Fermenting Beer


By Jack Horzempa

Fermenting Beer

History of yeast

In the Reinheitsgebot (German beer purity law) of 1516 there were three ingredients listed in brewing beer: barley (malt), hops and water. As any brewer today knows there is one ingredient missing from that list: yeast.

Yeast as a microorganism was not known in 1516 but there is little doubt that brewers were aware that something ‘else’ was needed to produce beer.

For example, from a Middle English dictionary (Editor: Sherman M Kuhn; The University of Michigan Press):

“Berme [yeast], otherwise clepid [called] goddes good [god's good], withoute tyme of mynde hath frely be goven .to ye value only of a ferthyng .bicause it cometh of ye grete grace of God.”

The above reminds me of another quote which is mistakenly attributed to Benjamin Franklin:

“God made beer because he loves us and wants us to be happy.”

Well the brewers of yore certainly thought that God provided to them yeast so they could produce beer. One of the tricks they would use is to have a favorite wooden paddle as Michael Jackson (the beer Michael Jackson) described it:

“Brewers in the middle ages had no idea about the presence of yeast and the role it plays in beer production. These rustic brewers would often stir a new vat of wort with a "magic" wooden paddle inoculated with yeast cells from previous batches. Fermentation would kick in within a few hours.”

Science to the rescue

Many folks are familiar with Louis Pasteur since he came up with the process of pasteurization to help milk keep longer.

But prior to coming up with the pasteurization process, in 1856 he studied quality problems some manufacturers of beet root alcohol were having. What the heck is beet root alcohol you might ask (as do I)? Well, I do not know what folks were drinking in 1856 but beets are a source of sugar (e.g., sugar beets) and we all know that it is sugars (i.e., the sugars in wort) which are the food for fermentation. During his studies Louis Pasteur discovered a number of single cell microorganisms involved in the fermentation process. The good microorganisms were the yeast (which are a fungus) and the bad microorganisms were bacteria which would create undesirable flavors like sour (spoilage) flavors – for example Acetobacter aceti.

Perhaps Louis Pasteur would have said that when it comes to fermentation it comes down to the little things?

Louis Pasteur

Brewers yeast

There are a number of types of yeast used to ferment beer:

  • Ale yeast (Saccharomyces cerevisiae)

  • Lager yeast (Saccharomyces pastorianus)

  • Brettanomyces (sometimes just called the shortened name of Brett)

Ale yeast (Saccharomyces cerevisiae)

It all started in ancient (Egypt or Mesopotamia or…) where some jar of grains and water spontaneous fermented from the yeast in the air. There were yeast cells found in Ancient Egyptian pottery (circa 3000 BC) that had fragments of the DNA of Saccharomyces cerevisiae. It could be that that the very first beers were fermented with wild strains of Saccharomyces cerevisiae. Needless to say but those ancient peoples knew that beer is a good thing so it was only a matter of time before their preferred strains of yeast would be domesticated via their favorite wooden sticks/paddles.

Brewing in ancient Egypt

One of the features of ale yeast is that it is a top fermenting yeast and for some breweries they will harvest this yeast for subsequent brews via skimming off this yeast during the active portion of fermentation – a practice known as top cropping.

Lager yeast (Saccharomyces pastorianus)

Compared to the ancient species of Saccharomyces cerevisiae, Saccharomyces pastorianus is a relatively modern species.

Nobody really knows when lager yeast first ‘happened’ but it is commonly described as being sometime in the 1400’s with lager beers being produced in Central Europe (e.g., present day Germany/Czech Republic).

German lager brewing in the 1400’s

Genetic studies reveal that lager yeast (Saccharomyces pastorianus) is a hybrid of Saccharomyces cerevisiae (ale yeast) and Saccharomyces eubayanus.

A number of years ago they discovered Saccharomyces eubayanus in the forests of Patagonia (South America). So some folks opined that this was the source for creating the hybrid Saccharomyces pastorianus. But how the heck did this yeast get from Patagonia to Central Europe over 500 years ago. Christopher Columbus did not arrive to the New World until 1492. I am not aware that there was any trading between South America and Europe before then.

