Why It Works

  • Selecting a microbially-rich SCOBY and starter tea jumpstarts fermentation, resulting in an active and vigorously bubbly beverage.
  • Storing at a slightly warmer temperature (around 80 degrees) ensures consistent and rapid fermentation.
  • A slightly increased amount of sugar in the primary fermentation provides just enough residual fuel for the secondary fermentation, which gives you greater chances of carbonation.
  • Bottling at a slightly warmer temperature (around 80 degrees) speeds up carbonation, shortening the time it takes to produce a bubbly, fizzy drink.

I have a love-hate relationship with kombucha. Don’t get me wrong—I love the odd bottle of GT’s Brand from time to time; how can you resist a beverage with names like “Mystic Mango,” “Strawberry Glow,” or “Guava Goddess”? On the other hand, I find that GT’s kombucha tends toward the aggressively tart side, which can overpower the subtleties of other flavors. And more than anything, commercial kombucha can put a serious dent in your wallet if you consume the stuff with any regularity.

Which brings us to brewing your own kombucha. Homebrewing opens multiple avenues for flavor and customization—tartness, fruitiness, fizziness, and sweetness to name a few. And for the price of one bottle of GT’s kombucha, you can brew an entire gallon of the stuff—not a bad deal. 

I also realize that kombucha evokes certain tropes: health-conscious, California-crunchy maker culture, or even that over-eager line cook who just got back from a stage in Copenhagen having discovered the marvels of fermentation*. But you know what—who cares? Brewing kombucha is awesome—and with the right direction, it’s easy.     

*I am guilty of this. I am a walking meme.     

Kombucha has seen a massive rise in popularity over the past decade. It feels like you can’t pass two aisles in a grocery store without running into a shelf loaded with bottles of the stuff. Heck, you can even find a respectable selection at the Speedway off the interstate. Maybe it’s the tart flavor, the slight sweetness, or that unmistakable effervescence. Or maybe it’s the hazy promise of ‘good health,’ conveniently packaged in a fancy bottle, ready to wash your mortal sins away. 

Serious Eats / Amanda Suarez


Whatever its appeal may be, kombucha comprises a more than 1-billion dollar market value globally.* By 2025, that number is projected to be anywhere from 6.5 to 8.15 billion dollars. For comparison, Sprite had a market value of just over 6 billion dollars in 2020. 

*In 2019, the global market value for kombucha was estimated to be 1.36 billion dollars, of which 600 million dollars was concentrated in North American markets.

But what is kombucha, beyond a trendy ‘health’ drink? Where does it come from? And how do you make it? 

What Is Kombucha?

Kombucha is a fermented beverage that is traditionally made from sweetened green or black tea. Its funk, tartness, and refreshing carbonation depend on the activity of various microorganisms—bacteria and yeasts. Under the right conditions, those bacteria convert sugar in the tea to different kinds of acid, lowering the pH and creating tartness, while the yeasts metabolize sugar to produce carbon dioxide under pressure. What you’re left with is a sour, bubbly, slightly sweet concoction that has been consumed for hundreds, if not thousands, of years for its various purported health benefits and its deliciousness. 

The Murky History of Kombucha

The origins of kombucha (a.k.a. tea fungus and mushroom tea) are unclear. Some believe that the drink originated in China as early as 220 BCE at the start of the Qin Dynasty, tracking the spread of tea as it made its way along the Silk Road (other sources set the date one year earlier at 221 BCE, which is why World Kombucha Day is celebrated on 2/21). Prized for its apparent healing properties, by 414 AD the beverage spread to Japan: According to lore, a Korean doctor named Kombu (or Komu-ha) brought the fermented tea to Japan in order to treat the ailing Emperor Inkyo. While this timeline would be fitting, it should also be noted that people didn’t regularly drink tea in China until the Tang Dynasty (618-907 AD), at least 200 years later. 

