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When making homemade vinegar it is important to reach a minimum pH of 4.0 as this acid level will prevent unwanted bacteria and molds from causing the vinegar to go off. Although most vinegar has a pH under 3.0 sometimes you may want to make vinegar which has a lower pH. To do that you need to understand what are the factors which increase or decrease the acid level of vinegar.
To make a strong vinegar there are a number of factors:
- The amount of sugar the yeast has to consume
- The starting pH of the medium
- The efficiency of the yeast to make alcohol
- The availability of oxygen for the acetic acid bacteria
- The time period given the fermentation process
Vinegar is made by lactic acid bacteria in a sugar solution of some form. It is a two step process where resident yeast uses the sugar to produce alcohol which the lactic acid bacteria use to produce lactic acid. To get the strongest vinegar you need to optimize the above conditions and keep the yeast and acetic acid bacteria thriving in the fermentation for as long as possible.
Basic steps for making strong vinegar
The most basic method of making extra strong vinegar is to use additional sugar than is naturally found in the fruit you are using. The extra sugar will provide additional fuel for the yeast to produce additional alcohol.
The method:
- Wash the fruit with warm running water and cut into small enough sizes to fit through your juicer.
- Juice your fruit and pour into a wide mouthed non-reactive container.
- Place a cloth over the mouth of the container and secure it with an elastic band.
- Aerate the juice twice a day until it begins to bubble
- Add additional sugar to the juice. Only add up to 2 tbsp for every cup/500ml of juice
- Stir and leave to ferment for 10-14 days
- Once the active fermentation has subsided, pour the liquid off the sediment on the bottom of the container, replace the cloth and leave to ferment for another 3-6 weeks.
Why additional sugar makes for stronger vinegar
When making vinegar there are two processes happening. For most homemade vinegar these processes are going on simultaneously but they can be separated into two separate processes. The first is the production of alcohol which requires a sugar solution and live yeast. The second requires oxygen, alcohol and the presence of acetic acid bacteria.
The strength of the vinegar is dependent on the amount of alcohol in the solution which the acetic acid bacteria consumes in the process of making acetic acid. The more alcohol the more potential acetic acid and the higher the acid level in the vinegar.
By adding additional sugar when making vinegar you increase the amount of alcohol which the yeast can produce and as a result the acetic acid bacteria make more acetic acid. This can be done a number of ways depending on your situation.
Add the sugar at the beginning of the fermentation
Fruits which are considered fruit in the grocery store such as apples, grapes and berries have enough sugar in them to produce vinegar with a pH of at least 3.8. To increase the acid level in your vinegar you can add a little sugar at the beginning of the fermentation. To do this simply add the sugar to the solution at the beginning of the fermentation time and stir it in.
Be careful to not add too much sugar as too much sugar acts as a preservative in high concentrations and will slow the growth of the wanted yeast and bacterial culture, leaving an opening for unwanted mold to get a foothold.
Add the sugar partway through the incubation time
One of the problems with using sugar to increase the acidity level of your vinegar is that sugar is also used as a preservative. Adding too much sugar at the beginning of the fermentation can stunt the growth of the yeast and acetic acid which leaves an open door for mold to start to grow.
To solve this problem you can add the sugar in doses as the vinegar ferments. By starting your vinegar and letting the yeast consume some of the sugar in the solution and produce alcohol two things happen which support a higher acidic level of the vinegar.
- The sugar content will drop allowing you to add additional sugar without concern that the additional sugar will slow the fermentation down.
- The acetic acid bacteria will consume the alcohol and produce acetic acid which helps the yeast to continue to grow uninhibited.
This leads to a more active fermentation and provides adequate resources for the yeast and acetic acid bacteria to continue acidifying the solution.
Separate the two processes by using a restricted oxygen environment
One of the main challenges with simultaneous vinegar fermentation is the yeast species which are commonly found on fruit are not efficient at producing alcohol with the available sugar in the solution. This means that they use more sugar to make the same amount of alcohol. Thest yeasts produce interesting flavors which provide homemade vinegar with depth and complexity not often found in store bought vinegar but they also reduce the vinegar’s acidity level.
This works because most of the low efficient yeast cannot survive high levels of alcohol. Whereas typical wine yeast () has a high tolerance for alcohol and is efficient at making alcohol, making more of it with the available sugar.
Don’t use too much sugar
Additional sugar in the solution will provide more food for the yeast to use to make alcohol but there is a limit. To make extra strong vinegar you can add sugar in batches until your vinegar reaches the acidity you want. To do this start your vinegar normally and let it ferment for a while, the amount of time depends on the method you are using to make your vinegar (For an explanation of the various methods read this article). Then once it has started to acidify add additional sugar for the yeast to consume and make more alcohol.
One of the methods of food preservation is to use sugar concentrations to prevent microbial growth. This is because although sugar is a source of energy it is also poisonous when concentrated too much. Yeast growth will begin to slow once a solution has reached a 6% sugar concentration. Too much sugar can slow and even stop the fermentation. By adding the sugar in batches you give the yeast a chance to consume the sugar and allow the acetic acid to use the resulting alcohol to produce vinegar.
