DIY Kitchen Science

Kombucha Brewing: The Process

Photo credit: Mgarten (Wikimedia Commons)

Photo credit: Mgarten (Wikimedia Commons)

At first glance, making kombucha sounds straightforward. After all, kombucha is fermented tea, which tells all you need to know about making it: take some tea and ferment it. Unfortunately, brewing kombucha is not that simple, as evidenced by the plethora of information and recipes found on the Internet. For those who have ever contemplated or even decided to begin brewing kombucha for the first time, don’t let the wealth of kombucha information intimidate you. Here, we break down the process of kombucha brewing and experimentation, supplying you with the scientific rationale for each step. Understanding the science of each stage may allow for a more successful and experimental brewing without having to rely on a recipe.

1. Making the tea base.

The tea base is nothing more than sweetened tea, so it is easy enough to make. However, the amount of tea and sugar used will affect the flavor of the resulting kombucha. The exact proportion of water to tea to sugar can be modified to suit personal tastes. For the varieties of teas and sugars suitable for making kombucha, check out our previous post on the ingredients that go into making kombucha.

In general, for every 1 cup of boiled water, steep 1 tea bag or 1 ounce of loose leaf tea; this should be left to steep for 3 – 5 minutes, with deviations depending on the type of tea and desired tea strength. Certain teas, such as green and white teas, have subtle flavor profiles that may result in a bland-tasting kombucha. To obtain a more concentrated flavor with delicate teas, use more tea bags, do multiple infusions, or combine the green or white teas with a more robust black tea.

If you use tea bags, adding more of them can help increase the amount of flavor compounds in the brewed tea, creating a more concentrated green or white tea flavor. Avoid steeping the teas for too long; steeping teas longer than the recommended time results in the extraction of more bitter compounds. This over-extraction will create a more bitter tea base. The same caution equally applies to loose leaf teas.

If you use loose leaf teas, multiple infusions will help concentrate the flavor without the risk of over-extraction. A proper method for multiple infusion involves steeping a large amount of tea leaves for 20 – 45 seconds in just enough hot water to cover the leaves. The brewed tea is removed, and another small amount of hot water is added to the leaves and steeped for another short amount of time. This can be repeated 3 – 15 times, depending on the type of tea. This method uses twice the amount of tea leaves with half the amount of hot water [1], essentially concentrating the flavor compounds that diffuse out of the tea leaves. Multiple infusions may not be as effective with tea bags; the tea fannings used for tea bags have small surface areas, and so most, if not all, of the flavor compounds will have quickly diffused into the water in the first steeping.

Sugar can be added to the boiling water before or after steeping the tea, as long as the sugar source completely dissolves. Typically, 1 cup of sugar is added for every 4 cups of boiled water.

2. First fermentation.

Once the tea is finished steeping and the sugar is dissolved, remove the tea bags or strain out the leaves – this is the completed tea base. Tossing the SCOBY (Symbiotic Culture of Bacteria and Yeast) into this freshly-completed tea base willy-nilly will negatively affect the fermentation process, as the microbes within SCOBY thrive best at specific temperatures and pH levels. To ensure a successful fermentation, the tea base has to be adjusted for temperature and pH create a suitable environment for the SCOBY.

  1. Optimum temperature. Recall that SCOBY is alive; wait for the tea base to cool down to at least below 90°F (32°C) before adding SCOBY. A hot tea base would destroy the SCOBY microorganisms, resulting in a complete lack of fermentation. Conversely, do not add SCOBY to a tea base that has been refrigerated to below room temperature, as this would encourage the microbes to go into a dormant state, leading to a very sluggish fermentation process. The optimal temperature to add the SCOBY is between 77°F (25°C) and 90°F (32°C); , as this is the range which SCOBY microorganisms such as Acetobacter and yeast grow best [2,3].
  2. Optimum pH. SCOBY bacteria are acidophiles, meaning that these bacteria thrive in acidic environments. Excluding herbal teas, the teas used for kombucha generally have low pH ranging from 2.9 to 6.3 [4,5]. While this is considered acidic, the pH of the tea base may not be at the optimal range for the Lactobacillus and Acetobacter that inhabit SCOBY, which thrive around pH 5.0 – 6.3 [6,7]. To remedy this, a starter liquid is added to the tea base, which is the liquid that the SCOBY was stored in. Since the starter liquid houses both Lactobacillus and Acetobacter, which produce acid by oxidizing sugar to lactic acid and ethanol to acetic acid, the starter contains a mixture of lactic and acetic acid at a buffered pH that is ideal for the SCOBY. In general, 1 cup of starter liquid is used for every 2 cups of tea base. If there is not enough starter liquid, then plain, store-bought kombucha can be used in lieu of the starter.

SCOBY is added to the tea base in a wide-mouthed container, often a glass jar, to allow for gas exchange and left to ferment for 7 to 10 days at room temperature. During this first fermentation, oxygen has to be abundantly available for Acetobacter, which requires oxygen to grow (it is an obligate aerobe) [7]. However, leaving the container uncovered puts the kombucha at risk for contamination by fruit flies. Covering the jar with a tightly-woven cloth or paper towel and an elastic band can keep out fruit flies while permitting oxygen availability for the fermenting kombucha. The longer the fermentation period, the more vinegary the flavor and the lower the sugar content.

