Kombucha has stormed onto the health food scene over the past few years, building the market space from basically one company (GT’s) to what currently seems like a hundred different brands occupying entire refrigerator cases. As it has grown in popularity the formulations have changed, and the marketing around their health benefits has evolved. To navigate the myriad of claims and evaluate their role in your own health regimen, it is important to know the truths and caveats of some of these claims.
This article introduces the science of fermentation and the process of making kombucha. Given that background information, the health benefits are then reviewed.
What is Fermentation?
Kombucha is a fermented tea, so the key to understanding kombucha is to understand the process and products of fermentation. Fermentation is the transformation of nutrients into energy and acid through microbial activity. The energy is usually manifested as carbon dioxide, and the acids can be alcohol or other types of organic acids.
The term “microbe” simply means “microscopic organism.” There are untold millions of types of microbes, consisting of bacteria, yeast, fungi, algae and many others.The microbes used in fermentation are predominantly yeasts and/or bacteria, and the resulting product of fermentation depends on the type of microbes used and the nutrients to which they are exposed.
There are millions of known strains of bacteria, and probably millions yet to be discovered. The genus of bacteria used for fermenting our food is predominantly lactobacillus. This is the same genus that lives in our guts and helps us digest food. Different strains of lactobacillus prefer different nutrients, so there are strains that process vegetable matter, others that prefer dairy, etc. Lactobacillus bacteria produce carbon dioxide and lactic acid. The kombucha starter (called a SCOBY) contains acetic acid bacteria, which turn ethanol (alcohol) into vinegar (acetic acid).
As you might imagine, there are also many strains of yeast. The types used in fermentation prefer sugars (such as white sugar or malt), and they convert those sugars carbon dioxide and ethanol (alcohol).
How Kombucha is Made
The core components
SCOBY - this is an acronym standing for “Symbiotic Colony of Bacteria and Yeast.” It is sometimes called a mushroom because of its appearance; however, it it is not a fungus, it is comprised of two kinds of microbes: bacteria and yeast. An in-depth discussion of these microbes is included later in this article.
Tea leaves (black, white or green) - the microbes in the SCOBY feed on the tannins in the tea, There are some techniques for using a few select other forms of tea, but the vast majority of kombuchas are made with real, caffeine-containing tea leaves for consistency and health of the SCOBY.
White sugar - As with the tea, the microbes in the SCOBY are the type that feed on refined sugar. A different type of SCOBY has been developed that feeds on honey, and that product is called “jun.”
Kombucha fermentation is aerobic - that is, it is done in the presence of oxygen (this is different from vegetable fermentation, which is an anaerobic fermentation). During fermentation the microbes in the SCOBY slowly transform the sugar into carbon dioxide and various acids, thus acidifying the mix (lowering the pH). The length of fermentation varies by the desired sourness of the final product.
This is the sequence:
Sweet tea is brewed and cooled.
The SCOBY is added.
The vessel is covered with a towel or other cover that lets oxygen in but keeps dust and flies out.
The vessel is allowed to sit at room temperature (or slightly higher) for a number of days to ferment (described below).
After the mix has reached the desired pH, it is drawn off the SCOBY and flavoring can be added. This could be spices, juice, chopped fruit, or other desired flavorings.
When making kombucha at home, the mixture is then often poured into bottles, sealed and left to ferment a while longer. During this time the microbes feed on the additional sugar (from the flavoring) and create carbon dioxide, which carbonates the drink. This is referred to as the “second ferment.”
Commercial kombucha is generally carbonated artificially (like sodas), bottled, cooled and there is usually not a second fermentation. This varies by producer and is usually proprietary information.
What happens during fermentation?
When the SCOBY is added to the sweet tea mixture, the yeast and bacteria go to work. The yeast first transforms the sugar into alcohol and carbon dioxide. The acetic acid bacteria then transform the alcohol (or at least a lot of it) into vinegar (acetic acid).
Kombucha and Health
Many claims have been published (and printed on labels) regarding the health benefits of kombucha. While there are good arguments for some of these claims, others are questionable. There are also a few down sides to kombucha that should be considered.
“Probiotics” are defined as "live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host." Probiotic microbes are a specific subset of all microbes. Most probiotic microbes are bacteria, with a very small number of yeast that may qualify (here is a nice survey of the microbes included in most drugs and supplements). Some obvious examples of bacteria that are not probiotic are strains such as E. coli and Clostridium botulinum (which causes botulism). Many other bacterial strains are not harmful, but are also not considered beneficial, so they are not considered probiotic. Just because a product contains live cultures (microbes) does not mean that it is probiotic. If this were true beer, wine, compost and even a Petri dish of e coli would be considered probiotic.
Many two main bacterial strains in a kombucha SCOBY are the acetic acid bacteria Acetobacter and Gluconobacter (which produce vinegar and gluconic acid, respectively). Acetic acid bacteria are not considered to be probiotic because they are aerobic (they need oxygen to live) and there is no oxygen in our stomachs.
During fermentation, the yeasts in the SCOBY produce ethanol. They are not considered to be probiotic because ethanol is not beneficial to the human body. Acetic acid bacteria then make use of glucose to produce gluconic acid and ethanol to produce acetic acid.
If a kombucha advertises that it contains other specific probiotics, they must have been added artificially to the brew post-fermentation (as they are not present in SCOBY) and it should list the specific strains of bacteria as ingredients. A popular yeast strain to be added to kombucha is Saccharomyces boulardii. There is preliminary evidence that it may help with digestion. It is mostly a laboratory-created strain (appears rarely in nature) and is not a part of the original SCOBY, it was added post-fermentation. If a kombucha advertises that it contains probiotics but does not list them, that claim should be viewed with skepticism. Some commercial products are also pasteurized for longer shelf life and to insure the stoppage of fermentation (specifically the production of carbon dioxide) so don’t contain any living micro-organisms at all.
Acids and Nutrients
As noted above, the fermentation process creates acetic acid (vinegar), which has been shown to have many benefits. Antioxidants, B vitamins and other nutrients have also been isolated in kombucha samples. A search of the research database from the National Institutes of Health reveals many of these.
Since refined sugar is a necessary nutrient for the SCOBY, sucrose will remain in the final product. Fructose will also often be present for the flavoring added at the end of fermentation (or as part of the second fermentation). Therefore it is important to check nutritional labels of commercially produced kombucha for sugar content, as it may vary significantly by ingredients and process.
As described in the section on Probiotics, the yeasts in the SCOBY do turn sugars into ethanol. The acetic acid bacteria in turn transform some of the ethanol into vinegar, but not necessarily all of it. Some trace of alcohol will be present in almost every kombucha.
There is still some debate about how much caffeine remains in kombucha after fermentation. There will certainly still be some, but research is ongoing.
Kombucha is almost certainly healthier than soft drinks, and might be a good replacement for bubbly drinks in one’s diet. It also has many components that have been shown to have beneficial effects, such as anti-oxidants, acetic acid and gluconic acid. However, kombucha also contains several unhealthy substances, such as alcohol, caffeine, and potentially high sugar amounts.
Most importantly, the industry-wide shift towards deceptively advertising kombucha as a source of probiotics needs to be raised, addressed and debunked.