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Anyone who has been making kombucha for a while has had the experience of forgetting about a batch in the back of a cupboard or left a bottle of kombucha soda fermented too long. Upon tasting this over fermented kombucha you will declare that it has turned to vinegar. It will be super sour and have no sweetness left from the sugar or flavoring you added when you made it.
Although you’re over-fermented, kombucha tastes really sour or the soda has a tanginess to it that makes it undrinkable. It may not be considered vinegar. Vinegar, like kombucha, is fermented with two primary microbial cultures, yeast and acetic acid bacteria (AAB). The yeasts make alcohol and the AAB converts the alcohol into acetic acid.
Commercial vinegar must have an acetic acid content of between 4% and 14%. This is because vinegar with a lower percentage has the danger of getting infected with pathogenic bacteria detrimental to your health. Although this is unlikely that it would happen to over fermented kombucha without the minimum acidity it still is not officially vinegar.
Here are seven steps to make kombucha vinegar with at least 4% concentration level:
Make strong tea
When making kombucha that is meant to be drinkable like kombucha soda the tea should not be too strong. Tea which is too strong can make the kombucha bitter and highly caffeinated but when making kombucha vinegar the flavor of the tea will quickly get overwhelmed by the acetic acid and lactic acid.
To counter this effect you need to make a strong tea which will still be detectable in a strong acidic solution. When intentionally making kombucha vinegar I double the amount of tea which goes into the same amount of water. After a few batches you will find a balance you like but for the first one add at least 1/3 more tea to the water as normal for you.
Let it steep longer than normal also as this will help the tannins and other nutrients disperse into the water more thoroughly. Do not be concerned with the caffeine levels or the bitterness it the leaves as both will mellow as the vinegar ages and you will want the additional body in the final vinegar.
Add the correct amount of sugar
Vinegar is a fermentation driven by the growth of acetic acid bacteria which to grow efficiently and produce acetic acid need the presence of alcohol. The alcohol comes from the yeast activity in the liquid which is dependent on the amount of sugar in the tea.
To get a vinegar with a 4% acetic acid level at least you need enough sugar for the yeast to produce an ABV of 8%. This means you need to add 3 ¼ cups of sugar for a one US gallon batch of kombucha vinegar. Which seems like a lot of sugar for kombucha and it is if you are going to drink it but you aren’t going to drink it you are going to make it into vinegar.
The yeasts in your starter liquid and SCOBY are not all high alcohol producing yeast species rather most of them are low alcohol producing species. The famous Saccharomyces cerevisiae which is the yeast which takes over when making wine or beer are very efficient but because kombucha is a low alcohol drink these less efficient yeasts can proliferate the sweet tea. As a result less alcohol is produced giving the acetic acid bacteria less alcohol to make acetic acid.
All that to say you need more sugar than normal to make kombucha vinegar. If you follow the ratio above your sweet tea will have a specific gravity of about 1.061 or above. This should give the yeast adequate sugar to produce enough alcohol for the AAB to convert to vinegar.
Add the starter liquid and SCOBY
The starter liquid should be from a recent batch of kombucha, not one which has sat for a while. You want an active starter liquid with lots of live yeast and AAB cells in it. This will kick off the fermentation quickly as the yeast can rapidly multiply in the presents of available sugar. With a sluggish starter liquid the tea will be at risk of unwanted bacterial and mold growth, giving the vinegar an off flavor.
The SCOBY will provide the needed active AAB cells which will begin to convert the alcohol in the tea faster than if it was a dormant SCOBY. This is necessary as you want to keep the alcohol content as low as possible to allow all the yeast species to contribute to the flavor rather than only the high alcohol tolerant ones.
Ferment until most sugar has been consumed
The concentration of the sugar in your kombucha vinegar will drop rapidly at the beginning of fermentation but as the easily fermentable sugar is depleted the population of the yeasts will diminish as well. This combined effect will slow the rate the sugar is used until there is little sugar and few active yeast cells in your vinegar.