Serious Eats / Amanda Suarez


By the time tea trade expanded to Europe, kombucha had gained popularity, particularly in Russia (as “kambucha”) around the end of World War I and into World War II, again for its supposed health benefits. In 1951, the Academy of Science in Russia and the Central Oncologic Research Institute in Moscow studied rural regions with low incidence rates of cancer, and eventually identified kombucha as a potential (though unproven) explanation, as it was popular in many of those areas. In the 1980s and early ‘90s, kombucha saw rising interest in the United States during the HIV/AIDS epidemic, as it was believed, according to 1996’s Kombucha: The Miracle Fungus, that the drink could bolster T-cell counts and support the immune system. But in 1995, kombucha hit a low point when the CDC reported two cases of severe metabolic acidosis, or a buildup of acid in the body due to kidney failure (one fatal) linked to the drink

Coincidentally, 1995 was the year that George Thomas “GT” Dave founded GT’s Living Foods, the largest and most successful kombucha producer in the world. And after many years, this beverage has grown into the wildly popular product we know today.

According to Kristin Hovzen, co-owner of the Cultured Pickle Shop in Berkeley, California, the story of kombucha is simply a story of probability. “[Kombucha] evolved with the tea plant,” Hovzen says. “Someone probably left a pot of sweet tea open and exposed to air. A fly landed in the cup, transferring acetic acid bacteria. Some film grew on top, preserving the tea, making it sour and funky.” The point is, there are many legends and plausible beginnings to kombucha, but as Hannah Crum and Alex Lagory write in The Big Book of Kombucha, “there is a kernel of truth at the heart of each one.”   

Are There Health Benefits to Drinking Kombucha?

Proponents of kombucha attribute all kinds of health benefits to the beverage: It’s a viable source of probiotics (healthy gut bacteria) that bolster beneficial gut flora, it boosts T-cell count and therefore strengthens the immune system, it cures cancer, it’s great for skin—it even does your taxes. But most, if not all of these claims have yet to be proven definitively. Officially, the FDA has not approved any probiotic for treating or preventing any health ailment. And even as the most serious of kombucha stans, I hesitate to make any such health claims. No, I’m here because I like the taste of kombucha and I like making it. 

How Do You Make Kombucha?

Making kombucha involves just three ingredients: sweetened tea, some starter kombucha, and a SCOBY, that floppy gelatinous thing that looks it came straight from the goo tank in the Matrix trilogy. You throw all of that into a big jar, cover it, and microbes get to work, converting sugars into acids and carbon dioxide. After seven to 14 days, (and after a couple more steps) you’re left with a tasty, tart, fizzy beverage.  

Serious Eats / Tim Chin



Of course, there are tons of details in this process, and plenty of opportunities to add flavor, but we’ll cover that later.

First, we need to understand the individual components of a kombucha brew. The most important are the SCOBY and the starter kombucha—the sources of microbes that drive fermentation.

What Is a SCOBY?

There is nothing more emblematic of brewing kombucha than a SCOBY. An acronym for Symbiotic Culture of Bacteria and Yeast, a SCOBY is the floppy, opaque, gelatinous layer that forms on the top of a brew of kombucha as bacteria and yeasts metabolize sugar. This biofilm layer, sometimes called a pellicle (or my favorite term, “zoogleal mat”), is largely composed of cellulose, a byproduct of fermentation in the presence of oxygen (which is why the layer forms at the exposed surface). It absorbs water readily—much like a sponge—and grows layer by layer. Since microbes are distributed throughout the solution, the sponge-like SCOBY is a valuable source of them, acting as an “inoculant” or starter for subsequent batches.

Serious Eats / Tim Chin


A SCOBY is sometimes described as a “consortium”—an association of different microbes that power fermentation. Its primary functions are twofold: serves as a source of microbes to jumpstart or power the fermentation, and as a protective layer for the liquid beneath that facilitates an anaerobic environment.

Technically, SCOBY is not a term exclusive to kombucha. Other examples of SCOBY’s include water kefir grains (tibicos), jun (a kombucha-like drink made with honey instead of sugar), and vinegar. Even a sourdough starter is a kind of SCOBY—a simple collection of bacteria and yeast. 

Do You Really Need a SCOBY to Make Kombucha?

There’s a certain mysticism and sentimentality behind owning, using, and sharing a SCOBY. Like a sourdough starter, a SCOBY is often thought of as an essential mother culture—the key source of microbes required to brew successive batches of kombucha. And due to its alien-like, slimy appearance, a SCOBY fits the ethos of crunchy, DIY homebrewers looking to share their “mothers” with friends and family alike. 

