Aerobic vs. Anaerobic & Kombucha Fermentation

Aerobic fermentation and anaerobic fermentation are two types of fermentation processes that differ in their use of oxygen.  Aerobic fermentation is a type of fermentation that occurs in the presence of oxygen, while anaerobic fermentation is a type of fermentation that occurs in the absence of oxygen.

Aerobic fermentation is a common process that is used in the production of many different products, including beer, wine, bread, and yogurt.  In aerobic fermentation, the microorganisms that are used to ferment the product consume oxygen in order to produce energy and convert the sugars or other organic compounds in the product into the desired end products.  This process typically produces a small amount of carbon dioxide (CO2) and alcohol, as well as other byproducts such as lactic acid, acetic, and gluconic acids.

Anaerobic fermentation, on the other hand, is a type of fermentation that occurs in the absence of oxygen. This type of fermentation is commonly used in the production of products such as pickles, sauerkraut, and kimchi. In anaerobic fermentation, the microorganisms that are used to ferment the product do 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.

The main difference between aerobic and anaerobic fermentation is the presence or absence of oxygen. Both types of fermentation are important in the production of a wide variety of products, and the type of fermentation that is used depends on the specific product and the desired outcome of the fermentation process.

So which is kombucha?

Like with most things kombucha, it’s complicated — kombucha contains both aerobic and anaerobic bacteria and yeast.  So some of the bacteria and yeast in the brew do require oxygen in order to grow and multiply.  This is why 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 sugars in the tea and 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.

And how does kombucha actually ferment?

At it’s most basic level, it’s pretty simple.  Sweet tea is fermented with the 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.  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.
3. 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.
4. 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.
5. 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.