This process can take a month to six weeks depending on the temperature, population of the yeast species, population of the AAB species and how much sugar was added. After a month the vinegar will have a low enough pH to ward off any unwanted microorganisms and the population of the various species in the vinegar will be declining.
At this point you can check to see how much sugar is left in your vinegar a number of ways:
Use a hydrometer
A hydrometer is a weighted glass float with a scale on the inside. When it is floated in a liquid how high it floats indicates how dense the liquid is.
The concept is that water has a certain density, which on the scale is 1.000 for distilled water, when you add sugar which dissolves into the water it becomes denser. The more sugar the denser the water. As the microorganisms in your vinegar eat the sugar the density of the water will go down.
This is used mostly in beer and wine making as an indication of the potential alcohol level in a batch of wine or beer and when to rack or bottle. For a complete explanation of the use of a hydrometer read “Determining the Concentration of Sugar(S) in a Soft Drink on the Basis of Density Using a Hydrometer”.
What to do:
- Fill a clean long thin flask with your vinegar
- Over the fermentation vessel gently lower the hydrometer into the flask
- Read the level where the hydrometer floats at the surface of the vinegar
- Pour the testing vinegar back into the fermentation vessel
A reading which is less than 1.000 indicates that the vinegar is less dense than water and that most of the sugars have been consumed. This is not completely accurate of course because your vinegar is no longer just water and sweet tea. It also has less dense acetic acid, alcohol and lactic acid in it which all are less dense than water.
Using a chemical color test
This method relies on reactions with special chemicals in the presence of sugar which changes the color of the solution in predictable ways. So if you mix the chemical with your vinegar the chemical will react with the sugar in the vinegar and change color. The color indicates the amount of sugar.
For a fuller explanation read “Using Simple Chemical Indicators”.
This method requires special chemicals and experience. Both of which can be hard to aquire.
Taste test
Taste the vinegar to see what it tastes like. If it is super tart and tangy then there will be little sugar left. Unfortunately this method is not very reliable as everyone has a different “sweetOmeter” but after a while you can be fairly certain when your vinegar is ready for the next step.
Pressure bottle test
Pour some of the vinegar into a pressure safe bottle and seal it. In about a week open the bottle. If there was any pressure in the bottle the vinegar is still active so leave it for another week or two.
Filter and age your vinegar
Filtering your kombucha vinegar at this point is important to remove the dead cells and other remnants found on the bottom of the jar. Without this step the vinegar will take on a yeasty off flavor as the flavors of these cells (which are mostly dead yeast cells) permeate the vinegar.
Pour the liquid through a tightly woven cotton cloth and let drain until most of the liquid has flowed through. Do not squeeze or otherwise force the liquid through as this will force some of the sediment through the cloth.
You can either discard the SCOBY or use it for another batch of kombucha or vinegar.
Once your vinegar has stopped actively fermenting it is time to filter it and put it into an ageing container. For this you have three choices:
A sealed container
A sealed container will prevent air flow in or out of your vinegar and allow all of the flavors found in the vinegar to meld together. The danger here is that there may be fermentation continuing for a while and pressure may build up in the sealed container. To avoid explosions and loss of vinegar only seal young vinegar in pressurizable bottles and every month or so burp the bottle to release any pressure. Extend the period of time you handle the bottles depending on if there is built up pressure or not.
An air locked container
These are mostly used for racking wine. They let pressure out (CO2) but no air in. This allows some volatile elements in your vinegar to escape along with the CO2 which can mellow the vinegar out.
Air locks are available at any wine or beer making store and are very inexpensive. Other methods such as a hose into a glass of water also works but are less reliable as the glass can often be knocked over allowing access to the vinegar to bugs (mostly fruit flies) which destroy the vinegar.
