Kombucha

Kombucha is a tea that is often drunk for medicinal purposes. There is limited scientific information supporting any health benefit and few studies are being conducted, although there are several centuries of anecdotal accounts supporting some of the health benefits attributed to the tea. Kombucha is available commercially and can be made at home by fermenting tea using a visible, solid mass of yeast and bacteria which forms the kombucha culture which is often referred to as the "mushroom" or the "mother".

Biology

The culture mainly contains a symbiosis of Acetobacter (acetic acid bacteria) and one or more yeasts.

The culture itself looks somewhat like a large pancake, and though often called a mushroom, a mother of vinegar or by the acronym SCOBY (for "Symbiotic Colony of Bacteria and Yeast"), it is scientifically classified as a zoogleal mat. It takes on the shape of its container, but varies in thickness depending on how long it has been allowed to develop and the acidity of the tea medium during the development period. The culture is leathery and non-elastic, similar to a thick calamari.

The yeast component of kombucha may contain any of Saccharomyces cerevisiae, Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii, and Zygosaccharomyces bailii, or another domesticated strain. Alcohol production by the yeast(s) contributes to the production of acetic acid by the bacteria. Alcohol concentration also plays a role in triggering cellulose production by the bacterial symbionts.

The bacterial component of a kombucha culture usually consists of several species, but will almost always contain Gluconacetobacter xylinus (formerly Acetobacter xylinum), which ferments the alcohols produced by the yeast(s) into acetic acid. This increases the acidity while limiting the alcoholic content of kombucha. G. xylinum is responsible for most or all of the physical structure of a kombucha mother, and has been shown to produce microbial cellulose. This is likely due to artificial selection by brewers over time, selecting for firmer and more robust cultures.

The acidity and mild alcoholic element of kombucha resists contamination by most airborne molds or bacterial spores. As a result, kombucha is relatively easy to maintain as a culture outside of sterile conditions. The bacteria and yeasts in kombucha may also produce antimicrobial defense molecules. Gluconacetobacter diazotrophicus, a bacterium related to G. xylinum, is known to produce an antimicrobial known as a bacteriocin.

History

In Chinese, kombucha is called hongchajun 红茶菌 (lit. "red tea fungus/mushroom"), hongchagu 红茶菇 ("red tea mushroom"), or chameijun 茶霉菌 ("tea mold").

In Japanese, the kombucha drink is known as "kōcha kinoko" 紅茶キノコ (lit. "red tea mushroom"). Both the Chinese and Japanese names incorporate the characters for hongcha or kōcha literally, "red tea," referring to what is known in the West as black tea rather than simply cha 茶 tea or (ryoku cha) 緑茶 "green tea".

Japanese kombu 昆布 "a Laminaria kelp; sea tangle" is dried and powdered to produce a beverage called kombucha (literally "kelp tea"). The English kombucha fermented tea is pronounced similarly, and is confused with the Japanese kombucha seaweed tea.

Some promotional kombucha sources suggest that the history of this tea-based beverage originated in ancient China or Japan, though there are no written records to support these assumptions (see history of tea in China and history of tea in Japan). One author reports that kombucha, famously known as the "Godly Tsche [i.e., tea]" during the Chinese Qin Dynasty (221-206 BCE), was "a beverage with magical powers enabling people to live forever". A Korean author claims an etymology from "a Korean doctor named Kombu treated the Japanese Emperor Ingyō in 415 A.D." The early Japanese history Kojiki does mention an envoy from the ancient Korean state Silla who was "deeply versed in the medical art" and cured the Emperor's sickness and his name was 金武 Komu Mu.

Components

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Kombucha contains multiple species of yeast and bacteria along with the organic acids, active enzymes, amino acids, and polyphenols produced by these microbes. The precise quantities of a sample can only be determined by laboratory analysis. Finished kombucha may contain any of the following components:

Normally kombucha contains less than 0.5% alcohol, which classifies kombucha as a non-alcoholic beverage. Older, more acidic, kombucha might contain 1.0% or 1.5% alcohol, depending on more anaerobic brewing time and higher proportions of sugar and yeast.

Health claims

A review of the published literature on the safety of kombucha suggests no specific oral toxicity in rats, although it has also been shown to increase the size of both the liver and spleen in mice. While no randomized case-controlled studies have been published in humans, several unsubstantiated reports have suspected liver damage, metabolic acidosis and life-threatening toxicity. Other reports suggest that care should be taken when taking medical drugs or hormone replacement therapy while regularly drinking kombucha. It may also cause allergic reactions.

Many claims have focused on glucuronic acid, a compound that is used by the liver for detoxification. The idea that glucuronic acid is present in kombucha is only based on the observation that glucuronic acid conjugates (glucuronic acid waste chemicals) are increased in the urine after consumption of kombucha. Early chemical analysis of kombucha brew suggested that glucuronic acid was the key component, and researchers hypothesized that the extra glucuronic acid would assist the liver by supplying more of the substance during detoxification. These analyses were done using gas chromatography to identify the different chemical constituents, but this method relies on having proper chemical standards (needs citation) to match to the unknown chemicals.