Kombucha undergoing the first fermentation. Photo credit: Amy Selleck (amyselleck/Flickr)

Kombucha undergoing the first fermentation. Photo credit: Amy Selleck (amyselleck/Flickr)

3. Remove the SCOBY.

To end the first fermenation, simply remove the SCOBY from the kombucha. From here, there are two options: reuse the SCOBY for another batch of kombucha or store it for later brewing.

Reuse: Make another tea base. For the starter liquid, it would be easiest to use the kombucha that the SCOBY was previously removed from.

Store: Store the SCOBY in a tea base/starter liquid mixture. This can be kept at room temperature for up to three weeks, depending on the volume of the storage mixture, before the microbes exhaust the nutrients. For longer storage, place the SCOBY mixture in the refrigerator. SCOBY become dormant in cold temperatures, but this does not mean the microbes cease activity altogether. Rather, in dormancy, cell division halts and the microbes’ metabolism slows significantly [9]. In storage, the SCOBY will continue to ferment its storage mixture, albeit at a slower rate than if left at room temperature. To maintain SCOBY viability, replenish the storage mixture every 4 – 6 weeks by removing 50 – 80% of the liquid and replacing that with new sweetened tea [8]. The main idea is to provide continuing fuel for the microorganisms. It is also possible to simply add ¼ cup sugar per quart of storage mixture every 4 – 6 weeks [8], but keep in mind that the dormant microbes are still carrying out cellular functions which require nutrients and water. The stored SCOBY will reduce the volume of its storage mixture, and so additional tea is required to prevent the storage mixture from drying up.

4. Second fermentation.

Pour the kombucha into bottles and cap them, leaving the bottles out at room temperature. If a flavored kombucha is desired, this is the step to add flavoring ingredients. Although SCOBY was removed at the end of the first fermentation, not all the microorganisms were attached to the cellulose matrix, especially if the microbes were newly-cloned during that previous fermentation period. There will still be kombucha microbes present to perform a second fermentation.

As this second fermentation occurs in a closed system, CO2 produced from the yeast cannot escape the kombucha as it did during the first fermentation. As a result, the kombucha becomes carbonated during this step. Further, the kombucha microbes will continue to metabolize any remaining sugar to produce lactic acid, acetic acid, ethanol, and CO2, so the kombucha will become less sweet but tangier.

After 1 to 3 days, depending on how quickly carbonation occurs, store the kombucha in the fridge. This stops fermentation and carbonation because the significantly decreased temperature causes the microbes to go into a dormant state. And voilá! You have your first batch of kombucha!

Photo credit: thedabblist (64636759@N07/Flickr)

Photo credit: thedabblist (64636759@N07/Flickr)

While making kombucha is a lengthy process that can take up to two weeks to complete one batch, and perfecting the recipe to your own taste will involve making many batches, there is perhaps nothing more satisfying than a successful and delicious kitchen experiment.

The process described in this post was based off of kombucha recipes from The Kitchn and Food52.

References cited

  1. Thoughts on Re-steeping. Teatrekker’s Blog. 22 Sept, 2013.
  2. Science of Bread: Yeast is Fussy about Temperature. Exploratorium.
  3. McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Simon & Schuster, 1997.
  4. pH Values of Common Drinks. Robert B. Shelton, DDS MAGD.
  5. Singh, S., Jindal, R. Evaluating the buffering capacity of various soft drinks, fruit juices and tea. Journal of Conservative Dentistry, 2013; 13(3): 129-131.
  6. Rault, A. Bouix, M., Béal, C. Fermentation pH Influences the Physiological-State Dynamics of Lactobacillus bulgaricus CFL1 during pH-Controlled Culture. Applied and Environmental Microbiology, July 2009; 75(13): 4374-4381.
  7. Hwang, J. W., Yang, Y. K., Hwang, J. K., Pyun, Y. R., Kim, Y. S. Effects of pH and dissolved oxygen on cellulose production by Acetobacter xylinum BRC5 in agitated culture, 1999; 88(2): 183-188.
  8. Take a Break from Making Kombucha Tea. Cultures for Health.
  9. Lahtinen, S. J., Ouwehand, A. C., Reinikainen, J. P., Korpela, J. M., Sandholm, J., Salminen, S. J. Intrinsic Properties of So-Called Dormant Probiotic Bacteria, Determined by Flow Cytometric Viability Assays. Applied and Environmental Microbiology, July 2006; 72(7): 5132-5134.

Alice PhungAbout the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.

Read more by Alice Phung


5 thoughts on “Kombucha Brewing: The Process

  1. Pingback: Here’s to optimal health! – homemaderemediesblog

  2. You said above: obligate anaerobe, meant was aerobe. Anaerobe is without oxygen, aerobe is with. Acetonacter requires oxygen.

  3. When I put my SCOBI into the tea and then came back a day later, the SCOBI had disappeared. Is that supposed to happen? It seems as if the SCOBI dissolved or something. Does it dissolve and then reconstitute itself?

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