Hobby forums on the internet often suffer from “parroting” where an idea is presented and seems reasonable enough that it finds purchase in the collective consciousness of the group. Once established, the information becomes undisputed folk-science. I had some doubts about commonly shared kombucha brewing wisdom so I dove into the research to try and parse fact and fiction.

“The SCOBY is the pellicle and necessary for brewing”

In conventional kombucha brewing wisdom, the pellicle (the gelatinous, floating mat of cellulose) is the S.C.O.B.Y. (Symbiotic Colony Of Bacteria And Yeast). This is inaccurate: the pellicle is an inert mat of cellulose produced by a few bacterial species, primarily Komagataeibacter Xylinus. The SCOBY is the liquid culture of the kombucha beverage itself and the only mat required to propagate a kombucha culture is the liquid. Including the pellicle certainly doesn’t hurt the brew so even “pellicle-skeptical” brewers often include it.

Thankfully, this misconception is starting to see some turnover in recent years with more emphasis being placed on the broth.

“The SCOBY requires real tea to stay healthy”

Sometimes presented as “the SCOBY requires caffeine” or that after a batch of herbal tea, the SCOBY needs to be “refreshed” with “real tea.” This was one of the first wisdoms that I was skeptical of particularly because in The Noma Guide to Fermentation David Zilber and René Redzepi detail many long-running kombucha cultures that have thrived in a wide variety of substrates such as mango juice, maple syrup, elderflower tisane, apple juice, lemon verbena tisane, and rose water.

It’s true that caffeine content decreases significantly over the course of brewing and may be used as a source of nitrogen. (Malbasa et al. 2006¹) However if it is a source of nitrogen it’s likely minor. Certain Gluconacetobacter and Acetobacter have been isolated from kombucha and shown to fix nitrogen from the air and into the solution. (Dutta & Gachui 2006²; Dutta & Gachui 2007³) This source of nitrogen is much greater than the trace nitrogen that could be provided by caffeine.

“Oils will inhibit pellicle formation”

Common wisdom will often caution against oily ingredients because they’re “bad for the SCOBY” indicated by inhibited pellicle formation. On its face this sounds plausible: K. Xylinus is an obligate aerobic fermenter so perhaps the layer of oil could create an anaerobic environment and halt cellulose production. However, the addition of vegetable oil has instead been found to drastically increase cellulose production. (Zywicka et al. 2018)⁴ There is also some oxygen dissolved in the substrate for the bacteria to utilize. I have also not observed this effect personally: batches brewed with a large amount of hops that produced a visible oil slick on the surface have been as capable at pellicle production as traditional black tea brews.

This notion is based in the idea that culture health is necessarily indicated by pellicle growth which is not necessarily true. Although SCOBY in anaerobic conditions do not create a pellicle, anaerobic fermentation continues while aerobic fermenters stay dormant. Reintroducing the SCOBY to aerobic conditions will revive aerobic fermenters to again produce a pellicle. Even if the purpoted pellicle-inhibiting effect of oil was observed it would not be a concern.

In-Bottle Pellicle Prevention Measures

One of the frustrations many kombucha brewers have is the production of a pellicle during secondary fermentation inside the final bottle. This “baby pellicle” can be unsightly and inconvenient, requiring the use of a strainer and a separate glass when consuming the final product. Commonly suggested prevention measures often follow from the wisdom above, advising the use of an essential oil, hops, or specific time, temperature, and sunlight conditions. I have not found evidence to support these. K. Xylinus cannot produce cellulose in an anaerobic environment so reducing the amount of available air during bottle is the best measure.

“Kombucha is just vinegar”

This was my own misconception before I got into kombucha brewing. I assumed that kombucha was essentially oxygen-exposed alcohol brewing that allowed acetobacter to convert ethanol to acetic acid. On the surface they appear quite similar: Kombucha often has the sharp bite of vinegar, vinegar also often produces a pellicle, and they even share some bacteria species. Similar to vinegar production, some acetic acid in kombucha is created when Saccharomyces Cerevisiae, brewers’ yeast, converts sugars into ethanol that acetobacter convert into acetic acid. However, bacteria and yeast in kombucha produce a much wider variety of acids including glucuronic, gluconic, citric (Neffe-Skocińska et al. 2017)⁵, and lactic (Reiss 1994)⁶ acids. This wide variety of acids contributes to the palatable acidity it is known for rather than the sharp burn of vinegar.

