Yeast & Fermentation

This post endeavors to briefly illuminate a particularly minuscule organism that since the dawn of mankind has exerted considerable influence over the human condition.  Found in the dirt, air and water some yeast also subside naturally – inside all vegetation, animals and humans.  All fungi are parasitic or saprophytic and cannot manufacture their own food.  Since yeast are fungi and all fungi are heterotrophs that live on preformed organic matter some yeasts have been using mankind for far longer than he has been using them.   To state that mankind has domesticated yeast for thousands of years is probably an erroneous statement.  Whether he knew it or not however mankind has been exploiting these individually invisible microorganisms for his own benefit for perhaps ten millennia or more.  The historic relationship between brewing and baking is more intertwined than most readers may appreciate.  Today yeasts are also used to produce food additives, vitamins, pharmaceuticals, biofuels, lubricants and detergents. The more one learns, the more his appreciation grows for these seemingly simple little life forms.  It doesn’t take a degree in organic chemistry or molecular biology to put these little critters to productive work.

Yeasts are more evolutionary advanced microorganisms than say prokaryotic organisms like viruses and bacteria.  Prokaryotes don’t have a nucleus.   Higher life forms like onions, grasshoppers, humans and yeasts are eukaryotes which means their cells store genetic information within a nucleus.  Simpler and more basic than human cells and easier to work with, bread yeast (Saccharomyces cerevisiae) was the first eukaryotic organism to have its genome be fully sequenced.  A genome is the hereditary information stored in an organism – the entire DNA/RNA sequence for each chromosome.

The S. cerevisiae yeast genome possesses something like 12 million base pairs and 6,000 genes compared to a more complex human genome with 3 billion base pairs and 20,000 -25,000 protein coding genes.  Although sequencing has become easier in recent times, 18 years ago the thorough examination of Saccharomyces cerevisiae’s (beer yeast) genome was no simple task.  That project inspected millions of chromosomal DNA arrangements, involved the efforts of over 100 laboratories and was finally completed in 1996 after seven years of hard work.

* The 6th eukaryotic genome sequenced was also a yeast (Schizosaccharomyces pombe – in 2002) and it contained 13.8 million base pairs. 

The mentioning of this 1st accomplished genome sequencing is significant because it was to cause an upheaval in the current accepted classification of yeast species.  There are probably a great number of yet undiscovered yeast species in the wild but presently only a small percentage (between 600 and 1,500 species depending upon your source of information) are currently cataloged.  One of the more important fungi in the history of the world, the classification of Saccharomyces cerevisiae species is very much in a malleable state of flux.  You may read about the many types of bread yeast, or the hundreds of “varieties” of beer yeast or the hundreds of “strains” of wine yeast – but for the most part these share the same DNA and therefore must be considered the same species.   With beer and especially with wines the choice of yeast (strain or variety and species where applicable) can profoundly influence the outcome of the beverage’s flavor profile.

Bad fungus

“Almost all yeasts are potential pathogens” but none of the Saccharomyces species or close relations have been associated with pathogenicity toward humans.   “Candida and Aspergillus species are the most common causes of invasive fungal infection in debilitated individuals”, with 6 species (Candida: albicans, glabrata, krusei, neoformans, parapsilosis & tropicalis) accounting for about 90% of those infections.

Other multi-cellular (non-yeast) fungi affect humanity in various ways: Trichophyton rubrum and / or Epidermophyton floccosum bring us athlete’s foot, ringworm, jock itch and nail infection.  A member of genius Penicillium (with over 300 species) brings us a life saving antibiotic which kills certain types of bacteria in the body.   Claviceps purpurea or “rye ergot fungus” – if not immediately lethal or debilitating, brought us a mind altering alkaloid similar to LSD.  One of the more important negative influences fungi exercise upon us is in the capacity to destroy food crops.  

Domestication ?

