Is Fermentation a Natural Process?

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Before we get into the answer for this question we first have to define what is meant by fermentation.

Fermentation has two generally accepted definitions

From a biochemistry position fermentation is a natural process occurring wherever sugar and fermenting microbes meet.  From a food science position fermentation is a controlled process which needs specific conditions to be successful including temperature, salt concentration and oxygen exposure.

If you leave some food out on the counter it will indeed ferment.  The carbohydrates in it will be acted on by the yeast and bacterial species found in the environment but the result of the fermentation will not be something you will want to put into your mouth. 

To transform the food into something delicious you will have to control the environment which the food is kept.  This is not a natural process.  No matter what method you choose, relying on naturally occurring microbes or cultivated ones, fermentation from a food science position needs careful attention to the process.

Five types of food science fermentation

There are many ways to transform fresh food into preserved staples and flavor sensations.  An apple can be made into cider, chutney or apple cider vinegar.  What it becomes depends on the environment it is placed in during fermentation. 

Yeast fermentation

This is what most people think about when they hear the word fermentation but it is only one aspect of it.  Yeast ferments are those which convert sugary substances into alcoholic food and drinks. 

The environment needed to produce a yeast fermentation:

High sugar content

Species of yeast have the ability to consume short chain carbohydrates (sugar) and produce alcohol and CO₂.  This is where the term fermentation originated which means to boil.  It was the action of the yeast on a sugary liquid which produced large amounts of CO₂ making the liquid appear to boil.

Without a high sugar content a yeast fermentation will not be able to increase the alcohol content high enough to protect it from spoilage.  With an alcohol percentage of only 2% can inhibit the growth of mold.

Limited access to oxygen

yeast can survive in an aerobic and anaerobic environment.  Some yeast species have a preference like ­–­____ prefers an anaerobic environment and others prefer it aerobic.  Is makes yeast very hardy in the environment able to withstand a wide variety of conditions.

When you are trying to produce alcohol there are different stages which should be followed depending on the method of alcoholic fermentation you are using.

Natural fermentation

Natural fermentation is a term used in wine making to indicate that only naturally occurring yeast species were used to produce the alcohol in the wine (along with many other conditions). 

When using this method the wine must (crushed grapes) is exposed to oxygen for a short period of time before it is pressed and fermented in an anaerobic environment.  This two-step process helps the yeast to grow more efficiently at the beginning of the fermentation period which prevents spoilage bacteria from dominating the fermentation.

Commercial yeast fermentation

This method uses dried yeast which has been grown especially for alcohol fermentation.  Most fermentations using commercial yeast do not require oxygen.  Once the yeast has been activated it is pitched into the juice and air locked.

Backslopping

Backslopping is a technique where some of an active fermentation is used to start another.  It is usually performed in an anaerobic environment but can be used to start country wines with solid fruit in an aerobic environment.

Wild fermentation

Wild fermentation is a term used for only using the naturally occurring yeast species found in the environment.  It is used to make beer, wine or ciders.  It starts with an aerobic fermentation which requires stirring to prevent mold growth.  Once the yeast population starts to explode the stirring rate can slow down.

After a period of about ten days or so the primary fermentation is pressed and placed into an air locked container for anaerobic fermentation.

Temperature controlled

The temperature range for yeast growth is varied.  Ranging from just above zero to 40˚C but to help yeast species dominate a culture maintain temperature between 25-30˚C.  The rate yeast cells double in population increases as the temperature goes up until it gets above 30˚C where it slows and stops. 

Lacto fermentation

Lacto fermentation is what is used most often to preserve vegetables and sometimes fruits.  It is the process of making cabbage into sauerkraut and cucumbers into pickles.  It is also used to transform milk into yogurt.

The environment needed to produce a lacto fermentation is different from that of a yeast fermentation. 

An anaerobic environment

Lactic acid bacteria do not need oxygen to live but mold and acetic acid bacteria do.  By putting your fermentation into a closed jar or some other container which prevents oxygen from entering you prevent the growth of mold and acetic acid bacteria. 