But here’s the truth: You don’t necessarily need a SCOBY to successfully brew kombucha. All you really need is good starting kombucha tea—liquid teeming with the very same bacteria and yeast contained in a healthy SCOBY. Given enough time and sufficient conditions, those microbes will form another perfectly viable SCOBY at the surface of the liquid.

Serious Eats / Tim Chin


A SCOBY isn’t precious—certainly not in the way that a sourdough starter might be treated. In a healthy brew, over time SCOBY’s tend to grow quickly and abundantly. So you can pick and choose the best ones for future projects. “I’m not going to say that SCOBY’s live beautifully and indefinitely,” says Hovzen. “I’m constantly recycling material and taking out pieces from ones I like. The idea that you have this one culture, that you use all the time…It’s not really how it works.”

What Kind of Fermentation Happens in Kombucha? Who Are The Players Involved?

Kombucha is the product of fermentation. The bacteria and yeasts that drive that fermentation inhibit the growth of unwanted pathogenic bacteria while producing byproducts that contribute to the flavor and texture of the finished beverage. It’s best to think of a kombucha culture as a diverse ecosystem of all kinds of microbes, each with different jobs, and existing in different numbers. The primary processes involved in kombucha brewing are aerobic respiration, alcoholic fermentation and acetic acid fermentation. Let’s look in greater detail.

Aerobic Respiration

In the presence of oxygen, yeast will break sugar (sucrose) down into fructose and glucose, in a process known as respiration. The main end products here are carbon dioxide (which escapes into the atmosphere) and water (which just stays in the solution). For our purposes, we are really only interested in the fructose and glucose in the initial stages of brewing kombucha.

Anaerobic Respiration a.k.a. Alcoholic Fermentation

Alcoholic fermentation is easy enough to understand: Yeast eats sugar, then poops out ethanol and carbon dioxide (that’s how most people put it, anyway), but—and this is key—only in the absence of oxygen. Kombucha brewing is interesting in that it utilizes these two byproducts at different stages of its development—similar to the fermentative process in beer making. Yeast converts fructose to ethanol, which is then metabolized by bacteria to produce acids. Additionally, during bottling, yeasts continue to convert residual sugar into carbon dioxide, carbonating the beverage. So yeast plays double duty (or triple duty, if you count aerobic respiration), so to speak.

Serious Eats / Amanda Suarez


There’s no definitive consensus on which kinds of yeast are most important to kombucha. Zygosaccharomyces, Candida, Torulaspora, Pichia, Brettanomyces, Schizosaccharomyces, Hanseniaspora, and Saccharomyces are just some of the more common genera found experimentally in brewing. In a sample of over 100 kombucha cultures, this study found that Brettanomyces was the dominant yeast overall. But for the average brewers like us, it’s not too important a distinction. No matter how many variables you adjust, you can’t really micromanage the relative populations of microbes in any given brew—microbes just do what they do, and you gotta live with that.

Does alcoholic fermentation affect the flavor of kombucha? Sure, but to a less obvious degree than acetic acid fermentation. In a perfect world, all of the ethanol produced by yeast gets converted to acetic acid. But if your brew is very yeasty, then that increased activity may result in a slightly boozy, sometimes bready or musty flavor (between 0.5 and 1% ABV). And depending on the species, some yeasts provide slight funk and background complexity.

How Boozy Will My Booch Get?

Alcohol is a natural byproduct of yeast fermentation. So no matter how you brew kombucha, there will always at least be trace amounts of alcohol. Can you control the amount of alcohol in your kombucha? Yes and no. For a more alcoholic result, you could increase the amount of yeast in the brew by adding yeasts from an external source (this would overwhelm the population of bacteria and encourage a surplus of ethanol). You could also increase the amount of sugar in the brew, which helps the yeast proliferate over time. But in the end, the relative population of yeasts to bacteria in a given brew is random, so precisely controlling ABV can be tricky.

