Do you know the correct facts about the SCOBY?

The SCOBY culture is now a standard fixture in health stores and home kitchens, flourishing on its reputation. If you ask someone who knows the correct facts about the SCOBY, you’ll get a similar response: “It’s a slimy, thick layer used for fermenting the tea in kombucha.”

Researchers knew about Kombucha as early as 1920. It had been used for centuries before that, though. It is a fermented tea with a culture called SCOBY. The SCOBY culture is a vital part of the process and is the most mysterious of the cultures.

It’s common to call the layer of microbial culture on the surface of fermented kombucha tea a SCOBY. The layer floats on the surface of the fermented tea.

Scientists define it differently. The definition does not agree with the information we have carried for ages.

The scientific facts about the SCOBY

According to the researchers, SCOBY is a symbiotic culture of bacteria (acetic acid and lactic acid) and yeast. The culture is like a colony where the neighbors thrive or survive with one another’s help, despite their cultural and religious differences.

In making Kombucha, the culture is mixed with the tea. At the end of the fermentation process, the tea separates into a microbiome mat layer and a fermented tea liquid.

Initially, the microbiome mat suspends in the liquid medium, but it floats to the surface by the end of the fermentation process.

The microbial cellulose forms the network of the mat, which entraps the acetic acid bacteria and yeast. The liquid part of the fermented tea carries the same culture, but it lacks thick cellulosic fibers.

What is microbial cellulose?

Microbial cellulose is a polysaccharide. A polysaccharide is a chain of sugars held together by a link, specifically a beta-1-4 glucan linkage, like cellulose, dextran, xanthan, etc. It can be a single type of sugar, like a chain containing only glucose molecules, or a combination, like glucose and galactose. Cellulose contains only one type of sugar, either glucose or fructose.

It is a component of the bacterial cell wall that grows outside of it, in the cell space between the cells. The polysaccharide growing outside the cell is an exopolysaccharide, where Exo stands for “out.” It is of two types: one with no definite boundaries, the slime, and the other with defined borders, the capsules.

Acetic acid bacteria initially produce a slimy film. The slime develops into more complex cellulose in the SCOBY.

How is microbial cellulose produced?

The bacterial cell wall facilitates microbial cellulose formation. A series of reactions take place. Firstly, glucose or fructose forms a cellulose precursor, uridine diphosphoglucose (UDPGlc), inside the cell wall.

Cellulose synthase acts on UDP-glucose. It catalyzes beta-1–4 glucan linkage formation. The linkage binds glucose molecules together to form a polymeric chain. It eventually extrudes out of the bacterial cell wall.

The pores on the bacterial cell wall are the export components through which the extrusion occurs. Once extruded, the chains of cellulose form thin fibrils, or protofibrils.

The protofibrils further aggregate into a ribbon-like structure. The ribbon-like designs are thin and not very stable. These are microfibrils.

Types of cellulose synthesized by acetic acid bacteria when forming a SCOBY

Bacterial cells synthesize two types of cellulose.

Cellulose I, The ribbon-like fibers of microfibrils

Cellulose II, These are thicker and more stable than cellulose I. There are two factors influencing the formation of cellulose II;

• The higher level of oxygen supply forms stronger 1,4-linkages.
• Few compounds provide additional bonds to the 1,4-linkages, like hydrogen bonds.
• The strong linkages redefine the organization of cellulose, making it more rigid and substantial.
• If the extruded protofibrils come into contact with the conditions mentioned above, they form cellulose II instead of cellulose I.

That explains why the layer near the surface is thick and stable.

Microbial Cellulose-Producing Bacteria

Many kinds of microbes can synthesize cellulose as a part of their metabolic process. Acetobacter, Aerobacter, Agrobacterium, Achromobacter, Azotobacter, Rhizobium, Sarcina, and Salmonella are a few among them.

Acetobacter xylinum produces microbial cellulose more efficiently than other acetic acid bacteria. So, it is the first choice of preference for the research, which proved to be ground-breaking work in microbial cellulose application.

It has been classified as Gluconacetobacter and, more recently, as Komagataeibacter. Like A. xylinum, Komagataeibacter xylinum synthesizes microbial cellulose with excellent efficiency. So, it is a crucial bacteria in the fermentation of Kombucha.

The population of acetic acid bacteria is predominant in microbial cellulose. As it grows, it entraps the yeast colonies in the network. The complete conglomerate is the biofilm or pellicle that floats on the fermented liquid of a similar culture.

The lactic acid bacteria are present in fewer quantities. It is an inconsistent microbe. Sometimes it is present in traces.

Fact check

• SCOBY is a microbial culture.
• It occurs in both the liquid medium and the mat.
• The mat is the biofilm or pellicle.
• The microbes synthesize cellulose.
• The microbial cellulose forms the primary network or structure of the pellicle.
• This structure holds the microbes in the pellicle.
• The inoculum is a fermented tea with a pellicle.

Citation

• Silvia Alejandra Villarreal-Soto, Sandra Beaufort, Jalloul Bouajila, Jean-Pierre Souchard, and Patricia Taillandie, Understanding Kombucha Tea Fermentation: A Review, Journal of Food Science, 2018, Vol 83, pp. 580-587
• Prashant R. Chawla, Ishwar B. Bajaj, Shrikant A. Survase and Rekha S. Singhal, Microbial Cellulose: Fermentative Production and Applications, Food Technol. Biotechnol., 2009, Vol 47 (2), pp. 107-124.
• Nyhan, Laura & Lynch, Kieran & Sahin, Aylin & Arendt, Elke, Advances in Kombucha Tea Fermentation: A Review, Appl. Microbiol., 2022 10.3390/applmicrobiol2010005.