Now, Saccharomyces eubayanus exists in other parts of the world (e.g., China) and there has been regular commerce between Europe and China since the times of Marco Polo (13th century). Maybe some yeast ‘hitchhiked’ on some trading goods?

Below is from an Abstract from a technical paper published in 2019:

“Our genome analysis together with previous reports in the sister species S. uvarum strongly suggests that the S. eubayanus ancestor could have originated in Patagonia or the Southern Hemisphere, rather than China, yet further studies are needed to resolve this conflicting scenario. Understanding S. eubayanus evolutionary history is crucial to resolve the unknown origin of the lager yeast and might open new avenues for biotechnological applications.”


Perhaps with further genetic studies we will learn more here.


Brettanomyces is Greek for British yeast and its discovery is often attributed to N. Hjelte Claussen, working at the Carlsberg laboratory. He was the first to publish a paper on this topic (1904).

His paper discusses the importance of Brettanomyces for secondary fermentation and for producing the flavors of English stock (keeping) beers. It turns out that Brettanomyces would take residence in the oak barrels used to store/transport British beers which would then further ferment the beer. One example is that the British brewed IPAs (circa 1800) that were sent to India would result in lower final gravity values as a result of the fermentation by Brettanomyces in the wooden barrels during long storage and long transport of these beers.

In contemporary brewing Brettanomyces is often associated with Belgian Lambic brewing and also the Wild Ales of craft brewing. But beer can be produced solely with Brettanomyces in the primary fermenter and these single strain beers tend to be clean in flavor unlike the Lambic/Wild Ales where Brettanomyces is used as a secondary fermentation yeast and funky (e.g., barnyard) flavors are often produced via the secondary fermentation process.


Back in the time of Louis Pasteur’s studies the presence of bacteria and the spoiling tastes (e.g., sour) were not desired and for many contemporary beer drinkers that is still the same. Having stated that there are a number of craft beer drinkers who enjoy drinking Wild/Sour beers and the bacteria which produce these flavors are wanted vs. being undesired. There are a number of bacteria products available from yeast vendors. A few examples:

  • Lactobacillus delbrueckii

  • Pediococcus damnosus

  • Lactobacillus plantarum

Brewers (both commercial and homebrewing) need to take great care in their breweries when brewing with bacteria in order to avoid cross contamination when brewing non-sour beers.

How the brewers set the stage for fermentation

While it is indeed the case that the microorganisms (yeast, bacteria) do the work of fermenting wort into beer it is the brewer’s decisions which greatly influence how the resulting beer will taste.

Wort it is liquid that the brewer produces via mashing the grains and boiling with hops in the brew kettle. In simple terms it is sugar water that has been spiced with hops during the brewing process. The microorganisms utilize the sugars as food and they replicate once they are fed. Two byproducts of yeast fermentation are ethyl alcohol and carbon dioxide but there is more that occurs here which will be further discussed.

Yeast strain selection and format of the yeast (dry vs. liquid).

During the beer recipe formulation process the brewer will decide upon a specific yeast strain to use and which format to utilize.

There are two formats for brewers yeast: dry and liquid.

Dry yeast is sold to homebrewers in small sachets (e.g., 11.5 grams) and to commercial brewers in bricks (e.g., 500 grams). A benefit of the dry yeast format is that it is a very hardy product and if stored properly (e.g., refrigerated) it will be good for several years. When pitched the dry yeast has high viability and a single 11.5 gram sachet can provide over 200 billion viable yeast cells.

At the moment there are limited yeast strains available in dry format but every year more and more yeast strains become available.

Liquid yeast comes in plastic or mylar packages in a liquid solution. In contrast to dry yeast there is a large selection of yeast strains from a number of yeast manufacturers. The downside of liquid yeast is that it is not hardy with best by dates of just a few months, and decreased viability at the end of its shelf life. Liquid yeast must be stored cold (refrigerator temperatures) at all times or else yeast viability will decrease at an accelerated rate.