Acetic Acid Fermentation

Acetic acid fermentation results in the tartness that characterizes kombucha, as well as the formation of the cellulose biofilm or ‘pellicle’ that comprises the SCOBY. Central to this fermentation are acetic acid bacteria (AAB for short), which convert ethanol or sugar to acetic acid, gluconic acid, and other byproducts. In any given brew, there may exist several species of AAB, such as Acetobacter aceti, Acetobacter pasteurianus, Gluconobacter oxydans, Komagataeibacter xylinum, and Komagataeibacter oboediens, each performing a slightly different fermentation task. For example, A. aceti converts ethanol (from the alcoholic fermentation) to acetic acid (the acid in vinegar), producing an aggressive acidity that we can taste. Other species like G. oxydans produce gluconic acid from glucose, which similarly lowers the pH—but without increasing the sensation of sourness. Gluconic acid is generally milder and sweeter tasting, lending complexity to the overall flavor of kombucha. Finally, some bacteria such as K. xylinum or K. oboediens also produce cellulose in addition to acetic acid*. The Komagataeibacter genus is perhaps the most abundant and important group of bacteria associated with kombucha—central to the development of the SCOBY and that characteristic tart flavor.

*K. xylinum is often used in the Philippines to make nata de coco, a jelly-like dessert grown on coconut water. So yes, technically the SCOBY is edible and commonly eaten!

Other Microbial Fermentations

In addition to acetic acid bacteria, lactic acid bacteria may be present in a given brew, but typically they occur in smaller amounts. In that same study of over 100 commercial kombucha starter cultures, Komagataeibacter was the most prevalent and abundant. LAB bacteria (such as Lactobacillus) are sometimes even added in commercial production to enhance the beverage’s probiotic benefit.

A Symbiotic Dance: How It All Comes Together

It’s easy to get lost in the weeds of all the specific microbes involved and all of their varied functions. But all you need to know is this: All of these fermentative processes are constantly occurring in a complex system that isn’t easy to describe in a linear way.

Here’s a simplified flowchart to help you visualize:

Graphic by Serious Eats / Amanda Suarez


So to review: When you first mix a fresh batch of sweetened tea with a dose of an existing batch of kombucha and a small piece of SCOBY, yeast get to work immediately, breaking sucrose into glucose and fructose through aerobic respiration. Cellulose-producing bacteria use those sugars to build a biofilm at the top. Once this new SCOBY forms and fully covers the surface of the liquid, oxygen levels decrease, which encourages yeast to ferment fructose into ethanol (and carbon dioxide), which bacteria then metabolize into acetic acid, turning the brew sour. That’s one heck of a dance. 

Brewing Basics

Now that I’ve clobbered your brain with microbial theory, let’s get on to brewing. There are typically two stages to kombucha brewing: the primary fermentation and the secondary fermentation. For many brewers and commercial producers, these stages are commonly known as “F1” and “F2.” The first fermentation is the longest stage of brewing, and primarily responsible for the overall tart flavor of kombucha. The second fermentation is generally shorter in duration (and even optional), and is mostly intended for carbonation and additional flavoring. 

The Primary Fermentation (F1)

The primary fermentation is where the magic happens. “Most of the work is in that primary fermentation,” says Alex Hovzen. If you hit F1, then the rest of the process should go smoothly. The bulk of the kombucha’s flavor and acidity develop at this stage, as acids accumulate and yeasts proliferate. To begin, you have to brew and mix the initial tea, and there are several considerations to make before starting out. 

Water

As with many fermentations, it’s best to use filtered, bottled, or distilled water when brewing tea for kombucha. Tap water can contain trace amounts of chlorine or chloramine as well as other hard minerals like calcium, which can impede microbial activity.

Sugar

In theory, the key here is to select a sweetener largely consisting of sucrose—which can be readily broken down by yeast into glucose and fructose. Refined granulated sugar is almost 100% sucrose, and it works great for brewing kombucha. For artisanal brewers like The Cultured Pickle Shop, raw cane sugar crystals are the sweetener of choice. But anecdotally—and in my own experience brewing—it’s entirely possible to make kombucha with alternative sugar sources. Maple syrup is mostly sucrose, and it performs well. But fruit or vegetable juices like mango, carrot, and strawberry juice can also work—sources which are mostly fructose. Even honey works—to the chagrin of some armchair kombucha purists—and that’s backed up by Korean recipes for jun, a fermented beverage virtually identical to kombucha. 