Acetic acid is the acid responsible for making vinegar tangy which is great for tangy ferments but in a lacto-fermentation you want lactic acid which is less tangy and more flavorful.  By putting your lacto-ferment in an anaerobic environment you limit the acetic acid growth.

High salt concentration

Lactic acid bacteria are salt tolerant or halotolerant, whereas yeasts and mold are not.  Lactic acid bacteria have a high degree of tolerance to acid and salt because they can withstand ionic stress. 

By adding salt to your ferment you are inhibiting the growth of most common yeast and mold species which cause spoilage.  It is recommended that a 2.5-3% salt concentration be used for best results. 

Cool environment

A cool temperature slows the growth of yeast and mold as well as helps to keep the food crisp.  This gives the lactic acid bacteria an edge over the yeast and mold helping them to dominate the culture. 

Higher temperatures also helps the bonds which make the food stiff break down causing it to become soft and mushy (not something most people want).

Acetic acid fermentation

Although in some situations like wine or beer making acetic acid bacteria are spoilage bacteria they can produce some of the best foods like vinegar and soy sauce. 

Acetic acid fermentation is used to make vinegar out of any sugary liquid.  It will convert wine into vinegar or apples into apple cider vinegar.  Acetic acid bacteria are found in every environment on Earth so they can be wild fermented as well as introduced into an alcoholic solution to produce vinegar.

Environmental conditions for an acetic acid fermentation

Aerobic environment

Acetic acid bacteria require oxygen to survive, which is why wine and beer are fermented in an anaerobic environment.  When trying to make vinegar you will want to expose your solution to oxygen to encourage the growth of acetic acid bacteria. 

This is done in a number of ways:

• Use a wide mouthed vessel with a porous cover
• Agitate the liquid by stirring or transferring between containers
• Aerating the liquid by forcing air into the bottom of the container

Presents of alcohol

Acetic acid can live on sugar but grow much faster when they have access to alcohol.  They convert the alcohol into acetic acid and CO₂ which quickly acidifies a solution. 

This type of fermentation is truly a mixed fermentation as it requires the action of yeast to convert the sugar into alcohol before the acetic acid bacteria converts the alcohol into acetic acid.  When making vinegar it is best to start with a solution which either has an alcohol content of at least 6% or has a high sugar content. 

The presents of alcohol prevents the growth of mold while promoting the growth of the acetic acid bacteria.  Once the solution is acidified it is further protected from mold growth.

Warm environment

Acetic acid bacteria have an optimum temperature range of 18-25˚C.  Providing a warm location for your vinegar fermentation to live will decrease the time it takes for acidifying the liquid. 

It also helps the yeast to produce more alcohol as their temperature range is higher than that of the other microbes which are used in fermentation.

Mold fermentation

It may seem strange to you that all of the above methods of fermentation want to prevent the growth of mold but in this section you are going to encourage its growth.  Mold is a microorganism which can break down complex carbohydrates into simple sugars.  This differs from all the other microorganisms used in fermentation.

Mold has been used to transform and preserve food for as long as lactic acid bacteria has been used to preserve cabbage.  Mold is used to make tempeh, kogi and cheese.  Penicillium camemberti is used to make camembert and blue cheese is made with Penicillium roqueforti and Penicillium glaucum.  These molds are used because of their ability to add flavor to the cheese.

Specific mold cultures are to be used not wild mold which can be dangerous so when fermenting with mold get a source of mold spores from a reliable source and follow the directions on the package exactly for best results.

Environmental conditions needed for using mold to preserve and transform food

High humidity

Mold spores need moisture to begin to grow (which is why you keep things dry when not in use).  The humidity provides the mold with enough liquid on the surface of a substrate to grow.  For the growth of Aspergillus oryzae (koji mold) a relative humidity of 90% is preferable which enables the mold access to the surface of all the substrate. 

Temperature controlled

Depending on the type of mold you are wanting to encourage the temperature needs to be warm or cool.  By keeping the temperature at a steady temperature the mold growth is encouraged while spoilage bacteria growth is inhibited.

