Homemade Vinegar Trouble Shooting

Five bottles of in various sizes and shapes filled with different types of vinager

If you have ever tried to make vinegar but instead ended up with moldy fruit in a jar or your vinegar just doesn’t taste right then you must have wondered why.  Some of the typical problems when making vinegar are mold growth, low acid production, too high acid production and stagnant or stubborn fermentation.  Although these problems can be avoided if you don’t understand what is happening you may become frustrated and quit trying.

Making vinegar is a two step process which can be separated into two time periods or performed simultaneously but to avoid problems you need to understand each step and provide the proper conditions to allow the appropriate microorganism to thrive.  These processes are:

  1. Production of ethanol

Ethanol is produced by yeast cells found in the environment.  If you are making fruit vinegar then most of the needed yeast is found on the fruit itself but additional yeast species can be found within the environment at all times.  

  1. The production of acetic acid

Acetic acid is made by acetic acid bacteria.  These microorganisms use ethanol as their main source of energy, although they can use sugar as well their preferred energy source is ethanol.  In the process of consuming the ethanol produced by the yeast cells they produce acetic acid which prevents unwanted spoilage bacteria from colonizing the fermentation.

Most vinegar making problems disappear once it is understood what the roles of the two types of microorganisms are and how they should be supported during the early stages of the fermentation.  This includes how much sugar to start with, how long the fermentation period is and what should be done to the fermentation during each stage.

There are many ways to make vinegar (check out this article about the five main methods used) but they all boil down to the above two processes.

How to tell if your vinegar has gone bad

When making vinegar there are a few times when it can spoil but once your vinegar has reached a pH below 4.0 it is rare that it will go off.  Acetic acid bacteria produce many other organic compounds which can protect the vinegar from spoilage which include additional acids and biofilms but before the acetic acid bacteria can gain a foothold mold and other spoilage bacteria can sometimes begin to grow.  

Vinegar which has an off taste, seems to be losing its acidity or has fuzzy growths on its surface has spoiled.  This can happen when the pH of the vinegar is not low enough or the vinegar gets contaminated with debris.  When exposed to oxygen it can become less acidic leading to spoilage.

Homemade vinegar has a few times when it is susceptible to spoilage prior to the growth of acetic acid bacteria.  During the initial period when the liquid medium mostly made up of sugar and water mold can easily grow if left alone (for a complete step by step process on how to make vinegar read this article here).  Another time when vinegar is at risk is when the medium is left to ferment once the yeast has started to grow.  This often happens when there is too much organic matter in the vinegar giving mold and other spoilage bacteria a place to live.

Why does my homemade vinegar smell like alcohol?

Vinegar is made in a two step process the first being alcohol production.  Yeast found on the fruit or in the air use the sweet liquid to grow.  The waste product yeast cells produce is alcohol.  If your vinegar fermentation smells like alcohol it may be that the acetic acid bacteria have not had a chance to convert the alcohol into acetic acid yet.

Acetic acid bacteria need oxygen to live.  One common mistake is to restrict the amount of oxygen the vinegar is exposed to by fermenting the vinegar in a narrow necked container, using air locks rather than breathable cloth covers or sealing the container with a non porous lid.  

When making vinegar it is important that the acetic acid bacteria get enough oxygen.  Most commercial vinegar makers use incubators which have the ability to oxygenate the liquid much like a fish tank is oxygenated with a bubbler.  Other methods include alternating the vinegar from container to container or using barrels which  have a wide surface area to volume ratio.  For further information on vinegar making methods check out this article here.

Another reason your vinegar smells like alcohol is that it is at the early stages of fermentation.  At the beginning of the process acetic acid bacteria will not have enough time to convert the alcohol into vinegar.  This can take up to a week to ten days before your vinegar begins to smell like vinegar, sometimes longer depending on the ambient temperatures of the room.

What is floating around in my vinegar?