What sugar concentration should you use? The common standard ratio—championed by The Big Book of Kombucha (and even members of the r/kombucha subreddit)—is one cup of sugar per gallon of brewed kombucha. Can you use more, or less? Absolutely. Using less sugar means less potential acid in the finished beverage, and possibly a fermentation that isn’t so vigorous. On the other hand, using more sugar means you can have greater potential acidity (lower pH), but you also have an excess of sugar, so the balance of sweet-to-tart may be skewed in the sweet direction. It may take a longer period of time to reach the balance of sweet–sour that you’re looking for in your kombucha, since there’s a larger amount of sugar for microbes to metabolize.

If you want to get really nerdy, you can measure your starting sugar concentration in brix (°Bx), which is a unit of measurement to describe the sugar content of a liquid, measured as grams of sugar per 100 grams of water. Starting concentrations for kombucha can range from 6°Bx to as high as 15°Bx. For instance, many of the recipes in The Noma Book of Fermentation use a concentration of 12 brix or higher. To be honest, I found this concentration produced kombucha that was a bit too sweet in some cases (and the kombucha subreddit seems to agree). I found better results in the 8-9°Bx range, which (after doing some math) lines up with 1 to 1 ¼ cups of sugar per gallon of brewed kombucha.      

Tea (And if You Even Need it)

Traditionally, dried tea is essential to making kombucha, and for many it’s the only choice for brewing. The argument here is that tea provides trace amounts of nitrogen, as well as nutrients like caffeine, tannins, and theanine (a non-essential amino acid analogue that gives green tea its slightly savory flavor), all of which encourage microbial activity. Empirically, I found this to be true: Kombucha brewed from green, black, or jasmine tea was reliably sour and bubbly, and it was easy to brew successive batches from such tea cultures. But what if you’re caffeine sensitive? What if you don’t want or enjoy the tannic qualities of tea? Do you even really need tea to make kombucha?

Purists argue that if it doesn’t have tea, it isn’t kombucha. I won’t argue with you if you’re willing to die on the tea-only hill, but I will offer this: It’s absolutely possible to make a fizzy, fermented drink—one that looks and tastes like real-deal kombucha—without using tea. Just look at the entire catalog of kombucha recipes in The Noma Guide to Fermentation. None of those recipes utilize traditional teas, but instead rely on herbal tisanes and fruit or vegetable juices. 

“We brew herbal batches all the time,” says Hovzen of The Cultured Pickle Shop. There, they employ all kinds of fresh herbs in the first fermentation: Shiso, fennel, celery, and parsley are just some of their flavors. “I think herbs produce some of the more interesting, unique flavors that we do. Obviously, [the tea plant] Camellia sinensis is an important plant in the anthropology of food. But there are just so many different options for plant-microbial relationships that I think one could explore through kombucha.” 

Technically, you could even sustain a healthy SCOBY in sugared water. The bacteria, yeast, and residual acids will survive and grow in a kind of stasis; over time, the SCOBY will form new layers and grow in size. 

SCOBY

Typically, the best source for a SCOBY is a friend or someone in your neighborhood who is reliably brewing their own kombucha: SCOBY’s grow quickly and can easily take over a brewing vessel, so more likely than not, a kombucha brewer will be happy to offload excess SCOBY material (and if you’re lucky, some microbe-rich tea as well). 

If you can’t find a friend—or if you don’t feel all that comfortable with someone handing you a suspicious jar of gelatinous goo—then online or through a vendor is your next best option. But keep in mind that not all commercially-sold SCOBY’s are equal. SCOBY’s sold dry or unsubmerged in starter liquid tend to perform weakly in comparison to those sold fresh and packaged in starter tea. Finally, it’s absolutely possible to grow a SCOBY from scratch—provided that you have enough viable kombucha starter tea, and enough time. But for our purposes, we’ll stick to the traditional SCOBY method. 

The thickness of the starting SCOBY doesn’t matter too much—more SCOBY isn’t necessarily better for brewing, since a thick disc of SCOBY can displace enough liquid to overwhelm the jar. It’s also not clear if having more SCOBY at the start speeds up or improves fermentation: It would depend on how many microbes populate the culture. What may be important is the diameter of your SCOBY: If it completely covers the surface of the liquid, it acts as a slight barrier to oxygen, which helps to inhibit spoilage in the initial stages. Later on, that cap can partially trap carbon dioxide as it accumulates, which is helpful in determining the success of your brew when you taste it later down the line.    