When making koji the temperature is kept at a warm 36˚C whereas cheese mold is kept cool at 12-15˚C. 

Aerobic atmosphere

Mold requires oxygen so it is important to provide adequate ventilation when making a mold based fermentation.  Special rooms are built for the fermentation of koji.  Cheese aging rooms are built to encourage access to oxygen.  Some cheese is even hung to allow airflow all around the cheese.

This access to oxygen will encourage the growth of the mold.  As mold grows it produces chemicals which are toxic to other organisms which discourage predators from eating them.  An example of this is penicillin which is a toxin which kills bacterial species.  

Mixed fermentation

These are fermentations which combine several types of fermenting organisms to produce unique food products.  These include kefir, kombucha and sake.  Some of these types of fermentations require the introduction of starters such as water and milk kefir whereas some like kombucha can be made through wild fermentation methods and still others like sake need a series of steps to be successful.

Environmental conditions needed to mixed fermentations

Adequate food reserves

A mixed culture like kefir needs enough food reserves so that the various microbial species found in the culture can find a niche to survive.  In the case of kefir the milk needs enough lactose and proteins to feed the yeast, lactic acid bacteria.  The yeast produces alcohol which provides a ready source of energy for the acetic acid bacteria.

If the culture is not fed adequate food it may become off balance with a higher population of one of the species found in the culture.

Sake needs enough kogi mold to break down the carbohydrates in the rice which provides energy for the yeast and lactic acid bacteria.  Without enough starch the alcohol production is reduced.

Each type of mixed fermentation requires specific food sources.  As an example of this kombucha which has been fed black tea has a different makeup then kombucha fed with green tea. 

Consistent environmental conditions

A consistent temperature, humidity and access to oxygen is important for mixed fermentations to provide the desired species a chance to dominate their specific niches. 

This is why moving a kefir grains from the fridge to the cupboard regularly is hard on the grains and produces kefir which has off flavors.  To get consistent flavors maintain the environment which your ferment the culture for a long period of time.

Once the culture has balanced out (the population of the various microbial species have found their place) you can alter the environment to change the flavor in some way.

An example of this is making kombucha in a hot environment increases the alcohol in it once the yeast species establish their dominance.  If you want lower alcohol kombucha lower the temperature (for more ways to lower its alcohol content read “How much Alcohol is in Homemade Kombucha?”)

Microorganism culturing

Some mixed fermentations require culturing a microbial species before adding the other species to the mix.  This is done when making Sake where the mold Aspergillus oryzae is fed to steamed rice and cultured for a period of 2-3 days until it has fully penetrated the rice grains.  This process provides large amounts of simple sugar which is then fed to the yeast to make alcohol. 

Other examples of this are cheese making where the mold is introduced after the milk has formed whey and the making of soy sauce where rice and soybeans are fermented together with Aspergillus oryzae and then salted and aerated to help the acetic acid bacteria acidify and liquefy the bean mash.

The process of fermentation

As you can see the process of fermentation depends on the type of fermentation you are doing.  Each type requires a different process with specific temperatures, acidity levels and food sources.

Alcoholic fermentation

For an alcoholic fermentation you want to encourage the yeast to multiply.  This is done by providing a high sugar liquid with a warm temperature out of oxygen exposure. 

Once the yeast has consumed the sugar it can be bottled or put in a cask for a long time.

Lacto-fermentation

Lacto-fermentation needs a cool environment without access to oxygen with a high salt concentration.  This provides the lactic acid bacteria the best chance of dominating the container and acidifying it.

This preservation method can preserve food for months to years depending on the food source and its method of storage.

Vinegar fermentation

To make a good vinegar you will need an alcoholic fermentation which is then exposed to oxygen for a period of time. 

With a high enough alcohol content (5-10%ABV) the acetic acid will adequately acidify the liquid to enable an unlimited storage time.  Some vinegars have been aged up to fifty years and are some of the most prized and expensive condiments you can find.