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Kombucha is a fermented beverage whose taste reminds of apple cider: it is slightly sweet, acidic, and sparkling. Kombucha consists of two components: a sour liquid broth made of fermented tea in the presence of sucrose, and a flat cellulose pellicle layer.
Kombucha’s popularity is now spread all over the world and is forecast to increase. Half of its consumption is attributed to North America, followed by the European market, with the United Kingdom and Russia leading the consumption. It is also gaining increasing interest in China, where the search for probiotic drinks is very diffused, and in the Middle East and Northern Africa, thanks to this beverage’s health benefits and non-alcoholic nature. [1]
Even though its success is mainly due to Western markets, Kombucha’s origin is set in Eastern Asia, probably in the region of Manchuria, in Northern China, where it was prized by the Tsin Dynasty around 220 B.C thanks to its detoxifying and energizing properties. As a matter of fact, another name for the microbial colony that is responsible for the beverage is “Manchurian mushroom”, along with “tea fungus”.
The etymology of the word “kombucha” is unclear: some theories say it’s the union between the Japanese words kombu (seaweed) and cha (tea), while others support the hypothesis that Kombu was the name of the Korean physician who cured the digestive problems of the Japanese Emperor Inkyo with the tea fungus. With the expansion of trade routes, kombucha arrived in Russia and East Germany, and after World War II it became popular also in France and Italy. [2], [3]
Fermentation Parameters
Kombucha’s fermentation lies on many different factors and not all of them are easy to control, such as the microbiological composition of the broth. However, some of them are particularly crucial for the final taste and properties. One fundamental factor is which substrate is used for fermentation.
The most common is black tea, which represents 75% of total tea production and is obtained by Camellia sinensis’s leaves, after being rolled, oxidized, and dried. Another common substrate is green tea, where leaves are steamed to prevent oxidation of catechins. [1] The absence of the oxidation process gives green tea a lighter flavor and a different polyphenol content.
Even though tea is usually used for Kombucha’s fermentation, other substrates have been tried as an alternative, obtaining similar results. For example, sweetened Echinacea (Echinacea purpurea L.) and winter savory (Satureja montana L.) have given characteristics comparable to traditional Kombucha, but reduced fermentation time; fermented coconut water with Kombucha’s consortium presented different biological activities.
Recently, after only six days of fermentation, Kombucha with improved sensorial and functional properties was obtained by grape juice, which leads to the conclusion that studying therapeutic effects of kombucha obtained by alternative substrates might be a promising direction.
Another key parameter is fermentation time, which usually ranges from 7 to 60 days, when the excess of CO2 creates a starved environment for bacteria to live. After 7 days all kinds of Kombucha’s nutrients are present in the beverage and after the 10th day its taste starts to assume a vinegary taste, due to the increasing concentration of acetic acid.
Furthermore, the pH decreases, due to organic acid concentration: it is important that pH does not get below 3, which is the one of the digestive tract. Finally, fermentation temperature ranges from 22 to 30 °C, but some metabolites, such as vitamin C, were obtained in higher concentrations with temperatures above this threshold. [4]
Biological Composition and Microbial Ecosystem
As previously anticipated, Kombucha’s fermentation is due to a gelatinous consortium of bacteria and yeast that floats on top of the liquid phase and takes the shape of the bowl. [2] Its name is SCOBY, which stands for Symbiotic Culture of Bacteria and Yeasts, and its composition is not unique since it depends on the source of the inoculum for the tea fermentation.
The principal components are both Saccharomyces and non-Saccharomyces osmophilic yeasts, acetic acid, and lactic acid bacteria. [4], [5] The metabolic pathway also remains undefined, since the precise interactions between different microorganisms are unknown. Yeasts are responsible for the first sucrose degradation into glucose and fructose, which can undergo both aerobic and anaerobic fermentation, giving either CO2 or as metabolites. [4]
On the other hand, bacteria produce acetic acid and gluconic acid from ethanol and glucose through aerobic fermentation, respectively. During the first period of fermentation, a biofilm of cellulose is produced thanks to the genus Gluconacetobacter xylinus which synthesizes uridine diphospho-glucose (UDPGlc) – a cellulose’s precursor – from gluconic acid.