Starter Kombucha (Inoculant)

Starter tea is an integral part of the initial brew. Not only is this tea a rich source of microbes, but it also lowers the starting pH of the solution, discouraging unwanted microbial growth and favoring yeast respiration and eventual fermentation. Adding just two cups of healthy, acidic starter tea to a one-gallon brew can lower the pH safely below 4.5, which lessens the chances of pathogenic growth in the initial stages of fermentation. The practice of adding starter tea from a previous batch (inoculant) to a fresh batch of sweet tea is known as backslopping.

Container 

The standard for primary fermentation is a one-gallon, wide-mouth glass jar. While it’s entirely possible to ferment in food-safe plastic, many people have reservations about trace chemicals leaching into the liquid, or abrasions that can harbor unwanted bacteria. Metal containers are a less popular alternative, since reactive metals like aluminum can disrupt fermentation. Non-reactive stainless steel is perfectly acceptable. (For those brewing larger batches of kombucha, a large brewing container with a spigot is a popular choice. The kombucha is poured off as needed, and continually topped off with sweet tea.)

Temperature

Like many fermentations, the optimal working temperature is ambient temperature. That can range from anywhere between 70 and 85°F. For instance, recipes in The Noma Book of Fermentation recommend a relatively high fermenting temperature of 82°F. At The Cultured Pickle Shop, the ambient temperature in the Bay Area sits right at 70°F. In general, the warmer the temperature, the faster the fermentation. Yeasts thrive at warmer temperatures, but acetic acid bacteria tend to favor lower temperatures. For a more tart, pungent kombucha, it’s better to ferment at the lower end of this range where the bacteria thrive. “If it’s too warm, you can get some pretty funky stuff,” says Hovzen. Flavors like uric acid can build up in the brew, which can taste unpleasant. On the other hand, if time is an issue and you just want kombucha fast, then fermenting around 80°F is better.

Time

Timing for the primary fermentation can range from one week up to one month. Why the discrepancy? There are many factors to consider: warm or cold ambient temperatures, the starting concentration of microbes in the starter tea and SCOBY, or the type of tea or plant material you use. Ultimately, you can only control so many factors. “If you think you have so much control, then you are vastly overestimating your role in this process,” says Hovzen. But over successive brews with your specific tea culture, you will get a feel for the timing.

How Do You Know When F1 Is Complete?

pH

One of the goals of F1 is to lower the pH of the brew enough to limit unwanted microbial growth, in addition to giving us that sour taste. We’ve mentioned that the starter kombucha lowers the pH instantly to 4.5 or even 4.0. But what is the final, target pH after fermentation? I’ve seen final pH values ranging from 2.5 to 4.5, but according to FDA guidelines, kombucha must have a final pH below 4.2, and greater than or equal to 2.5—measured either with pH strips or with a pH meter. At The Cultured Pickle Shop, Kristen Hovzen aims for a pH between 2.8-2.9. “It’s pretty acidic,” says Hovzen. “But it also has to be well-rounded.”    

Taste and “Texture”

While pH may be a more precise metric for knowing when the primary fermentation is complete, the best indicator is far more subjective. “When I pull kombucha to bottle it, it’s all done by taste,” says Hovzen. “I’m looking for a particular taste and I know what that is, but I certainly couldn’t put that in a language of metrics for you.” She describes a balance of sweetness and acidity that can be highly dependent on personal taste. Unlike GT’s brews, which Hovzen describes as aggressively sour, she prefers more rounded, tempered acidity. 

Properly fermented kombucha also has a certain lightness or bubbly texture on the palate when you taste it. While the jar is open to the air, there is still some carbon dioxide generated and trapped beneath the SCOBY during the first fermentation. “You can see bubbling underneath, and when you push the SCOBY aside to taste the tea, there should be a lot of activity going on in there.” That dryness, or bubbliness, is something you can feel on your tongue—not quite carbonation, but almost there, Hovzen says. 