A defining characteristic of domestication is artificial selection by humans.  Domestication means altering the behaviors, size and genetics of animals and plants.  These things were not done to yeast in antiquity.   Isolation of certain beneficial yeast strains was only beginning some 200 years ago, in breweries.  Only recently (by 1938) was one scientist was able to cross two separate strains of yeast and come up with a new one.  Although by the 1970’s scientist were beginning to mutate and hybridize yeast, it may be with the more recent attempts to engineer yeast to convert xylose (a wood sugar) into cellulosic ethanol that some additional yeast species can confidently be described as domesticated.  Even then “engineering” is a strong word.  Yeast mutate all the time without human help.  Scientist didn’t create a new fungus but started with examples that already decomposed dead trees or other cellulose containing plant material.  By attenuating the selection process for yeast with numerous cellulase enzymes, scientists hope to produce economical automotive fuel from sawdust and other normally wasted biomass.  The quest for an ideal yeast and bacterial biomass consuming combination is still ongoing.  This particular process defines artificial selection, not gene modification.

Right now, this very moment anyone can capture wild yeast from vegetable matter or from the very air to make bread or to ferment beer or wine.  In antiquity the women folk who cooked and then later bakers, brewers and tavern keepers likely kept a portion of a previous dough or barm yeast culture as a ‘starter’ simply to hasten the development of the next batch.  While this process might support claims of artificial yeast selection throughout history, one might also be reminded that sanitation during those bygone days was questionable and that exposure to wild yeast and bacteria was probably persistent.  It has always been easy to just whip up a new yeast culture from scratch, as will be explained shortly and as revealed in several recipes from a 120 year old cookbook.


Bread, Beer & Wine

The discovery or invention of wine, beer and bread were unavoidable and early man deserves no special intellectual credit for the achievement because omnipresent yeasts and bacteria did all the work.  Consider the cavewomen that picked a bountiful harvest of wild grapes and then carted these back home in animal skins or clay-lined baskets to be consumed later.  In a few days time wild yeast and bacteria would begin breaking down the fructose and glucose from juice released from crushed grapes at the bottom of any impermeable container.  The oldest available archeological evidence of a fermented beverage comes from 9,000 year old mead (honey wine) tailings found in northern China.  Here probably someone had originally, unknowingly enabled the enzymes from yeast to work by adding water to get all the sticky honey out of a container.  Likewise the inescapable discovery of bread and beer are no mystery.  Raw fresh grain is soft and easily chewable foodstuff.  Dried grain is next to impossible to chew so ancient man was soon mashing it between two rocks to make the powder called flour.  Dry flour is not very tasty so the next obvious experiment would be to add water and later perhaps to cook the gruel in a fire – eventually inventing bread.  Obviously the first breads were probably flat breads.  The proper leavening of bread actually requires several hours of rest for fermentation to create carbon dioxide bubbles which get trapped in gluten to make bread rise.  Had someone boiled a wet soup from the flour instead and then abandoned it because it wasn’t very good, it would have eventually turned into a beer in a few days.  Perhaps the first beer or ale resulted simply from someone’s bread falling into a pot of water.  Regardless, our encounter with fermentation and the invention of both bread and alcoholic beverage was inevitable.

Briefly, Saccharomyces cerevisiae (or sugar fungus) is typical of many yeast species but is a particularly successful species because it can live in many different environments.  Few of the other 64,000 or so members in the Ascomycota fungal phylum can reproduce both sexually and asexually while also being able to break down their food through both aerobic respiration and anaerobic fermentation – all at the same time.

budding yeast

 Under favorable conditions most, but not all yeasts reproduce asexually by budding where one cell splits into two.  On average a particular yeast cell can divide between 12 and 15 times.  In a well controlled ferment aerobic (with oxygen) respiration allows “sugar fungus” yeast cells to reproduce or double about every 90 minutes.  During respiration carbohydrates donate electrons, allowing cell growth, CO2 and water (H2O) production.   During anaerobic fermentation carbohydrates undergo oxidization while ethanol and CO2 are produced.  One yeast cell can ferment approximately its own weight in glucose per hour.  Favorable ferment conditions in this context imply moisture, mineral nutrition, a neutral or slightly acidic pH environment and a narrow temperature range of 50° F to 99° F.  Most yeast cells are killed at temperatures above 122°F.