Cellulose’s synthesis increases as long as the presence of oxygen at the medium/air interface is guaranteed, allowing only the bacteria on the surface to maintain their activity, whereas the ones that are in the liquid are kept in a dormant state due to the progressive lack of oxygen and can be used for further inocula. [4] [6] However, when SCOBY becomes too thick and starts to sink into the liquid, the oxygen supply is insufficient for bacteria to carry out their metabolic activity and cellulose’s production stops.
On the other hand, while bacteria perform aerobic fermentation on the surface, yeasts are kept on SCOBY’s bottom, where oxygen supply is reduced and anaerobic fermentation leads to the production of ethanol and other fruity organic compounds, that are responsible for kombucha’s aromas. [5]
The presence of both Saccharomyces and non-Saccharomyces yeasts avoids the risk of stuck fermentation and the production of organic acids, and the consequent pH decrease, inhibits pathogenic bacteria’s growth, enabling safe drinking. [4]
Chemical Composition and Properties
Many bioactive compounds in Kombucha are the ones contained in green and black tea. These plants are renowned for their high polyphenol content, the most abundant antioxidants in the diet. The main polyphenols in green tea are catechins, that in black tea are oxidized to thea-flavins and thea-rubigins, causing a less bitter taste and a darker color.
Polyphenols are associated with:
- the prevention of cancer
- increased immunity
- Arthritis prevention
- and inflammation reduction.
Another class of abundant compounds due to bacteria fermentation are organic acids and their content strongly depends on the starter culture used. Acetic acid is responsible for kombucha’s vinegary flavor, and its concentration reaches its peak of 11 g/L on day 30 of fermentation and then starts to drop since, after the depletion of sugar and ethanol, bacteria start to use acetic acid as a carbon source. Lactic acid is mainly found in Kombucha made from green tea.
The metabolism of glucose produces glucuronic acid which is the most significant detoxifier in the human organism. [1], [6] Naturally produced by a healthy liver, its function is to conjugate different lipidic compounds through a process called glucuronidation, producing more hydrophilic molecules that can be excreted through the kidneys or the digestive tract.
Glucuronic acid is also important as a precursor in the biosynthesis of L-ascorbic acid (AA), also known as vitamin C, in acetic acid bacteria. AA is a water-soluble vitamin that, like all the other vitamins, is not synthesized within the body and has to be supplemented in the diet. Vitamin C is found in many vegetables and fruits such as citrus fruits, peppers, broccoli, and strawberries.
It has numerous functions, first of all protecting the immune system, reducing cholesterol levels, and collagen synthesis. Collagen is the principal fibrous protein of animals’ connective tissue, hence AA plays a crucial role in wound repair and healing/regeneration process. [1], [7]
Amino acids belong to another class of compounds that is commonly found in food and beverages. Many of them have been identified in tea, the most abundant being theanine, accounting for 50% of total amino acids. Even though this class of compounds is essential for the human organism, one of their byproducts, biogenic amines (BAs), represents a threat to health when contained in high ratios.
BAs, such as histamine, putrescine, or serotonin are produced endogenously through of amino acids and are essential for several metabolic activities. However, BAs can also be produced by decarboxylase-positive microorganisms in fermented food and beverages. In this case, BAs content might exceed the toxic threshold and some toxic effects might occur.
Hence, in non-fermented beverages, no BA content is a quality indicator, stating the absence of unwanted microorganisms. However, even though three potential BAs precursors have been identified in Kombucha (lysine, phenylalanine, and tryptophan), no BAs have been found in the analyzed samples.
Ethanol is present in Kombucha in a range of 0.7-1.3% by volume, as a byproduct of yeast fermentation, while caffeine serves as an essential nitrogen source for yeasts and bacteria to build new cells. [1]
Alongside all the beneficial properties attributed to the above mentioned compounds, the presence of the whole plethora of Kombucha’s microorganisms provides this beverage with probiotic effects, i.e. live microorganisms that, when consumed, cause beneficial changes in a person’s gut microbiota. [1]
However, there is a lack of scientific data about Kombucha’s beneficial properties and a few cases of adverse effects have been reported. However, in all cases, side conditions, such as existing pathologies, might have contributed to the acuity of the intoxication. A very important factor to keep under control to avoid unwanted contamination is to work under hygienic conditions [6].