Visual

We’ve already given one visual: The formation of tiny bubbles in the kombucha as carbon dioxide builds and slowly escapes; we see this activity as the bubbles rise to the surface. For kombucha brewed with tea, you can also look at the color. As fermentation progresses, the kombucha lightens in color relative to its starting point, a result of dark colored tannins in tea being metabolized over time.

The Secondary Fermentation (F2)

The Secondary Fermentation is entirely optional. Technically, after the primary fermentation, your kombucha is ready to drink. But a secondary fermentation provides two things: carbonation and potential flavoring. For many home-brewers of kombucha, these qualities are the entire point of making kombucha. Primary fermentation is the boring but essential foundation of the brewing process; secondary fermentation is the flashy, attractive step that gets all the credit—the iconic bottle, the big bubbles, the wacky flavors.  

Here’s the general process: After the primary fermentation, the kombucha is siphoned or divided into glass bottles with an airtight fit. Sometimes fruit or vegetable juice, whole fruit or vegetables, or sugar are added to the bottle (or mixed with the kombucha in a large bottling bucket prior to bottling). The bottles are left to ferment for a shorter period of time—generally 3 to 7 days (though timing could take longer)—until the kombucha is sufficiently carbonated. The bottles are then refrigerated, slowing any additional fermentation, until ready to drink.  

There are generally three broad decisions you have to make for a secondary fermentation: the type of bottle, the type of juice or additional plant material, and the type of sugar. 

Bottle Type

Most people bottle in glass exclusively. Why? Glass is inert, does not stain or scratch easily, and if properly cast it can withstand the pressures exerted by gas as it accumulates. It’s also transparent, so you can easily view the contents (and the pretty color) inside. Plastic is a far less popular alternative: It stains, it can sometimes contain trace chemicals that leach into the liquid, and if it is scratched, then those micro-abrasions can harbor unwanted bacteria. 

As far as shape, round is the way to go. It’s generally unwise to select a square or any sharp-sided shape, since the distribution of pressure is unequal—creating weak points in the glass. At worst, using square-shaped glass can result in unintended explosions. Plus, those fancy flip-top bottles with square sides aren’t specifically designed for carbonation in the first place.

For homebrewers, flip-top or swing-top cylindrical “Grolsch” bottles made of thick glass are the classic choice for bottling, and for good reason: They are affordable, widely available online or at homebrewing stores, durable, and their rubber stoppers form an airtight seal for reliable carbonation. The only downside? Flip-top bottles have an all-or-nothing switch, so they can explode when opened if the contents of the bottle contain too much carbon dioxide. In other words, there’s no way to gradually degas the contents of the bottle. For this reason, Hovzen uses twist-cap bottles for bottling. And as long as the twist caps are durable and the threads form a tight seal, you can easily capture good carbonation. 

Fruit or Plant Material

Adding any additional fruit or vegetable—in the form of whole chunks, puree, or juice—is a simple way to flavor kombucha, but it also supplies an extra source of sugar and sometimes wild yeasts to jumpstart carbonation. The existing population of yeasts metabolize these excess sugars, producing plenty of carbon dioxide, which is trapped in the liquid in the airtight bottle. Popular options include chunks of pineapple, ginger, berries, even herbs like mint. Ingredients can be fresh, frozen, or even cooked. Here are a few tips to keep in mind:

  • Whole chunks of ingredients produce a clearer finished result, but carbonation tends to be slower; you also have to strain out those chunks at the end when serving if you want a smooth, clear drink.
  • Purees increase the surface area of materials exposed to microbes, as well as the availability of sugar for fermentation; carbonation is generally faster, but the kombucha is often cloudy and usually contains a raft of pulp and increased sediment at the bottom.  
  • Strained juices contain less pulp, so they can be a better option than purees if you want a clearer result.
  • Purees and juices should not exceed 20 percent of the total volume of liquid, since the resulting solution might exceed the recommended pH range to be considered safe for consumption. 
  • Cooked syrups or jams offer another avenue for flavor; but they do not provide any added wild yeasts—just added sugar and flavoring. Carbonation may be slower compared to a fresh fruit juice or puree, since there is a lower relative population of yeast.

Sugar

Depending on how far you have pushed the primary fermentation, there may not be too much residual sugar left in your kombucha available for yeasts to metabolize. Presumably, there should be enough sugar in whatever added fruit you are using. But if you’re using a vegetable, or you want to increase your chances of successful carbonation, then it’s not uncommon to add a teaspoon or two of sugar to each bottle.

