis mechanism of cellulose I and cellulose II by* Acetobacter xylinum *[16].*

Cellulose I is synthesized to form cellulose complexes that are linked to one another outside the cytoplasmic membrane, while cellulose II is formed to resemble free cellulose fibers outside the cytoplasmic membrane. The structure content of cellulose I and II will affect the tensile strength, polymerization rate and crystallinity index of cellulose fibers. The microfibrils produced by *Acetobacter xylinum* have dimensions of about 3-4 nm in length and 70–80 nm in width [16].

Cellulase is an enzyme that can degrade cellulose by breaking the 1,4-glycosidic bonds in cellulose polymers. Naturally, cellulases can be obtained in nature as metabolites of microbial metabolism, such as bacteria and fungi. Microbes that can produce cellulase enzymes usually have habitats in the soil, where these microbes play a role in the degradation of cellulose in plants. Cellulase is known to be one of the most widely used enzymes in the industrial sector, such as bio-stoning in the textile industry, extraction of fruit and vegetable juices in the food industry, and bleaching in the paper industry [18]. In the manufacture of nanocellulose, cellulase can also be used to degrade the structure of cellulose fibrils into crystalline, so that it can change the size of cellulose to nanocellulose.
