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When someone is asking this question they are usually wondering if they can seal the fermentation container for the first fermentation and restrict the microorganisms in the culture access to oxygen doing this essentially cuts off their air supply which affects various microorganisms differently.
Kombucha Fermented without oxygen is less tangy and more alcoholic. Acetic acid bacteria, which requires oxygen to grow, gives kombucha its sharp tangy flavor which would be missing without oxygen. The yeast species survive with or without oxygen but produce more alcohol in anaerobic conditions.
There are three main fermentation microorganisms in a kombucha culture which all react to the presence of oxygen in different ways. Acetic acid bacteria (AAB), lactic acid bacteria (LAB) and yeast. Each of these species of microorganisms contribute to the flavor of kombucha and their exposure to oxygen will affect their growth.
Why Kombucha Cannot be Made Using an Airlock
Airlocks are great for making alcohol because yeast becomes less efficient in anaerobic environments which produces more alcohol. The acetic acid bacteria in the culture cannot grow without oxygen which prevents them from consuming the alcohol and producing vinegar. When it comes to kombucha this is not usually what you want.
Airlocks are designed to let carbon dioxide to escape from the container but prevent any oxygen from entering. This will raise the alcohol level, reduce the tangy flavor produced by the acetic acid bacteria and increase the sour flavor from lactic acid bacteria.
Kombucha needs oxygen to maintain a balance of acetic acid bacteria, lactic acid bacteria and yeast species. All of which add to the flavor and health properties of this ancient drink.
How Oxygen Affects Acetic Acid Bacteria in Kombucha
Limited Oxygen Alters Kombucha Flavor
The familiar tang of kombucha is produced by the acetic acid bacteria found in the culture. These bacteria produce vinegar as a byproduct of their respiration which acts to acidify the liquid. The lower pH prevents unwanted bacteria and mold growth that can cause spoilage of the kombucha.
Without AAB the flavor will be less tangy and more yeasty with different esters and flavor components which taste and smell more like beer or sour depending on the lactic acid content.
Higher Alcohol Content
Acetic acid bacteria are important for keeping the alcohol content of kombucha down. They do this by consuming the alcohol which the yeast produces to make acetic acid. It is exactly the same process which converts alcoholic beverages into vinegar.
This is why when making wine or beer you want to restrict the fermentations access to any oxygen. Without the oxygen the alcohol produced remains in the liquid rather than being used as fuel.
Slow SCOBY Growth
The subspecies of AAB, gluconobacter, is credited with the production of cellulose. This cellulose is what makes up the structure of the SCOBY which helps protect the fermentation from contamination and contains many of the bacterial cultures needed to make kombucha.
Without oxygen there would be little if any SCOBY growth. The SCOBY will most likely sink to the bottom of the container where the aerobic bacteria found in and on it will die or go dormant making it no longer valuable for starting another batch.
How Oxygen Affects Yeast Species in Kombucha
Various yeast species live in a kombucha culture. All of which produce various flavors and esters which produce the rich flavor found in most mature kombucha cultures. In winemaking the yeast is a main component of the terroir of a vineyard and is unique to each location, year and current weather conditions. The same is true with kombucha making with the exception that the yeast culture tends to mature the longer it has been used.
Kombucha made in an aerobic environment provides a low ABV level in the primary fermentation stage. This helps the yeast species which are not alcohol tolerant to thrive and compete with more alcohol tolerant species helping to maintain a balanced, stable culture.
When yeast is placed into an anaerobic environment its respiration alters to compensate. When it does this it becomes less efficient which produces more alcohol. As the alcohol level increases and the low alcohol tolerant yeast species become stressed they die off. This die off does two things:
Produces a yeasty flavor in the kombucha
The yeasty flavor found in some alcoholic beverages is the product of dead yeast bodies found in the liquid. This is prevented in the winemaking by racking the wine and leaving the lees in the bottom of the carboy but in beer and kombucha where it is desirable to carbonate the beverage yeast becomes a flavor component.
Too much contact with the lees in kombucha produces a highly yeasty flavored beverage which can be unpleasant.
Raises the Alcohol Content in the Kombucha
Most yeast species found in kombucha cultures are not very alcohol tolerant with an upper end of about 4% ABV. Once this level has been reached they begin to die off and are replaced by high alcohol tolerant yeast species, the main player being Saccharomyces cerevisiae. This is the yeast most often found in alcoholic fermentations like wine and beer. It has an alcohol tolerance of up to 18-20% before it dies off.
How oxygen affects the Lactic Acid Bacteria in Kombucha
Lactic acid bacteria in kombucha is usually only active at the bottom of the kombucha container where the oxygen level is the lowest and there is less competition for food from the acetic acid bacteria. Yeast species are also present in this environment so they are kept under control and do not produce large amounts of lactic acid.
Produces a sour flavor
When kombucha is fermented in an anaerobic environment the lactic acid bacteria has a chance to compete with the yeasts directly throughout the fermentation which produces different flavors not usually found in kombucha. Lactic acid has a sour flavor rather than the usual tangy flavor produced by AAB.
Can Kombucha Get too much Oxygen?
Although oxygen is needed for kombucha fermentation it is possible for it to get too much oxygen.
When kombucha has too much access to oxygen the balance of the various microorganisms change. Yeast reproduce rapidly in the presence of oxygen and can quickly overwhelm the culture. Some highly acid tolerant species of acetic acid which grow exceedingly fast quickly convert the alcohol produced by the yeast into vinegar.
As a result the kombucha will take less time to reach the proper sweetness but will lack the depth of flavor which the other microorganisms provide. It is like comparing industrially produced red wine vinegar with aged balsamic vinegar. The kombucha just does not have enough time to mature.
How Your Kombucha May Get Too Much Air
Surface area too large
When making kombucha it is good to use a wide mouthed vessel so there is enough surface area for oxygen to be absorbed into the liquid and carbon dioxide can escape. If the depth of the container is less than the diameter of the opening then the surface area is too large. Either add more liquid or restrict the airflow around the container.
Too Much Airflow
By placing your kombucha fermentation by an air vent or heating grate the air movement can lead to a high level of oxygen exchange especially if it is warm air. The old carbon dioxide laden air coming off the surface of the SCOBY is replaced too quickly with fresh oxygen laden air.
Remedy this by either moving your kombucha fermentation elsewhere or covering the vent or heating grate. You could also use a thicker cloth on the surface of the container or use a container with a narrower opening.