Ale yeast strains will have varied characteristics. Some yeast strains are quite flavor neutral while other yeast strains will produce notable amounts of esters (e.g., fruity flavors). Also, some ale yeast strains are POF+ (Phenolic Off Flavor positive) and will create phenols (e.g., spicy flavors); examples include some Belgian ale yeast strains and German Wheat Beer strains.

Lager yeast strains are not quite as variable in comparison to ale yeast strains but they will have varying characteristics. Also lager yeast strains are typically fermented at cool temperatures which encourage a very neutral flavor; a lager yeast strain fermented cool will have greatly reduced levels of esters as compared to ale yeast strains. The lagering phase (cold conditioning) following fermentation further aids in producing a beer that is described as crisp and clean, a very different flavor and quality in the resulting lager beer as compared to ale.

Brettanomyces and Bacteria are not widely available in dry format so liquid is the choice here.


Yeast pitch rate/amount

When it comes to how much yeast (number of yeast cells) to pitch into the wort an important figure here is George Fix. In his book An Analysis of Brewing Techniques he recommends pitch rates of:

  • 0.75 million cells/ml/°P for ales

  • 1.5 million cells/ml/°P for lagers

There are a few aspects to further explore concerning the above:

George Fix arrived at the above values based upon the assumption that the yeast has already fermented a batch of beer. Brewers refer to this as a re-pitch. When yeast goes through a round of fermentation they have been stressed and consequently they are not as vital as a factory fresh package of yeast. In order to account for this stressful situation it is prudent to pitch more yeast cells.

Let’s consider a calculation on how much yeast to pitch for an example homebrewing batch of ale:

For a batch of about 5 gallons let’s consider a metric equivalent of 20 liters (20,000 ml) and an original gravity of 1.050 specific gravity which equates to 12.5 °P.

Then the amount of yeast needed per the George Fix pitch rate is:

Amount of yeast (cells) = 0.75 million X (milliliters of wort) X (degrees Plato of the wort)

Amount of yeast (cells) = 750,000 X 20,000 X 12.5 = 187,500,000,000 cells.

So 187.5 billion cells are need per the George Fix recommendation.

But note that the above calculation applies to re-pitching yeast which has already produced a batch of beer. Since the products from yeast vendors are factory fresh and unstressed it is reasonable to expect that a lesser amount could be used to conduct a healthy fermentation but how much less you might ask.

The Brewers Friend website provides a suggestion with their yeast pitch rate calculator for manufactured yeast:

  • 0.35 million cells/ml/°P for ales

  • 0.5 million cells/ml/°P for lagers

Let’s repeat the above example calculation for a 1.050 (12.5 °P) ale:

Amount of yeast (cells) = 350,000 X 20,000 X 12.5 = 87,500,000,000 cells.

So, 87.5 billion yeast cells per the Brewers Friend manufactured yeast recommendation.

As brewers we all get to decide what works in our individual breweries. Use the above as guidance and decide what is best for you.

Now, some brewers like to tailor their pitch amounts based upon a beer style/brand and what they want to achieve for that style/brand. It is critical to keep in mind that each yeast strain is its own unique product and thereby will vary accordingly but there are some general rules of thumb when deciding upon pitch amounts:

Pitching less yeast can increase higher alcohols (fusel oils) formation during fermentation. It used to be a thought that pitching rates will impact the amount of esters in a linear correlation but there have been some later studies which indicate that there is no definitive answer here. For some strains/conditions overpitching can result in increased esters but for other strains/conditions under pitching can result in increased esters. A situation of trial and error while you improve your knowledge of yeast strains. For commercial brewers who utilize a house yeast strain they are in a position to learn the nuances of their house strain.

Fermentation temperature(s)

Yeast vendors provide a recommended temperature range for their products. Some brewers view this more like a guideline but I a view it as a rule since I figure the yeast vendors know their product line better than I do.