There are a few possibilities depending on how your vinegar was made, how old it is and how it has been stored.  Homemade vinegar is most often unpasteurized because most fermenters are after what is called the mother or the bacterial culture found in most fermentations.  

Unpasteurized vinegar contains the vinegar mother which can present itself as a cloudy amorphous stop floating around in the vinegar.  It is made up of the biofilm which the bacterial culture of the vinegar produces during the fermentation process.  

The vinegar mother is actually the collection of yeast and acetic acid bacteria found in the vinegar culture.  A vinegar mother can produce a cellulose mat like that found in kombucha and serves as a barrier between the fermenting vinegar and the environment.  It also will produce yeast strings which are dead yeast cell bodies held together loosely by biofilm.  It is these which are often found in commercial apple cider vinegar.

The mother of vinegar has been used to indicate that it is a living product as evidenced by the biofilm produced after the vinegar has been filtered and bottled.  Although this is true to some extent properly fermented vinegar has little acetic acid bacteria activity as the available sugar which feeds the yeast which makes the alcohol the acetic acid bacteria require for growth is depleted in properly aged vinegar.  This gives the acetic acid little chance to grow.

To learn more about what the vinegar mother is and what its purpose is check out this article about kombucha SCOBY which are similar to that of a vinegar mother.

Why vinegar fermentations get moldy on top

If you have been making vinegar for any period of time then you may have had the experience of checking on your vinegar jars only to find one or two growing a nice colorful carpet of mold on the surface.  Mold comes in many types and can grow in a wide variety of conditions so it can sometimes be difficult to avoid this from happening but with a few precautions it is easy to control.

Vinegar gets moldy when yeast and acetic acid bacteria are not active enough to prevent mold growth.  Mold grows in warm places with a neutral or slightly basic pH.  To prevent mold growth in a vinegar fermentation, start with an acidic solution and stir it regularly.

Mold is everywhere and although it is usually an unwanted visitor in your kitchen it is still a valuable organism which provides us with a great service.  That said, when making vinegar it is important to prevent its growth.  Mold produces mycotoxins which are meant to prevent its environment from being disturbed and its food source from being consumed.  These are antibacterial agents like penicillin which deter the growth of other organisms.  

Mold is an aerobic microorganism which means it needs oxygen to survive, it also has no method of movement but rather relies on the sheer number of mold spores it produces to ensure some will end up in an environment conducive to its growth.  This is why stirring a vinegar fermentation is important while the yeast and acetic acid bacteria grow.  By stirring the solution any mold spores which are on the surface get submerged in the liquid where there is limited oxygen.

Since mold grows best where the pH is basic you can prevent most mold growth by acidifying your vinegar fermentation at the beginning of the incubation period.  This is done simply by adding a little vinegar which is the same or close to the type of vinegar you are making.  This will lower the pH of the solution which slows mold growth.  For more about how to make vinegar check out this article about making fruit vinegar from scratch.

Michael Grant

Mike has been an enthusiast of fermentation for over ten years. With humble beginnings of making kombucha for himself to the intricacies of making miso, vinegar and kefir. He makes a wide variety of fermented foods and drinks for his own consumption and family and friends. Being a serial learner he began experimenting with a wide variety of fermented products and learning widely from books, online from content and scientific studies about fermentation, its health benefits, how to use fermented food products in everyday life and the various techniques used to produce them both traditionally and commercially. With a focus on producing his own fermented products in an urban environment with little access to garden space he began Urban Fermentation to help others who want to get the benefits of fermentation in their lives. He provides a wide variety of content covering fermented drinks like kombucha and water kefir, milk kefir and yogurt, vinegar production and lacto-fermentation such as pickles, sauerkraut for those who have to rely on others for food production. With an insatiable hunger to know more about fermentation from all nations and cultures he also has learned to make natto, miso and soy sauce, with more to come as the body of knowledge about fermentation is constantly expanding and becoming more popular as time passes.

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