* (No yeast yet known is completely anaerobic nor is fermentation necessarily restricted to an anaerobic environment).

Under harsh or unfavorable conditions yeasts like S. cerevisiae can become dormant and reproduce sexually by producing spores.  Spores can survive for hundreds of years, perhaps indefinitely, and like many other infinitesimal items can remain airborne for years before coming back into contact with the surface of the earth.  Anyone questioning this assertion should have a look at Lyall Watson’s book, titled “Heaven’s Breath: A Natural History of the Wind”.


A typical yeast cell measures about 3–4 µm (microns or millionth of a meter) in diameter.   Dry packaged yeast as imaged above can survive a long time when refrigerated.  The 3 large bakers yeast packages pictured at the bottom are labeled as containing 21 grams of yeast each.  The 3 brewers yeast packages on top are labeled 5 grams.   Compressed yeast which would contain less yeasts per gram because less water has been removed, is estimated to contain between 20 and 30 billion living organisms – per gram.  The physical volume of that gram would be about the size of a pencil eraser.


In general, bacteria are to be avoided during normal food and beverage production, but as usual there are exceptions.   Many of the approximate 125 species of lactobacillus bacteria are closely associated with food spoilage.  Without the assistance of beneficial bacteria (several of which are lactobacillus members) however we would have no vinegar, chocolate, cider, cheese, kim-chi, pickles, sauerkraut, sourdough bread or yogurt.  Bacteria can drive fermentation by themselves.  More preferably, certain beneficial bacteria can assist yeasts in the fermentation reaction for breads, beers or wines and are sometimes deliberately used to do so.


In baking or brewing it is the enzymes that yeasts or bacteria possess or produce which catalyze chemical reactions and drive fermentation.  A mixture of enzymes might be needed to successfully break down complex longer chained carbohydrates, before either bread leavening or ethanol production is achieved.  In alcoholic fermented beverages, enzymes might be acquired from sources beyond yeast and bacteria, such as from human saliva where for a thousand years descendants of the Incas have chewed maize and spit into common vats to produce the wine called “Chicha”.  The rice wine “Sake” is made with the help of enzymes from a (non yeast) fungus mold named Aspergillus oryzae.  The enzymes used to create the Mongolian horse milk wine known as “Ayrag” or “Kumis” came from the lining of a bag sewn from a cow’s stomach.   There are far too many types of enzymes to list here but the names of some important ones often end in the suffix “ase” (as in: lactase, saccharase, maltase, alpha amylase or diastase, zymase or invertase and alpha-galactosidase).    

Sugar or starch

To briefly outline and oversimplify a topic that deserves more attention: there are many names for, and many types of, starches and sugars and enzymes needed to break them down.  There are simple sugars, complex sugars and very complex sugars or conversely one could say: ‘there are: monosaccharides, disaccharides, oligosaccharides, and polysaccharides’.  Glucose (or dextrose), fructose (or levulose), galactose, and ribose are monosaccharides and examples of the simplest sugar molecules.  Two monosaccharides are found combined in a disaccharide – as in sucrose, lactose or maltose.  Table sugar is almost pure sucrose.  An enzyme like zymase (also called invertase or a dozen other names) is needed to split sucrose into two mono or simple sugar molecules (glucose and fructose) before fermentation of ethanol and CO2 can commence.  Oligosaccharides generally contain anywhere between 3 and 9 monosaccharides.  Polysaccharides are even longer, linear or branched polymeric carbohydrates and may sometimes contain thousands of monosaccharides.  Starch and cellulose are examples of polysaccharides.