Open air container
A wide mouthed jar with a cloth held on with an elastic band works fine for this method. It allows free flow of air in and out of the vinegar and produces the most smooth tasting kombucha vinegar. Storing your vinegar this way allows the acetic acid in the vinegar access to oxygen allowing it to continue to convert any alcohol into acetic acid. It will also continue to grow a SCOBY as it is mostly the AAB which produce it in kombucha.
Bottle and age
Once the vinegar is a year old or so it is time to bottle and store it. Bottle it in sealable containers which can be opened and closed often without a lot of fuss. Swing top bottles are great for this. Store the bottles as you would wine in a cool dark location.
Alternatively you could get a wooden barrel to store it in. This will add another level of flavor to your kombucha vinegar. The type of wood will give it a deeper texture and add its own flavors as the acid in the vinegar reacts with the wood in the barrel. This evens out the vinegar and adds esters and other flavor compounds to it.
As the vinegar ages it changes flavor, texture and feel. It is a good idea to keep a log to keep track of the changes in your kombucha vinegar as it ages. This will not only be interesting but also help you make new batches which have specific flavors at different ages.
Enjoy at your leisure
At this point your kombucha vinegar will last almost indefinitely. It has a high enough acetic acid content and low usable nutrients and food energy in it that there is little or no microbial activity in it. You can use it in salad dressings, dips, sandwich spreads or anything else you use vinegar for.
Kombucha vinegar is especially good at activating the baking powder when making quick breads. This is because there is more than one type of acid prevalent in kombucha vinegar. Acetic acid is the main player but lactic acid is also present and provides an additional boost to the base in the baking powder when it is heated. If you like really fluffy slightly tangy quick breads then add a little vinegar to your liquids when making biscuits, pancakes or muffins.
Making vinegar from over fermented kombucha
This is a little different. You didn’t start out with the idea of making kombucha vinegar it just seemed to happen when you left a batch ferment a little (or a lot) too long and it became a tangy undrinkable liquid. It is no longer suitable for making into kombucha soda but isn’t really vinegar either.
If you want to convert it into vinegar which will last a long time then you will have to increase the amount of acetic acid concentration in it. Here is a method which will ensure your over fermented kombucha has enough acetic acid content in it.
- Make a new batch of strong tea
- Add enough sugar for the acetic acid content needed
- Add some starter liquid, a SCOBY and ferment for a week
- Combine the two batches and add more sugar
- Ferment another 3 weeks before checking the sugar content
- Once most sugar has been consumed filter and store in an open container
- Bottle and age
This is a less exact process and the amount of acetic acid will not be predictable but it will produce a vinegar which can last a long time and age without incident.
Use the sugar ratio of 3 ¼cups per US gallon as a guide for the new tea. This will give the culture enough sugar to convert into acetic acid for this amount. By fermenting it for a week you will have allowed the yeast and AAB cells to multiply enough to restart the fermentation of the over fermented kombucha.
Add enough sugar to total that 3 ¼ cup per US gallon ratio for the original batch. For example if you make your kombucha with the regular 1 cup per gallon ratio then you need to add 2 ¼ Cups more sugar.
The fermentation time is unknown here so begin checking it for sugar content after three weeks or so and when you are ready filter and store the vinegar only in an open container. The reason for this is that there may be more sugar present than you expect and you don’t want any explosions when storing in sealed containers and you also don’t want the vinegar to have extra alcohol.
The open container will allow the AAB access to oxygen and they will convert any extra alcohol into acetic acid quickly. You will notice a large SCOBY forming on the surface of the container as it ages. Just leave it alone for at least 3 months and then you can filter it again. If the SCOBY forms slowly you know the acetic acid has slowed their action and the vinegar is almost ready to bottle and age.
At any time you can use some of your kombucha vinegar so don’t think that you have to wait a year to try it. Make a large batch and use some right away, some in six months and some at a year. If you make enough you can age it a long time and compare the experience at each point.