High sugar concentrations also inhibit the growth of acetic acid bacteria. Although they can use sugar directly to produce vinegar it is much less efficient and the sugar concentration needs to be lower than that of what yeast can tolerate.
Why the type of yeast affects the strength of vinegar
Yeast is a single celled organism which can be found in every environment. It exists in a variety of types and all use simple sugars as a food source. As a result of their respiration they produce alcohol. So surely having any yeast in your vinegar fermentation will produce enough alcohol right?
The most common yeast used to make wine or beer is Saccharomyces cerevisiae. It is chosen because it is highly efficient at making alcohol, can survive in low pH solutions of 2.8 and high concentrations of alcohol. Which makes it ideal for alcohol production but when making vinegar other yeast species are present. These add flavors and depth to the vinegar and are therefore valuable in making homemade vinegars but they are less efficient at producing alcohol from the amount of sugar available.
This means that less alcohol is produced by these yeast species which results in less alcohol available for the acetic acid bacteria to produce acetic acid.
To produce the highest strength vinegar you can divide the fermentation process into two portions:
Make the alcohol first
This requires some specialized equipment which prevents the fermentation from being exposed to oxygen during the fermentation process. This does two things, it prevents the growth of acetic acid bacteria and it narrows down the dominant forms of yeast in the fermentation.
Most yeast species cannot survive in an alcohol solution above 6%and as a result will die off when the alcohol content reaches this level and more but wine yeast and a few other species are highly tolerant to high alcohol solutions and will continue to ferment the sugar until the alcohol content approaches 18% and in some cases 22%.
Convert the alcohol to acetic acid (vinegar)
To convert the alcohol to acetic acid you need to expose the solution to oxygen and the natural process of acetic acid bacterial growth will begin to increase. Vinegar can be made with any solution which has an alcohol content between 6-12%. Obviously the higher the alcohol content within this range the lower the potential pH of the vinegar.
Why the availability of oxygen affects the acidity level
Oxygen is needed for the growth of acetic acid bacteria. It is this family of microorganisms which convert the alcohol into acetic acid so without enough oxygen they will not be able to do their job.
When using active methods of making vinegar like the Orlando method, using an aerator or pumping the solution over a wide surface area additional oxygen is available to the acetic acid bacteria which grow quickly. As a result vinegar is produced more efficiently and it will have a lower pH.
More passive methods of making vinegar will often produce vinegar with less acetic acid as the solution is exposed to the air over a long period of time (weeks, months or years) and some loss in alcohol from the solution due to evaporation is inevitable. This is why most homemade vinegar is less acidic than commercially made vinegar.
When making vinegar the fermentation is exposed to the environmental oxygen in one way or another. Some methods require forcing air through the liquid while others are more passive where the fermentation is just left exposed the the air with a cloth over the surface of a wide mouthed container.
All methods require the exposure of the solution to the air. When an alcohol solution is exposed to the air some of the alcohol will evaporate. This will reduce the amount of alcohol in the solution and lower the potential acidity of the vinegar. Solutions with a higher amount of alcohol will lose more alcohol due to this evaporation process.
Why fermentation time affects the acid level in vinegar
Homemade vinegar will have a wide variety of acetic acid bacteria species in the mother. Each has a niche in the community and each has ranges of acid level, sugar tolerance and alcohol concentrations which they will thrive in. This means that throughout the fermentation process different acetic acid bacteria will thrive while others will die off.
A long fermentation time increases the acid level as in time more of the sugar in the solution will be converted to alcohol and then into acetic acid by the various acetic acid bacteria species. This process will continue until the vinegar is pasteurized which kills the mother and stops the fermentation process.
Unpasteurized vinegar will continue to ferment as long as there is some alcohol being produced by yeast in the vinegar. Once the main fermentation process is complete it may seem like it has stopped fermenting but it will continue slowly almost indefinitely. This is evident by the slow growth of the “Mother” on the surface of the vinegar as long as it has access to oxygen.
To stop the acetic acid bacteria from continuing the fermentation process you can place the vinegar into an anaerobic environment. You can do this in two ways:
- Place an airlock on the vinegar container
This is the safest method as it allows any pressure which can build in the container to escape. Since active yeast and acetic acid bacteria produce carbon-dioxide it can be dangerous to contain the pressure in non-pressure safe bottles.
- Bottle the vinegar
When bottling your vinegar it is difficult to determine how much carbon-dioxide it is producing. If it is too much it can build up and burst non-pressure safe bottles. There are methods of testing the vinegar to see how much sugar is left in the solution but it is complicated and time consuming.
Instead you can simply place your vinegar into pressure safe bottles which can be opened and closed easily. This will prevent any oxygen from getting in and will stop the acetic acid bacteria from further acidifying the vinegar.
Ensure the bottles you use are pressure safe as if the yeast is still active it can build up a fair amount of pressure and explode regular bottles. I recommend flip top beer bottles. They can be found at beer making stores, online and, if you are patient, at thrift stores or from friends and family.