Sugarcane was originally indigenous to Southeast Asia and was slowly spread by man to surrounding regions.  In ancient times sugar was exported and traded like a valuable spice or medicine – not as a food commodity.  There was some spread of sugarcane cultivation in the medieval Muslim world but otherwise cultivation did not blossom until the 16th century when colonials reaped their first sugar harvest in the New World (Brazil and the West Indies or Caribbean Basin).  Sugar from sugar beets was never realized until a German chemist noticed that the beet roots contained sucrose.  The first refined beet sugar commodity appeared around 1802.



Leaven” is the ancient equivalent term for yeast and it caused bread to rise.  Leaven was mentioned in the Bible when Moses led the Israelites out of Egypt, and where they all left in a hurry without waiting for their bread to rise.  Flat, unleavened, unremarkable bread is served during Passover, which is not a Jewish feast or celebration but a remembrance of deliverance, simplicity, haste, and powerlessness.  “Yeast” is a younger word with roots from Indo-European and Old English words meaning surface froth, bubble, foam and boil.  In times past and probably for many centuries, housewives and or cooks usually made both bread and beer on a frequent basis, from a leaven-yeast starter that they maintained in the kitchen.  In both Medieval Europe and colonial North America many households also maintained a constant supply of “small beer” on hand for servants and children or for general consumption.  Small beer had low alcohol content but some taste and since it was pasteurized it was usually much safer to drink than the local water.  Two centuries ago some children drank small beer with breakfast just like today’s children might drink orange juice.

Almost all bread before the 1840s was probably a form of sourdough bread.  Without the help of either bacteria or refined sucrose, S. cerevisiae yeast alone cannot properly break down the starches (polysaccharides or carbohydrates) in flour, work its fermentation or cause bread to rise.  In the early 1800s, for the fist time, collective bakers began making sweet breads (as opposed to sour) by using bottled yeast skimmed off and collected from ale (beer) vats.  This renaissance in baking quickly spread outwards from Vienna, Austria.  In general, bakers started buying top-fermenting beer yeast from brewers.  Initially the yeasts were collected by skimming barm or krausen off the top of a beer vat and putting it into bottles.  In about this same time frame another renaissance or revolution was occurring in the beer world.   German brewers were learning to make lagers, which employed different (bottom dwelling) yeast and much cooler and longer fermentation periods.  At the time lagers were a taste sensation and considered a great improvement over the heaver ales.  With many brewers ‘changing horses in mid stream’ to use different yeast and processes in order to jump on the lager bandwagon, bakers in Vienna and elsewhere were left without convenient sources of sweet yeast.  To fill that void ‘press yeast’ was developed.  The forerunner of modern baker’s yeast, press yeast was first skimmed from the top of a dedicated grain mash and washed and drained carefully before being squeezed in a hydraulic press.  Modern baker’s yeast have pretty much been selected for optimum carbon dioxide production.  Such yeast would still make good ale.  Bread dough makes alcohol while fermenting but that escapes when it is baked.

* The grains corn and rice have no gluten.  To make breads with these grains rise, flour with gluten must be added. 

* “Quick breads” like biscuits, pancakes, bannock, scones, sopapias and cornbread are made with “self rising flour” or regular flour with the help of a baking power.  Self rising flour merely contains its own baking powder.  Baking powder is a mixture of soda, acid salts and starch (which helps keep the other two ingredients inactive).  Baking powder is basically a little bomb, a little electrochemical reaction for making gas bubbles; waiting only to be triggered by the addition of liquid.  