Let’s consider Wyeast 1056 (American Ale). For this product Wyeast recommends a fermentation temperature range of: 60 – 72 °F. As a general rule of thumb fermenting cooler would produce lower levels of esters so for a super clean beer you would choose to ferment on the cooler side of this temperature range. Wyeast 1056 has a reputation for being a more neutral yeast strain so even if you choose to ferment at the upper range the resulting beer is likely to have subdued levels of ester.

I have a lot of experience in brewing with Wyeast 3787 (Belgian High Gravity) yeast which has a recommend temperature range of: 64 – 78 °F. I have noticed in brewing my Trappist style ales that this strain will produce a notably different flavor profile (esters, phenols) based upon fermentation temperatures. For my batches which were fermented under 70 °F the flavor profile was more muted as compared to my batches brewed warmer (73 – 74 °F). I much preferred the more flavorful profile of the warmer fermented beers – lots of fruity (esters) and spicy (phenol) flavors. But I personally make sure to not ferment warmer than 74 °F since increased amounts of higher alcohols (fusel oils) tend to be produced at higher temperatures.

Some brewers prefer to utilize a fermentation temperature profile with a popular choice being to pitch the yeast on the cooler side of the range and let the fermentation temperatures free rise during the exothermic process that fermentation provides. I personally prefer to use more of a constant fermentation temperature when I ferment my beers.

Wort composition & nutrition

The wort that is produced during the mashing process has a combination of sugars. In order of greater to lesser a typical sugar composition will be: maltose, glucose, maltotriose, sucrose, maltotetraose, fructose.

sugar make up of wort

The relative amounts of those sugars will depend upon the mash conditions (e.g., malt/grain composition, mashing scheme, etc.). One sugar worth highlighting is maltotriose since some yeast strains are not capable of fully processing that sugar and consequently a beer fermented with those stains would result in a higher final gravity. As a rule wort with a higher original gravity will result in increased esters during fermentation.

Wort contains the sugars for the yeast to metabolize but it is deficient in zinc which is a necessary nutrient for the yeast. Because of this I choose to add Wyeast Beer Nutrient Blend which includes zinc for the last 10 minutes of the boil. Providing the yeast with the necessary nutrients improves the health of the fermentation. A consequence of greater amounts of zinc is increasing amounts of esters and higher alcohols (fusel oils). In my opinion this ester/higher alcohols aspect should not be a consideration since ensuring that there is proper nutrition for the yeast is of higher importance – do not starve the yeast of proper nutrition.

Wort oxygenation prior to pitching the yeast

Liquid yeast needs oxygen at the beginning of fermentation; liquid yeast cells rely on oxygen for the biosynthesis of cell membranes. Prior to pitching there is a need to aerate/oxygenate the wort. A target for the amount of oxygen is 8 to 10 ppm dissolved oxygen in the wort. To achieve this amount an oxygen tank with a diffusion stone is needed. You would place the sanitized diffusion stone into the wort and provide bursts of oxygen. Some folks choose to just pump in air as is done in an aquarium but this is a longer process since air is just 20% oxygen,

In my homebrewery I choose to aerate (vs. oxygenate) using a very large egg whisk whereby I aggressively stir/whip the wort for over a minute. When I see a couple of inches of foam on top I then pitch this yeast. I am not achieving the target values listed above but this process results in a healthy fermentation for me,

Whatever method is selected something must be done to aerate/oxygenate the wort prior to pitching so the liquid yeast cell membranes are properly synthesized.

Increasing the oxygen amount (e.g., 10 ppm dissolved oxygen) will decrease the level of esters developed during fermentation while increasing the amount of higher alcohols (fusel oils).

If pitching dry yeast there is no need for aeration/oxygenating since the dry yeast cells have been manufactured such that the same sort of biosynthesis of membranes for liquid yeast is not needed. Therefore one less step is needed, another advantage of utilizing dry yeast vs. liquid yeast.