Trà kombucha và sức khỏe

Health claims

A review of the published literature on the safety of kombucha suggests no specific oral toxicity in rats. ^[11] While no randomized, case-controlled studies have been published in relation to its effect on humans, there has been suspicion in isolated incidents of its effect on the central nervous system, liver, metabolic acidosis, and toxicity in general,^[12][13] though no specific links have been established. Acute conditions, such as lactic acidosis, caused by drinking of kombucha, are more likely to occur in persons with pre-existing medical conditions.^[14] Other reports suggest care should be taken when taking medical drugs or hormone replacement therapy while regularly drinking kombucha.^[15] It may also cause allergic reactions.^[16]

Many claims have focused on glucuronic acid^{[citation needed]}, a compound used by the liver for detoxification. The idea that glucuronic acid is present in kombucha is based on the observation that glucuronic acid conjugates (glucuronic acid waste chemicals) are increased in the urine after consumption. Early chemical analysis of kombucha brew suggested 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 chemical constituents, but this method relies on having proper chemical standards^{[citation needed]} 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. Instead, the active component is most likely glucaric acid. This compound, also known as D-glucaro-1,4-lactone, helps eliminate the glucuronic acid conjugates produced by the liver. When these conjugates are excreted, normal gut bacteria can break them up using a bacterial form of the enzyme 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.^[17]

Glucaric acid is commonly found in fruits and vegetables, and is being explored independently as a cancer-preventive agent.^[18] The bacterial enzyme beta-glucuronidase can interfere with proper disposal of a chemotherapeutic agent, and antibiotics against gut bacteria can prevent toxicity of some chemotherapy drugs,^[19] 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,^[20] 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.

Some evidence from small case studies has shown kombucha to aggravate symptoms of ulcerative colitis in patients taking remicade, potentially causing life-threatening side effects, such as causing the appendix to burst.^{[citation needed]}

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.^[21]

Several firms market kombucha capsules and tea bags purportedly containing some form of dried kombucha. No evidence supports any health benefits to such products.^{[citation needed]} The addition of boiling water to dried kombucha in teabag form is likely to kill any remaining live culture. Another suspect product being marketed is Kombucha extract. Most such extracts are no more than small amounts of kombucha tea that has turned vinegary.

Some firms market their kombucha mushroom cultures based on their size, charging more for larger cultures. The size of a mushroom culture does not really matter; smaller ones will ferment a new batch of prepared tea just as well as larger ones.

Safety and contamination

Mold contamination on the culture surface.

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 healthy metabolisms need not be advised against cultivating kombucha tea cultures. However, those suffering from immunosuppression should preferably consume controlled commercial kombucha beverages.^[22]

In every step of the preparation process, it is important that hands and utensils (or anything that will 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 containers in which it is fermenting if they are not food-grade.^[20] Food-grade glass is very safe. Other acceptable containers may also include lead-free china or glazed earthenware, raw wooden bowls, and stainless steel.^[23] Keeping cultures covered and in clean environments 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.^[24] 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 the correct pH is achieved and the brewing cycle is complete.

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