Sourdough bread

Sourdough is a vague term.  There are many ways to create a sourdough starter.  While the name implies a sour taste due to contribution of bacteria and / or wild yeast, some sourdoughs taste little different than normal commercial sweet bread.  Some sourdough starter recipes actually call for baker’s yeast to be used while others might begin with pineapple juice, potatoes or even yeast captured in an opened can of beer left on the kitchen counter top for about a week.  A characteristic practice of sourdough bread making is that a portion of the ‘sponge’ is to be retained after each dough batch and is stored in a cool place to be used as the next starter.  ‘Sour mash’ whiskey has the same connotation – part of the original yeast and enzyme culture is retained and used in the next batch – maintaining consistency of product.   In brewing “re-pitching” the yeast is similar to using a sourdough starter; a portion of the live yeast from the bottom or top of a wine must or grain mash is saved to be reused again.

In the 1840s as the first Bavarian lager technology was reaching America, gold miners were about to congregate in the California Gold Rush.  San Francisco is a modern bastion of sourdough bread patronage with some restaurants or bakeries claiming to have maintained the same starters since the Gold Rush days.  One species of lactic acid bacteria found in some sourdough is actually named after the city: Lactobacillus sanfranciscensis.  Also these starters might include species of yeast (like Saccharomyces exiguous or Candida milleri) that can leaven bread by working on polysaccharides instead of simple sucrose.

Homemade yeast

While fresh compressed yeast was becoming common in the urban food markets of Europe and America by the 1870’s, many individuals (especially those in remoter areas) simply made their own yeast.  The “White House Cook Book” was an authoritative publication ((c)1887 and before) used by ambitious housewives across the country.  The book gives several recipes for starting a yeast culture, including the use of milk or salt and even drying the yeast into cakes for later use.  One of the book’s recipes for yeast is simply titled “Unrivaled Yeast” and it resembles the following (actual recipe is on p.242):

– boil 2 oz. of hops in 4 qts. of water for 30 minutes, strain and let cool

– mix this water in large bowl with 1 qt flour, ½ cup salt and ½ cup brown sugar –let stand for 3 days

– mix this with 6 boiled and mashed potatoes – let stand for another day, stirring frequently.  

– ready to use or to be stored in bottles for future use (good if kept cool for about 2 months).

Obviously the yeasts native to the potatoes were killed by boiling, so yeasts from the atmosphere and perhaps flour as well were the ones captured.  Sanitation and sterilization of utensils was and still is important to limit the procreation of undesirable bacteria.   Hops (flowers of the Humulus lupulus plant) are frequently mentioned in these older recipes because hops which were also used as herbal medicine, act as an antiseptic \ antibiotic preservative by inhibiting bacterial growth but not beneficial yeast growth.

* The Reinheitsgebot or Bavarian Purity Law of 1487 – specified the use of only water, barley and hops – for the brewing of beer.   The contribution of yeast was not appreciated but the antibacterial benefits and virtuous bitter flavor components of hops were.  Evidence suggests that hops were being used in Bavarian beer as early as 736 in an abbey outside Munich.  The Reinheitsgebot also had the effect of discouraging competing imported Belgian beers which preferred to use gruit and of preserving the wheat harvest for those needing to bake bread.  

 * Actually there may be much more to the story of the Reinheitsgebot.  Conveniently coinciding with beer brewing politics during this period was the growing religious dichotomy between the Catholic Church and theologians like Martin Luther.  The arrival of the Gutenberg printing press in 1439 helped spread Protestant literature, leading to the Protestant Reformation and ultimately the widespread replacement of gruit flavoring, with hops.   Secretive, controlled and monopolized gruits would enhance sexual drive while Protestant approved hops did the exact opposite.  The motivations (for hop’s substitution of other herbs) were religious and mercantile.

There are many, many other interesting facts to discuss about yeast, enzymes and bacteria in regards to fermentation but this post has to draw a conclusion or come to an ending somewhere.  No more time will be taken to examine yeast killing sulfides in wine, the alcohol tolerance of different yeasts, turbo yeast or how Champagne is created by secondary fermentation.  Somehow it seems that yeasts have used us just as much as we have used them.  We have changed their nature little – if at all.  For the small percentage of yeast species we have identified, we are on the verge of understanding the true nature of just a few.


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