Fermentation in Theory and Practice

Refr Fiachson – Culinary Symposium 2011

Until Pasteur’s work in the mid-19th Century, nobody knew what caused fermentation.  To people in SCA period, food (under the right conditions) magically became something different in ways that are endlessly fascinating (at least if you are me).  And in the words of Terry Pratchet, “it doesn’t stop being magic just because you know how it works”.

Fermentation was part of everyday life in many parts of the world, and it still is today.  To understand what food was like in period, we have to understand both the theory and practice of fermentation and how it applies to food production.



We are surrounded by microscopic life forms constantly and in every aspect of our lives.  They live on us, around us, in us, and in many of the traditional foods our ancestors ate.  Some of those microorganisms are out to get us, but most of them mean us no harm, and a few of them can be made to serve us.  These little biological minions can be extremely useful in preparing and preserving food.  Our ancestors relied on these little helpers, even though they did not understand them, in order to save food for the winter, make their bread rise, make inedible foods edible (or more palatable), and made already edible foods into something fantastic.


In our modern world of antibacterial soap and germophobia, we are used to thinking of bacteria as something to be feared, but they can be harnessed to produce a number of beneficial and tasty effects when applied to foodstuffs.  While there are lots of bacteria involved in producing flavors in food (think cheese) there are two main categories that are instrumental in preserving food.


Lactobacilli, one of the most prevalent kind of bacteria used in food production,  eat sugars (usually lactose) and produce lactic acid, as well as other compounds that influence flavor.  The most common lactobacilli are those used in milk products like yogurt, cheese, buttermilk, and sour cream.  Other strains of lactobacilli make sauerkraut sour, and turn ordinary cucumbers into Kosher-style sour pickles. Others make lambic beer and sourdough bread sour, and add flavor and leavening to traditional grain products like South Indian idli and Ethiopian injera.   Lactobacilli are also responsible for the sour flavor of traditional salami and other cured sausages.


At least as instrumental in modern food production are the acetobacter.  These bacteria consume alcohol and produce acetic acid to make vinegar.  Vinegar has replaced lactic acid fermentation in many modern food products because it is cheaper and more predictable than relying on lactobacilli.  Most pickles today, for example, are pickled with vinegar instead of soured in the traditional way.  Because acetobacter eat alcohol, vinegar is a two stage fermentation.  First yeast are used to convert sugar to alcohol, and then acetobacter convert that alcohol into acetic acid.  Wine becomes wine vinegar, beer becomes malt vinegar, etc.


There are also bacteria that produce ammonia, and these play a role in products such as Brie cheese, and other red cheeses like Munster and Limburger.


More prevalent in Asia, there are still some common molds that are used to make cheese in the West.


Tempeh originates in Indonesia, and is made by introducing a fungus to cooked soybeans.  The fungus feeds on the soy beans (or other substrate) and produces a mass of threads that tie together the beans into a cake.

Cheese mold

Many of the soft cheeses depend on mold to create their rinds and “ripen” the interior.  Brie, camembert, etc. are good examples.


Koji is a mold that is used in many Asian ferments to break down rice starch into sugars.  Koji is introduced to cooked rice, breaking down the starches in the rice, much as the malting process does in wheat or barley.  Once that process has occurred, yeast can be introduced to produce fermented beverages like sake or shao xing.  Koji is also used in the initial stages of miso production.  Some of the earliest fermented beverages found in Asia probably used koji or something similar to prepare rice for alcoholic fermentation.


Yeasts are single celled organisms that are classified along with fungi.  Most of them reproduce by budding, and they can reproduce extremely rapidly.  The majority of the yeasts that are used in food preparation are varieties of Saccharomyces cerevisiae, although other types are used as well.


The production of beer, wine, and mead are all very similar.  Some sort of sugar is dissolved in water and yeast is introduced to turn the sugar into alcohol.  Whether that sugar is from malted grain, grapes, apples or other fruit, or honey makes little difference in the actual processing.  Once the yeast starts to produce alcohol, it prevents most other organisms from gaining a foothold.


Central Asian Kumiss is fermented from milk, traditionally the milk of mares.  This yeast is slightly different from beer yeast, adapted specifically to eat lactose in milk.  This may be similar to the yeast used in Scotland to produce blaand, another fermented milk beverage.


