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Aerobic vs. Anaerobic Fermentation & Kombucha

Kombucha: The Aerobic-Anaerobic Enigma

In the fascinating world of fermentation, two processes are the stars of the show: aerobic and anaerobic fermentation. These processes are like the yin and yang in the universe of microbial activity, differentiated mainly by their relationship with oxygen. 

And in the world of kombucha brewing, a fascinating interplay of both takes center stage, orchestrating a symphony of flavors and health benefits that have enchanted enthusiasts for centuries. This dual process is a delicate dance where oxygen plays a pivotal yet enigmatic role, guiding the transformation of simple sweet tea into a tantalizing elixir of life.

Aerobic vs. Anaerobic Fermentation in Kombucha

Aerobic fermentation means fermentation in the presence of oxygen. And in kombucha it sets the initial stage, inviting a lively ensemble of gluconobacter and acetobacter to flourish, initiating the vibrant fermentation fiesta. These aerobic artists weave a rich tapestry of flavors through a meticulous conversion of sugars into a harmonious blend of beneficial acids.

Yet, as the aerobic celebration reaches its peak, a mysterious transition occurs. The bacteria ingeniously form a pellicle, a protective shield that seals the environment, subtly nudging the process into a more anaerobic realm. 

Anaerobic fermentation means fermentation in the absence of oxygen, and this is phase less explored in kombucha science. In anaerobic fermentation, the microorganisms not require oxygen in order to produce energy and convert the sugars or other organic compounds in the product.  Instead, they rely on other electron acceptors, such as nitrogen or sulfur, in order to produce the energy they need to carry out the fermentation process. This type of fermentation is commonly used in the production of products such as pickles, sauerkraut, and kimchi. Alcohol production is also considered an anaerobic process, but many yeasts that produce alcohol can also function and thrive in the presence of oxygen.

And this anaerobic chapter, though not yet fully unraveled in scientific literature for booch, is something experienced brewers can tell you exist.  While it's the yeast that forms carbonation in F2 (and bottle bombs), our taste buds tell us that when kombucha is in a sealed environment for a long period (even in the refrigerator), it becomes more tart and more funky.  How is this happening with bacteria that are supposed to be aerobic?

Well, we don't really know yet (scientifically speaking)...but it's a phenomenon that hints at a deeper understanding and respect for the brew's dynamic nature.  And we can make some educated guesses. So join us on a voyage of discovery where science meets tradition, unraveling the secrets of kombucha's aerobic-anaerobic enigma.

So what's going on in kombucha?

Like with most things kombucha, it's complicated.

Kombucha contains both aerobic and anaerobic microorganisms.  And many of the "anaerobic" microorganisms responsible for fermenting kombucha (and beer, etc.) actually do require oxygen in the initial stages of fermentation to synthesize sterols and unsaturated fatty acids, which are vital components of their cell membranes. So: aerobic bacteria + anaerobic yeast that need to breathe at the beginning (at a minimum) = why your kombucha needs to "breathe" during the fermentation process.  And because it needs to breathe, we consider it an aerobic ferment.

When kombucha is first brewed, the bacteria and yeast are added to the sweet tea along with some oxygen from the air.  This initial supply of oxygen allows the bacteria and yeast to grow and multiply, and also helps to kickstart the fermentation process.  As the fermentation process continues, the bacteria and yeast will continue to consume the sugar in the tea and produce the bacteria will produce the beneficial acids and enzymes kombucha is know for. They'll also continue to consume oxygen and need more of it as they use what's in the liquid -- which is why you use a breathable cover during F1.

F2 is a little bit different story.  It's similar in the first part, as when the finished kombucha is blended with fruits, juices, flavorings, etc. and transferred to the secondary fermentation vessels, it will pick up oxygen along the way.  But because the secondary fermentation vessels (usually bottles for the home brewer) are sealed, the fermentation process going on inside will eventually use up all of the oxygen in the kombucha, at which point the fermentation will become anaerobic.

Anaerobic Fermentation in Kombucha - The Great Mystery

While the aerobic nature of kombucha fermentation is well-documented, the potential role of anaerobic fermentation remains largely unexplored. We hypothesize that alongside the dominant aerobic fermentation process, an anaerobic fermentation pathway also plays a substantial role in kombucha fermentation, potentially mediated by certain strains of Gluconobacter and the presence of lactobacillus species — and possibly contributing to the complex flavor profile and the health benefits.

Gluconobacter species are traditionally considered aerobic bacteria, however, the full spectrum of metabolic capabilities of Gluconobacter remains understudied. Given that the pellicle formed during kombucha fermentation creates a barrier, limiting oxygen diffusion into the liquid, it sets a premise for a substantial anaerobic phase.

It is plausible that certain strains of Gluconobacter exhibit anaerobic properties, allowing them to switch between aerobic and anaerobic metabolic pathways depending on the oxygen availability. Additionally, Lactobacillus species are already known to be anaerobic microbes, and their presence in the kombucha microbial consortium suggests a potential role in anaerobic fermentation.