Furthermore, pregnant women should avoid drinking Kombucha because of the presence of heparin, a protein contained in the tea that inhibits the blood-clotting system. Tea also contains tannins, polyphenols that cause tooth stains by binding to the hydroxyapatite component of the enamel. However, this kind of stain can be removed by bleaching treatment. [1], [3], [6]
Cellulose Biofilm
As anticipated above, the species Gluconobacter xylinum is not capable of synthesizing acetic acid from ethanol, but instead one of its metabolic pathways leads to the formation of a cellulosic biofilm.
This microbial cellulose is secreted through 3.5 nm-diameter pores and lies extracellularly in the form of fibrils attached to the bacterial cell. Each newly produced film is called “daughter” and is the closest to the surface, while the SCOBY is also known as the “mother” and consists of all the previous overlapped film layers that are located below the most recent one. [5]
This kind of cellulose exists into two forms, cellulose I and II. The first is a ribbon-like polymer that forms crystals, while the second is an amorphous polymer, more thermodynamically stable. It is hypothesized that Kombucha’s colonies might produce a different cellulose polymer than the one obtained by standard sources.
The main difference between Kombucha derived cellulose and plant-derived one, is the first one’s purity. Indeed, polymers such as hemicellulose, lignin, or pectin are completely absent. Furthermore, this biofilm is 100 times thinner than the one obtained from plants, but its water-holding capacity is over 100 times higher. [4]
A similar cellulosic floating network obtained by the fermentation of coconut water is a popular delicacy in the Philippines called “Nata de Coco” and other microbial cellulosic biofilms have been known to be used in Brazil to heal skin burns and other dermal injuries since ancient times. [3]
However, its astonishing properties make the SCOBY a promising candidate for innovative materials, even though the production costs remain a limitation for such applications. This material has been studied as an alternative eco-packaging, a biomaterial for medicine, and a supplement for animal feeding. [8] However, it showed its most promising results in the bio-absorption of heavy metals in aqueous solutions such as wastewater. [3], [5]
These results make Kombucha an interesting topic to be further explored not only for its potential health benefits, about whom data are still insufficient, but also as a promising new generation material.
References:
[1] P. Bishop, E. R. Pitts, D. Budner, and K. A. Thompson-Witrick, “Chemical Composition of Kombucha,” Sep. 01, 2022, MDPI. doi: 10.3390/beverages8030045.
[2] J. Jarrell, T. Cal, and J. W. Bennett, “The Kombucha consortia of yeasts and bacteria,” Mycologist, vol. 14, no. 4, pp. 166–170, 2000, doi: 10.1016/S0269-915X(00)80034-8.
[3] R. Jayabalan, R. V. Malbaša, E. S. Lončar, J. S. Vitas, and M. Sathishkumar, “A review on kombucha tea-microbiology, composition, fermentation, beneficial effects, toxicity, and tea fungus,” 2014, Blackwell Publishing Inc. doi: 10.1111/1541-4337.12073.
[4] S. A. Villarreal-Soto, S. Beaufort, J. Bouajila, J. P. Souchard, and P. Taillandier, “Understanding Kombucha Tea Fermentation: A Review,” Mar. 01, 2018, Blackwell Publishing Inc. doi: 10.1111/1750-3841.14068.
[5] R. M. D. Coelho, A. L. de Almeida, R. Q. G. do Amaral, R. N. da Mota, and P. H. M. de Sousa, “Kombucha: Review,” Dec. 01, 2020, AZTI-Tecnalia. doi: 10.1016/j.ijgfs.2020.100272.
[6] J. M. Leal, L. V. Suárez, R. Jayabalan, J. H. Oros, and A. Escalante-Aburto, “A review on health benefits of kombucha nutritional compounds and metabolites,” CYTA – Journal of Food, vol. 16, no. 1, pp. 390–399, Jan. 2018, doi: 10.1080/19476337.2017.1410499.
[7] S. Chambial, S. Dwivedi, K. K. Shukla, P. J. John, and P. Sharma, “Vitamin C in disease prevention and cure: An overview,” Oct. 01, 2013, Springer. doi: 10.1007/s12291-013-0375-3.
[8] I. Diez-Ozaeta and O. J. Astiazaran, “Recent advances in Kombucha tea: Microbial consortium, chemical parameters, health implications and biocellulose production,” Sep. 16, 2022, Elsevier B.V. doi: 10.1016/j.ijfoodmicro.2022.109783.
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