Yeasts are also responsible for making bread rise.  Yeast eat the sugars in grain and convert it to carbon dioxide.  When those carbon dioxide bubbles are trapped in a protein (gluten) web, they expand the dough and raise the bread.  That same carbon dioxide produces carbonation in beer.  The biggest difference between these processes is that bread yeast are adapted to produce less alcohol and more CO2, while beer yeast produce more alcohol.  In modern “quick rise” yeast you can see the connection because quick rise yeast produces more alcohol as a byproduct of its quick rise times, and bread made with any amount of sugar and quick rise yeast can smell very alcoholic if left to rise too long.  In sourdough baking, wild yeast is captured from the air, or a receptive surface such as fruit or organic whole grain.  Wild yeasts tend to grow on the skins of certain fruits, including grapes and apples, as well as under the husks of grains like wheat and particularly rye.  Fruit skins or organic whole grain rye can be introduced to flour and water to capture sourdough starters.


Some fermented food products are produced by a conglomeration of one of more types of microorganisms working together.  Most often, this takes the form of  yeast and a bacteria working together, although many cheeses are made by combinations of bacteria and molds.


Traditional Central Asian kefir is produced by introducing “kefir grains” into milk.  These grains are composed of a matrix of yeasts and bacteria that work together to produce a sour, slightly effervescent milk product.  The grains can then be strained out of the kefir and reintroduced to more milk.  Over time the grains grow, and can be subdivided and shared.  This traditional form of kefir is quite different from the “kefir” that is found in most grocery stores today,a thin yogurt or cultured milk product that is not made with kefir grains.


Kombucha is another example of a matrix of yeasts and bacteria used to ferment a liquid, in this case usually sweetened tea.  The kobucha “mushroom” or “mother” grows into a rounded blob rather than a set of grains, but the effect is similar.  Like kefir, the kombucha mushroom will just keep growing, and can be divided and passed along.


Sourdough starters are also composed of one or more bacteria (lactobacilli) and one or more strains of yeast.  The bacteria produce in acidic environment in which the yeast can thrive.

Food preservation

Many cultures have used fermentation as an aid to food preservation.  This is possible because of the very nature of fermentation.  Any fermentation process is a race to see if one species (or partnership of several) can alter the composition of their host material into something that is inimical to other organisms that might compete with them.  We consider fermentation successful if the dominant species does something we like to our food rather than something that we do not like.  Thus, lactobacilli turn milk into yogurt so that it does not grow mold (which tastes bad), and yeasts make beer out of barley so that it does not mold or succumb to bacterial infection.

Pushing out the Competition


One of the most common ways to achieve food preservation with little biological minions is by encouraging them to produce acid.  Lactobacilli and their cohorts produce acid in foods like sauerkraut, kimchi, yogurt, Kosher dills, piima, buttermilk, etc.  By lowering  the pH of these foods, they keep out molds and other bacteria (such as the less pleasant streptococci) that might spoil the food.


Some bacteria produce a basic environment, such as the Micrococcus and Staphylococcus that make traditional Scandinavian fish products like Surströmming and rakfisk.  Several of the red-rinded cheese like Munster and Limburger also employ bacteria that produce ammonia, which raises the pH of the cheese and keeps out other bacteria.


In beer/wine/mead, the yeast produces alcohol that keeps out bacteria that can produce off flavors.  If the yeast is not established fast enough, bacteria can grow that will sour the beer or cause unpleasant flavors.  Introducing commercial brewing yeast, as is modern practice, aims to ensure that the yeast prevail over their competition.  In traditional period brewing, contamination was probably much more common.

Health benefits

There are a number of benefits to consuming fermented foods, and many more are attributed to them.

Micro flora

Many traditional fermented foods are “probiotic”, meaning that they introduce beneficial bacteria into our systems.  We need certain bacteria to thrive in our intestines in order to make the most of the food that we eat.  Some health problems, modern foods, and medications can harm these bacteria, and probiotic foods can help replace them.  Traditionally made yogurt or other diary cultures, sauerkraut, and other pickles are all good sources of beneficial micro flora.

Nutrient absorption

One of the side benefits of many fermentation processes is that they make food easier to digest and/or make nutrients in the food more easily accessible for absorption by our bodies.  Fermented sauerkraut, for example, has more accessible vitamin C than does raw cabbage.  Lactobacilli break down lactose in milk, making it easier for those with lactose sensitivities to digest.  Sourdough bread is not only easier to digest, but has a lower glycemic index than plain yeast risen bread, meaning that is does not raise blood sugar as fast.