Kombucha and vinegar also differ in process. Vinegar is produced in two distinct stages: S. Cerevisiae or other brewers yeast converts sugar into ethanol in anaerobic conditions, then the yeast is killed by pasteurization, then acetobacter are introduced and allowed to convert the ethanol in the presence of oxygen. Kombucha is produced in a single stage: yeasts convert sugar into ethanol, bacteria convert ethanol into acids, and both yeasts and bacteria convert sugar directly into acids. (Neffe-Skocińska et al. 2017)⁷

Studies conflict if and how much malic, lactic, and quinic acids are produced. My own kombucha frequently has an apple-y taste that is the hallmark of malic acid so I am inclined to support that malic acid can be produced by some kombucha cultures.

“Wrap your heater around, not under”

Heating mats are sometimes used to accelerate fermentation in colder weather. Common wisdom advises against placing the heating mat under the brewing vessel and instead suggests wrapping the mat brewing vessel. The concern is that placing the heating mat below the vessel will encourage yeast production and disrupt the balance of the colony.

I believe this claim is based on the visible yeast that has settled to the bottom of the vessel. This visible yeast has flocculated: a phenomenon where yeast clump and settle in some region of the solution when the majority of the sugar has been consumed. (This is the same phenomenon that prdocues yeasty strands and clumps.) This yeast does not continue to ferment at the same rate as the rest of the yeast still suspended in the solution. Bacteria generally do not flocculate but are still present in the same region of the vessel and would benefit from the warmer temperatures to the same degree as the yeast and keep the balance in check.

All of that said, unless the ambient temperature is significantly different from the desired brew temperature the temperature differential between the bottom and the top of the vessel is likely to be negligible.

Carbonation, Geysers, and Burping

A common pest of kombucha brewers are “geysers” where the opening of a kombucha bottle releases a violent torrent of kombucha and many brewers have kombucha-tinted ceilings. A few remedies and tactics to avoid geysers are commonly recommended, one such method being “burping” where filled bottles are opened daily to release built-up pressure. This is an effective remedy but it also prevents desired carbonation from building to any palatable level.

To prevent geysers it’s important to understand the mechanics of carbonation. When carbon dioxide is dissolved in water it cannot freely bubble out of the solution; it needs a nucleation site to nudge the transition from solute to gas into action. Carbon dioxide would have difficulty bubbling out of a solution in a completely smooth container.

Geysers most commonly involve kombucha made with pureed or whole chunks of fruit. The fruit matter provides an abundance of nucleation sites so that when the cap is released the carbon dioxide is able to rapidly bubble out and expand. They key to palatable, controlled carbonation is minimizing the amount of free matter in the kombucha. Juices are the best option for maximizing carbonation and minimizing the risk of geysers.

Alcohol brewers have the luxury of calculating the precise amount of sugar needed to produce a specific amount of carbonation. Kombucha yeast is too unpredictable and measuring sugar content in an acetic acid solution is not easily possible; kombucha brewers must simply be careful.

“Don’t add vinegar”

New brewers are cautioned against adding vinegar when starting a new SCOBY from a starter. Kombucha cultures persist by maintaining a low enough pH to prevent the growth of other competitive yeast and bacteria and the biggest factor in propagating new SCOBYs is insufficient acidity. Adding vinegar will not harm a kombucha starter; the bacteria and yeast are swimming in acetic acid already. If a kombucha starter is insufficiently acidic to lower the pH below 4.0 then adding vinegar can help give the starter a chance to get established before competition takes over. That said, a starter that is insufficiently acidic may be a sign that it also has a low concentration of bacteria and yeast and will be slow to grow.

“If it’s too sweet, ferment it longer”

I see this repeated frequently and while it’s not per se “untrue”, I think it’s a mistake. Continuing fermentation will indeed consume more sugar but often to the detriment of producing an overly acidic brew. Most people prefer a dry kombucha and this requires not just reducing sugar but also limiting acidity. Instead, brewers should taste for sweetness and acidity and halt the brew when one or both reaches the desired level. If it’s too sweet, use less sugar next time. If it’s too acidic, ferment it for a shorter amount of time. If it’s too acidic and too sweet, do both. The art of kombucha brewing is the continual adjustment of the initial conditions to produce a beverage that is the right amount of sweetness, acidity, and carbonation.

Final Thoughts

I went into this venture assuming that kombucha would be a highly niche research topic but I was happily surprised with the large amount of research available. Kombucha Brewers International offers a fantastic database of freely available kombucha research. Pubmed also had a great amount of available literature.

I hope this post is useful in making research more accessible to kombucha homebrewers. The alcohol homebrewing community has a great feedback loop between professional research and amateur homebrewers that rapidly refines the art and science of homebrewing and I hope the kombucha community is able to do that as well.