But delving deeper, recent studies have shed light on the fascinating microbial dynamics at play in the kombucha brewing process. The presence of cyanobacteria, known for their oxygen-producing photosynthesis, hints at a self-sustaining ecosystem within the kombucha environment. This suggests the kombucha system might be more self-contained than previously thought, with the pellicle potentially fostering and encapsulating a relationship between different microbial entities, facilitating a harmonious exchange of essential nutrients and gases, including oxygen.

Why such a difference in results?  That one's easy...different source cultures!

And this intricate microbial dance could also explain the continuous thickening of the SCOBY over time, even in the absence of sugar. We hypothesize that this phenomenon might be driven by the ongoing metabolic activities of the microbial community, which could potentially continue as long as there are sufficient nutrients available.  Nutrients derived possibly from the breakdown of other components in the tea -- or by the microbes themselves. The presence of a plethora of microbes in kombucha opens up avenues for many biochemical pathways.

One thing is for sure: it becomes increasingly clear that the process is a complex, multifaceted interplay.  Kombucha isn't just a brewing process; it's a journey of discovery and a frontier of knowledge waiting to be explored.  So let's brew, let's explore, let's unravel the secrets of kombucha together, with Raw Brewing Co. guiding you every step of the way.

And how does kombucha actually ferment, step by step?

At it's most basic level, it's pretty simple:

  • Sweet tea is fermented with naturally occurring yeast and bacteria, which convert the sugar to ethanol (alcohol). 

  • As the brew continues to ferment, the alcohol is converted to gluconic and acetic acids and other beneficial compounds. 
  • During fermentation a pellicle also forms as a film on the surface of the tea as it ferments, and it helps to protect the kombucha from contamination by other microorganisms. The fermentation process typically takes about 5-10 days, depending on the temperature and other factors. During kombucha fermentation, a variety of chemical and biological processes take place.  Below are some of the key steps that occur:
  1. Fermentation:  This is the process by which the bacteria and yeast feed on the sugars in the tea to produce a variety of compounds, including acids, alcohol, and carbon dioxide. The acids give kombucha its characteristic tangy flavor, while the carbon dioxide gives it its characteristic fizziness.  The specific compounds produced during fermentation depend on the type of bacteria and yeast present in the scoby, as well as the conditions of the fermentation process. However, some of the key compounds produced during kombucha fermentation include organic acids such as gluconic acid, acetic acid, and lactic acid, as well as alcohol and carbon dioxide.

  2. Formation of the pellicle:  This is a layer of bacteria and yeast that forms on the surface of the brew.  The scoby helps to protect the kombucha from contamination by other microorganisms, and it also contributes to the flavor and texture of the finished product.  It is formed through the process of bacterial cellulose production and cell aggregation.  This process involves the attachment of bacterial cells to each other, and it is facilitated by a variety of factors, including chemical signals and physical interactions. Bacterial cellulose is a unique form of cellulose that is produced by certain species of bacteria. 

  3. The biological processes involved in the production of bacterial cellulose involve the synthesis and secretion of cellulose by the bacteria.  The specific biochemical reactions involved in the production of bacterial cellulose are complex and not well understood, but they are thought to involve the creation and polymerization of saccharides (sugar) into long chains of cellulose and form the basis of the bacterial cellulose. This process is facilitated by enzymes found in the bacterial cell membrane.
  4. Production of beneficial compounds:  As the bacteria and yeast feed on the sugars in the tea, they also produce a variety of beneficial compounds. These include the organic acids listed above as well as antioxidants and other compounds that may have health benefits.  The production of these compounds is the result of a variety of biochemical reactions, including the breakdown of sugars and synthesis of new compounds.  The specific biochemical reactions involved in the production of beneficial acids by bacteria are also not well understood, but they are thought to involve the breakdown of glucose and other sugars into simpler molecules, such as pyruvate, which can then be further metabolized by the bacteria.  These processes are facilitated by a variety of glycolytic enzymes found in the cell wall -- and other biological molecules.

  5. Flavor development:  This is the result of a combination of factors, including the type of tea used, the sugar source used, the length of fermentation, and the presence of any additional ingredients such as fruit or spices. The specific flavor compounds will result in of a variety of biochemical reactions of their own.

  6. Maturation (F2): Once the kombucha has reached its desired flavor, it is ready to be bottled to continue to develop its flavor and fizziness.  It is also during this stage that any additional flavoring ingredients, such as fruit or herbs, are added to the kombucha.  The specific biochemical processes that occur during F2 depend on the ingredients added to the kombucha, as well as the conditions of the maturation process.  However, some of the key processes that occur during this stage include the breakdown of sugar and other compounds, the synthesis of new compounds, and the release of carbon dioxide -- the CO2 is what makes your brew fizzy and why it's called carbonation ('carbon'-ation).
Overall, kombucha fermentation is a complex process that involves a complex network of biochemical reactions that produce a variety of compounds that contribute to the flavor aroma, and health benefits of the finished product. The end result is a tangy, effervescent beverage that is rich in beneficial compounds and has a unique flavor.

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