Altered food composition

Some foods are not edible at all until they have been fermented, usually because the fermentation process breaks down some toxic substance in the food.  The infamous hakarl of Iceland is made from a species of shark whose flesh contains often toxic levels of uric acid.  The fermentation process that creates the pungent dish also breaks down the uric acid and makes the flesh edible.


In practice, fermentation at home is mostly a matter of care and patience.  Patience is often the hardest, and yet the most necessary component.  One of the reasons that fermentation has fallen out of favor from a commercial perspective is that it takes time.  It is much faster (and more predictable) to pickle cucumber in vinegar than it is to make Kosher full-sours.  Baking with commercial yeast takes much less time than creating full-flavored and complex sourdoughs.  Grocery store sauerkraut is cured with vinegar and contains no beneficial bacteria.

Aside from patience, cleanliness and temperature control are the other most important factors.  In order to give the organisms we want a favorable environment, we can start be using clean instruments and containers that are free of unwanted contaminants.  Depending on the micro flora you are hoping for, you may need to increase or decrease the temperature at which you are fermenting.  Yogurt bacteria (l. acidophilus and friends) like warmer than room temperatures, while the lactobacilli in sauerkraut are more likely to beat the competition at lower than standard room temperature (< 70°).  Most sourdough cultures perform best at around 80-85°.


Most vegetable pickles are easiest to produce at cool room temperatures.  You may have a harder time with them in high summer when the ambient temperature is above 80°.  Keep your containers clean to help keep out unwanted flora.

Making Vegetables a Hospitable Environment for Friendly Minions

A little salt added to vegetables will help keep out unwanted bacteria or mold and help the lactobacilli we want to take hold.  In perfect conditions, salt can be left out almost entirely, but particularly when just starting out you will have an easier time using a little salt.

It also helps if you can introduce some beneficial bacteria early on in the process rather than waiting for them to establish themselves.  A little dairy whey, particularly from a cultured source like yogurt of buttermilk will help encourage the right lactobacilli.

Cooler temperatures below 75° or so will help keep mold from growing while your little minions are taking over.  If it is too warm, mold or unwanted  bacteria are more likely to grow and ruin your pickles.


One of the easiest and most edifying vegetable ferments to make at home is sauerkraut.  Shred your cabbage, and add a tablespoon of salt per average sized head of cabbage.  If you have some whey, add 3-4 tablespoons worth to the salted cabbage.  Pound the cabbage with a wooden mallet or pestle, or something similar.  This helps to break down the fibers in the cabbage, and releases the water to help cover the cabbage in its fermentation vessel.

Once the cabbage looks wilted and has started to release its water, pack it into clean mason jars and continue to pound it down into the jar until the liquid rises above the level of the cabbage.  Use some sort of weight to keep the cabbage submerged under the liquid.  I use a ziplock bag filled with filtered water packed into the top of the mason jar to keep the cabbage submerged.

Leave in a cool place for at least 3-4 days until bubbles appear and the cabbage starts to look dry and open.  You can put it in the refrigerator at that point, or leave it out to grow more sour if you like it that way.

You can add all sorts of spices to your cabbage during pounding, including dill, caraway, juniper berries, or pepper.  You can also include other vegetables or fruits, such as apples, cranberries, carrots, turnips, or sea vegetables.

Whey pickles

Root vegetables lend themselves particularly well to being fermented in brine.  Beets, carrots, turnips, and kohlrabi can be sliced thin and covered in salted water (again, adding whey helps here), then weighted to keep the roots submerged.  They are done when sour and slightly bubbly.  This can take at least 4-5 days.

In modern Scandinavia, the whey is strained from a cultured buttermilk like fil mjolk, and vegetables are submerged in the resulting whey, with or without salt.  These will be quite sour in just a few days and will keep well.


Brewing can be as simple as mixing honey with water and leaving it in an open container covered with cheesecloth.  However, that will not yield very predictable results.  It might make mead, or it might just spoil.  There are many excellent works on home brewing, so I will not add much here except to say that in period, brewing was probably a bit more hit or miss, and that yeast would have been either introduced from a previously successful brewing or by using the same wooden container impregnated with yeast to brew successive batches.  All brewing relies on yeast of one kind or another to turn sugars into alcohol and CO2.  Beer, cider and mead were popular in Northern Europe, while wine was more prevalent further south.