More Tips for Reliable Secondary Fermentation

Assuming you’ve got a vigorous and healthy primary fermentation, here are some extra tips to ensure you get a fizzy, bubbly result.

Stir Before Bottling

Stirring might seem like a simple act, but it can be an essential step for successful carbonation. The microbes in a jar of kombucha are not evenly distributed within the solution: You can see this as yeast strands dangle from the bottom of the SCOBY and collect at the bottom of the jar in a sediment layer. When you stir kombucha prior to bottling, the yeasts are more evenly distributed, so there’s a greater chance that there’s a healthy population of yeasts in each bottle.

Give a Little Headspace

It’s often recommended to leave a little headspace in the bottle. Why? Yeast are sensitive to both pressure and acidity; high pressure and high acidity cause the yeast to go dormant. If you don’t leave enough headspace, then pressure will build faster as carbon dioxide remains in the fluid (but there is less actual gas overall in the bottle). Carbon dioxide dissociates to carbonic acid in the liquid, raising the acidity of the solution. The combination of high pressure and acidity stresses the yeast, so the yeast go dormant before the bottles are properly ‘conditioned’ or carbonated. 

For this reason, it’s best to leave about 1 inch of headspace in the bottle, which allows gas to accumulate in the empty space, and then be forced down into the liquid. On the flip side, if you leave too much headspace, the gas may never force itself into the liquid, so you won’t get bubbles in your drink.  

Make Sure You Have a Tight Seal

A tight seal is essential for trapping gas in the bottle. Swing-top bottles usually feature a rubber stopper that creates a tight, flush seal. But it’s important to make sure that the lip of the bottle is dry and clean before closing. Similarly, twist caps should be tightened as far as possible, and the threads should fit the bottle cleanly.

Maintain Temperature

The ideal temperature range for secondary fermentation is between 70 to 80℉. Yeast thrive at warmer temperatures, so the fermentation typically goes quickly at the top end of this range—from 3 to 5 days.

Give It Time

The time range for secondary fermentation is usually less than 1 week. But if at the end of this period, your kombucha isn’t sufficiently bubbly, then you can leave the bottles to ferment for up to 2 additional weeks. Most of the time, carbonation does take place—but for a given batch, the concentration of yeasts might not be sufficient for a fast turnaround.

Just Keep Brewing

If all else fails, and you can’t seem to get any carbonation, then often times it’s best to just keep brewing. For new homebrewers—with a freshly procured SCOBY and unreliable starter tea—the community of microbes in the first brew may not be as robust and developed as you want. This community is constantly changing, and improves in activity over time, over successive brews. So if your secondary fermentation completely fails—despite starting with a fermented base that is pleasantly sour and sweet after the primary fermentation—just keep brewing. 

Storing SCOBY for the Long Term

In an ideal world, you might brew kombucha indefinitely and continuously: you brew tea, let it ferment, decant what you need into bottles, and brew more tea to continue the process. But people get busy, and it’s totally understandable if you don’t want to be tied to a constant cycle of brewing and bottling. If you can’t or don’t want to make another batch right away, storing a SCOBY for the long term is simple: Brew some more sweetened tea (or even sweetened water), add it to the jar with the SCOBY and starter tea, and let everything ferment at room temperature. Stored in this way, the SCOBY will thrive for upwards of eight to twelve weeks. The resulting tea is very acidic—generally far too acidic for the second fermentation. But it’s still microbially rich starter tea perfect for brewing subsequent batches of kombucha. 

Putting It All Together

By now you should have a complete idea of the steps and science behind brewing your own kombucha. Fortunately, putting it into practice isn’t all that heady. What follows is a simple recipe to get you started, for a truly foundational kombucha: jasmine tea. Once you’ve gotten comfortable with the rhythm of brewing and can produce consistent results, you should have the confidence to branch off into different flavors, like this recipe for blackberry-mint kombucha, and variations of the method, as I demonstrate in this recipe for strawberry, rhubarb, and parsley kombucha. And more than anything, this guide should give you all the tools to craft your own flavors, tailored to your tastes and preferences.  

Tim Chin

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