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There are few things in the kitchen which are as versatile as vinegar. It is used in recipes, cleaners, food preservation, sanitation and in some cases even medicine. It is prevalent in almost everything in the kitchen and has even made it to the garden by way of weed killer!
Acetic Acid (vinegar) in concentrations of 3% w/v essentially kills yeast fermentations. Vinegar at concentrations above 0.6% w/v have inhibitory effects on growth of brewers yeast and some spoilage bacterial species at a concentration of 3% w/v but does not adversely affect lactic acid bacteria.
Vinegar can be used to slow or stop fermentation when used in high enough concentrations but in most cases vinegar is used to provide inhibitory effects on spoilage microorganisms in the first stages of fermentation.
How Vinegar Effects Yeast Fermentations
Vinegar is the bane of alcoholic fermentations. If it begins to form in an alcoholic medium it can cause spoilage quickly by lowering the pH and the alcohol content. This is why the beer and wine industries go through such extreme measures to prevent the growth of acetic acid bacteria in their products.
Acetic acid (vinegar) inhibits the growth of yeast by causing yeast cells to use energy on repair rather than growth and by altering specific chemicals within the cell to signal cell death. Yeast is affected by vinegar depending on food availability, temperature and the pH of the solution.
Not only does live acetic acid bacteria sour a fermentation but even the presence of vinegar in a yeasted fermentation has negative effects on the fermentation process. Yeast species are varied and can survive in varied conditions from elevated temperatures to high ethanol concentration. Most yeast species are negatively affected by the presence of acetic acid.
Effect of low to high acetic acid concentrations on yeast
When yeast is present in a low acetic acid solution it has to protect itself from the detrimental effects of the acid. It does this by increasing its activities to strengthen its cell membrane and repair its mitochondria. As a result of this there is less energy to spend on energy intense activities such as cell duplication. This reduces the growth rate and the fermentation slows.
Low acetic acid concentrations are affected more when the temperature of the fermentation is higher. This could be due to increasing the acid activity on the cell membrane requiring additional energy expenditures on healing activities.
As the pH of a solution drops the action of the acid on the cell membrane increases, requiring more energy to be spent on repair and defence until the yeast can no longer absorb enough energy to keep the process going and it dies.
Effect of acetic acid on programmed cell death in yeast
The concentration of the acid does not explain all of the inhibitive effects of acetic acid on yeast fermentations. There is another process involved which has been studied which may account for some of the discrepancy.
Programmed cell death or apoptosis is where the cell may still have enough energy to continue to survive but becomes too damaged to repair itself. Acetic acid can penetrate the cell membrane and alter the chemical balance in particular ways. This leads to the mitochondria of the cell to decide to produce natural killer chemicals which can trigger cell death.
How Vinegar Effects Bacteria Fermentations
The bacterial tolerance of acetic acid depends on the type of bacteria and the environmental conditions in which it is found. Some bacteria are very sensitive to acid whereas some are highly tolerant to low pH solutions.
The Effect of Vinegar on Acid Sensitive Bacteria
Many types of bacteria are sensitive to acetic acid and are strongly inhibited once the pH drops to below 4.0. This is why a pH below 4.5 for long term food storage is recommended. Unwanted bacteria not only spoil the food but can also be toxic. The use of vinegar to pre-acidify a fermentation is a common method of protecting a ferment from spoilage in the early stages.
How Vinegar Affects Acid Tolerant Bacteria
Acetic acid is produced by a family of bacteria called acetic acid bacteria (surprising I know). This type of bacteria is very tolerant of acetic acid. Unlike yeast which produces alcohol as a waste product, acetic acid has the ability to use acetic acid as energy. This happens in vinegar solutions when all other sources of food energy are consumed.
Other types of acid tolerant bacteria like lactic acid bacteria are unaffected by acetic acid due to their ability to resist low pH solutions which they also produce to protect their environments from unwanted competitive bacteria. Lactic acid bacteria and acetic acid bacteria are frequently found together in traditional fermentations like kombucha, kefir and sourdough.
Not all acid tolerant bacteria are beneficial. Some species like E. coli can cause sickness as well as food spoilage. Adding vinegar to fermentations does not prevent the growth of these types of bacteria so other methods of preservation need to be present to ensure safety.
Fermentations like kombucha, kefir and vinegar production use other environmental conditions to prevent the growth of unwanted bacteria. To learn more about how fermentation improves the safety of food read Is Food Fermented with Yeast and Bacterial Cultures Safe. The addition of vinegar to unfermented food without further processing (whether by intentional environmental control or through pasteurization) is unsafe and can lead to sickness.
How Vinegar Can be Used to Ferment Vegetables
Although vinegar is a product of fermentation it does not act as a fermentation medium for vegetables. The use of vinegar in fermentation is limited to its inhibitory effect on unwanted microbial growth in the first stages of fermentation and for adding flavor once a fermentation is complete.
Vinegar can be used in vegetable fermentations to reduce the chances of mold and yeast growth in the early stages of fermentation. Vinegar reduces the pH of the substrate making it harder for unwanted microbes to grow.
Vinegar is used to pickle vegetables by submerging the vegetables in a vinegar solution and pasteurizing the container to prevent spoilage bacterial growth. This is not the same as fermenting the vegetables. Fermentation only happens when there is microbial growth in the vegetables.
This is a common misconception when fermenting vegetables, especially recipes which include tomatoes or peppers (which are actually fruits). The recipe usually reads something like: “Add two tablespoons of raw vinegar to add live probiotic bacteria and to help prevent mold growth.” Only half of that is true.
Vinegar in concentrations of less than 3%, raw or otherwise, prevents the growth of mold and other unwanted bacteria in vegetable fermentations due to its acidic nature but live acetic acid bacteria in finished vinegar are very low in number and cannot qualify as a probiotic. In fact there will be more live acetic acid bacteria found on the vegetables themselves than in raw vinegar.
Vegetable fermentation is done in anaerobic conditions in a salt brine. This prevents the growth of acetic acid bacteria and other bacterial and yeast species. The fermentation of vegetables is done by lactic acid bacteria which is both salt tolerant and anaerobic.
Fermentations which use Vinegar
Vinegar is prevalent in fermentation not only as a product of fermenting ethanol with acetic acid bacteria but as an ingredient to help protect a fermentation in the early stages.
Recipes which use Vinegar as an Inhibitory Agent
- Fermented tomato salsa
- Ketchup
- Moustard
- Fresh Fruit Vinegar
- Fermented Italian Tomato Sauce
- Wine Vinegar