Yogurt of various types is easy to make at home.  For true yogurt as it comes from the grocery store, you must ferment the milk at a warm temperature, 90+ degrees for a long period.  Yogurt makers usually either keep the milk warm using electricity or provide an insulated container in which a warmer temperature can be maintained.However, there are a number of yogurt-like dairy cultures that are easy to culture at room temperature.  Fil mjolk, piima, viili, matsoni, and some buttermilks will grow successfully at normal room temperatures.  Add a culture to milk and leave it at room temperature, usually overnight until it sets up as desired.  Cultures such as these are available from Cultures for Health, GEM cultures, and Moowiseherbs.com.  Cultures for Health also sells through Amazon.com.  Any of the above can be recultured indefinitely if care is taken to avoid contamination.  Depending on what kind of milk is used, different textures can be obtained.  Piima cultured in skim milk is like buttermilk, while piima in half & half is more like crème fraiche, and in heavy cream like sour cream.  For a simple cultured cheese, milk soured with one of the above cultures can be heated until it curdles, then drained, pressed and salted.  It will continue to ripen over several days if left in a cool place.


Sourdough cultures can be captured at home, or purchased from a number of reliable sources.  Sourdo.com, Cultures for Health, and others provide a wide range of cultures adapted to different flours and baking styles.  To capture one yourself, mix equal parts flour and water and leave in an open container in a cool dark spot in your kitchen.  Use organic whole grain flour for best results, since there are wild yeasts present in the hulls of many grains, particularly rye.  I personally have had the best luck with whole grain rye flour.  Each day, throw out half of the mixture and add additional flour and water in equal parts.  After a day or two, you should start to see bubbles forming on the surface, and notice a pleasantly sour smell.  After 3-4 days you should have a viable starter culture.  Once you have a good culture, you should be able to use it indefinitely if you keep it from being contaminated and continue to feed it.  If kept in the refrigerator and fed every few weeks it should last forever.  Baking with sourdough cultures is a wide a varied subject, and out of scope here.  See the references section for some good books on sourdough baking.


Curing meats with bacteria is a tricky process and has to be tightly controlled to avoid illness.  See Michael Ruhlman’s excellent book Charcuterie to get started.


The miraculous transformations engendered by fermentation at home are not reserved for professionals in laboratories.  There are a number of foods that are easy to ferment at home, and it is a very rewarding endeavor in terms of both process and product.

As hard as it can be to find commercially made fermented foods, it is easy to do at home with common materials, a little patience, and care.

Fermenting foods at home was something that most of our ancestors would have taken for granted, even though they did not understand the process.

Remember, it does not stop being magic just because you know how it works…


Bentley, Nancy. Truly Cultured : Rejuvenating Taste, Health and Community with Naturally Fermented Foods : A Cookbook and Nourishment Guide. 1st ed. [S.l.]  ;Indianapolis  IN: Nancy Bentley  Two Pie Radians Foundation ;;in association with IBJ Custom Pub., 2007. Print.

Campbell-Platt, Geoffrey. Fermented Foods of the World : A Dictionary and Guide. London ;;Boston: Butterworths, 1987. Print.

Fallon, Sally. Nourishing Traditions : The Cookbook That Challenges Politically Correct Nutrition and the Diet Dictocrats. Rev. 2nd ed. Washington  DC: NewTrends Pub., 2001. Print.

Katz, Sandor. Wild Fermentation : The Flavor, Nutrition, and Craft of Live-culture Foods. White River Junction  Vt.: Chelsea Green Pub., 2003. Print.

Mardewi, Yoke. Wild Sourdough: The Natural Way to Bake. New Holland Publishing Australia Pty Ltd, 2009. Print.

Reinhart, Peter. Peter Reinhart’s Whole Grain Breads : New Techniques, Extraordinary Flavor. Berkeley  Calif.: Ten Speed Press, 2007. Print.

Ruhlman, Michael. Charcuterie : The Craft of Salting, Smoking, and Curing. 1st ed. New York: W.W. Norton, 2005. Print.

Steinkraus, Keith, Symposium Workshop on Indigenous Fermented Foods. Handbook of Indigenous Fermented Foods. New York: M. Dekker, 1983. Print.

Tamang, Jyoti. Fermented Foods and Beverages of the World. Boca Raton: CRC Press/Taylor & Francis, 2010. Print.

Wood, Ed. World Sourdoughs from Antiquity. [Rev. ed.]. Berkeley  Calif.: Ten Speed Press, 1996. Print.