However, a more recent and thorough analysis of a variety of commercial and homebrew versions of kombucha found no evidence of glucuronic acid at all. Instead, the active component is most likely glucaric acid. This compound, also known as D-glucaro-1,4-lactone, helps eliminate glucuronic acid conjugates that are produced by the liver. When these conjugates are excreted, normal gut bacteria can break them up using a bacterial form of beta-glucuronidase. Glucaric acid is an inhibitor of this bacterial enzyme, so the waste stored in the glucuronic acid conjugates is properly eliminated the first time, rather than being reabsorbed and detoxified over and over. Thus, glucaric acid probably makes the liver more efficient.

Glucaric acid is commonly found in fruits and vegetables, and is being explored independently as a cancer preventive agent. It has also been discovered that the bacterial beta-glucuronidase enzyme can interfere with proper disposal of a chemotherapeutic agent, and that antibiotics against gut bacteria can prevent toxicity of some chemotherapy drugs, supporting the idea that glucaric acid is an active component of kombucha.

Reports of adverse reactions may be related to unsanitary fermentation conditions, leaching of compounds from the fermentation vessels, or "sickly" kombucha cultures that cannot acidify the brew. Cleanliness is important during preparation, and in most cases, the acidity of the fermented drink prevents growth of unwanted contaminants.

Other health claims may be due to the simple acidity of the drink, possibly influencing the production of stomach acids or modifying the communities of microorganisms in the gastrointestinal tract.

Safety and contamination

As with all foods, care must be taken during preparation and storage to prevent contamination. Keeping the kombucha brew safe and contamination-free is a concern to many home brewers. Key components of food safety when brewing kombucha include clean environment, proper temperature, and low pH. If a culture becomes contaminated, it will most likely be identifiable as common mold which is often green, blue, or black in color. Often novice brewers will mistake the brownish root filaments on the underside of the culture as a mold contamination when it is seen through the surface of a thinly formed culture. If mold does grow on the surface of the kombucha culture, or "mushroom," it is best to throw out both culture and tea and start again with a fresh kombucha culture.

Mold tends to grow especially when the kombucha mushroom is lifted out of the liquid by its own gasses. Keeping it covered with liquid in the later stages, i.e. when the new kombucha mushroom starts growing, can successfully prevent mold from growing.

The low rate of contamination by the home brewer might be explained by protective mechanisms, such as formation of organic acids and antibiotic substances. Thus, subjects with a healthy metabolism need not be advised against cultivating kombucha tea cultures. However, those suffering from immunosuppression should preferably consume controlled commercial kombucha beverages.

In every step of the preparation process, it is important that hands and utensils (or anything that is going to come into contact with the culture) be well cleaned to prevent contamination of the kombucha. Also, kombucha becomes very acidic (approximately pH 3.0 when finished), so it can leach unwanted and potentially toxic materials from the non food grade containers in which it is fermenting. Keeping cultures covered and in a clean environment also reduces the risk of introducing contaminants and insects.

Maintaining a correct pH is an important factor in a home-brew. Acidic conditions are favorable for the growth of the kombucha culture, and inhibit the growth of molds and bacteria. The pH of the kombucha batch should be between 2.5 and 4.6. A pH of less than 2.5 makes the drink too acidic for normal human consumption, while a pH greater than 4.6 increases the risk of contamination. Use of fresh "starter tea" and/or distilled vinegar can be used to control pH. Some brewers test the pH at the beginning and the end of the brewing cycle to ensure that the correct pH is achieved and that the brewing cycle is complete.

Brew

Kombucha is typically produced by placing a culture in a sweetened tea, as sugars assist fermentation. Black tea is a popular choice but green tea may also be used, as well as any other flavor. The container is often covered with a porous cloth to prevent contamination by dust, mold, and other bacteria, while allowing gas transfer ("breathing").

After a week or two of fermentation, the liquid is tapped. Some liquid is retained to keep the pH low to prevent contamination. This process can be repeated indefinitely. In each batch, the "mother" culture will produce a "daughter", which can be directly handled, separated like two pancakes, and moved to another container. The yeast in the tapped liquid will continue to survive. A second wait time for about a week can produce more carbonation.

Left entirely alone the kombucha settles into months of production time (the "daughter" thickening considerably), creating an ever more acidic and vinegar-dominated cider. At any point the kombucha can be tapped or have tea added. Liquid from the previous batch will preserve some of the culture.

A "daughter" SCOBY is produced on the liquid / gas interface during each fermentation. The surface area is the most favorable location for both aerobic bacteria on the top of the new "pancake" and anaerobic bacteria on the bottom. The surface area also has ideal concentration of oxygen for the yeast in the matrix to propagate readily.

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See also

References

Bibliography

External links

Category:Fermented beverages Category:Fermented foods Category:Carbonated drinks Category:Tea Category:Chinese tea Category:Mycology Category:Toxicology Category:Synthetic materials Category:Nonwoven fabrics