Buzz, Bubbles and Beer
Yeast Fermentation: A Journey

Yeast have long played a part in the creation of foods and drinks. This research paper focuses on the role of yeast and fermentation in the creation of beer. Saccharomyces cerevisae, Saccharomyces pastorianus and Brettanomyces lambicus are the three yeasts most commonly used in beer making. S. cerevisae creates an ale flavor, and is best used in top fermenting. S. pastorianus is used to create lager beers and flourishes in bottom fermenting. Lastly, B. lambicus is used to make strong, sour, dark beers and is spontaneously fermented.
Each yeast follows the same three phases of fermentation, Lag phase, exponential growth phase and the stationary phase. Throughout each phase, the yeast are absorbing nutrients, creating alcohol and carbon dioxide and routinely creating new yeast cells in order to produce the delicious beverage that is beer.

From the palest of Ales to the darkest of lagers, beer is a welcomed guest at almost any party. However, few people appreciate the role of fungi in the creation of their favorite crisp, flavorful and comforting beer. Yeast have played a role in the creation of beer since the beginning of its existence. The specialized organism is a fungus used in the creation of many food based products and different types of yeast ferment nutrients in different ways.
The 3 most common yeasts used in beer making are Saccharomyces cerevisae, Saccharomyces pastorianus and Brettanomyces lambicus. Each yeast differs slightly in it’s fermenting conditions and process and because of this, create different beers.Throughout this research paper, we will discuss the different aspects of the yeasts, how their fermenting process differ and what takes place in order to create the separate beer flavors.


The Creation of Beer
Beer making has been around for centuries. An accidental discovery in history, beer is present in the bible, engraved on walls and was once considered a healing agent for those thought to be ill. More recently, beer has become a calculated form of art, with individual steps that are necessary to get the best flavor and the best body. Beer consists of 4 main ingredients: water, a starch source (such as malted barley), a brewers yeast and flavoring, which varies with the type of hops used. Water serves as a happy medium for the yeast, which ferments the chosen starch in order to create the base, body and character of the beer. Hops come in last for the flavoring, sometimes fruity, other times woody, all varying with the ingredients used.

Yeast are nonfilamentous unicellular fungi and are responsible for the fermentative process that is required to make beer. The chosen yeast metabolizes the sugars from the starch source and create alcohol and carbon dioxide (the buzz and bubbles) turning the water, starch and yeast mixture into a basic beer. Each yeast used ferments at different temperatures, in correlation with the most beneficial environment for that particular yeast. The three most common brewers yeasts are as follows -

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  • Saccharomyces cerevisiae is a species of budding yeast and is the main fermenting agent used in many food processes (Legras 2007). In the beer world, this organism is also referred to as a top-fermenting yeast because it produces a foam during the fermentation process. The foam is created when the hydrophobic surface of the molecules causes its “flocs” (also known as flakes of the organism) to adhere to the carbon dioxide created during the fermentation, in turn floating to the surface. This yeast ferments better at higher temperatures, 18-21 °C, and can create a fruitier flavor. This strain is responsible for the lighter Ale beers.

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  • Saccharomyces pastorianus is a close relative of S. cerevisiae and is synonamous with Saccharomyces carlsbergensis. This yeast is referred to as a bottom-fermenter, operating best at lower temperatures (around 5 °C). The major difference between S. pastorianus and S. cerevisiae is the ability to metabolize all the sugars present in the starch. S. cerevisiae only processes the fructose present, leaving the Ale beers slightly sweeter. S. pastorianus process the fructose, glucose and galactose leaving the beer with a more bitter taste, hence its correlation with the production of Lager beer.

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  • Brettanomyces lambicus is a yeast found in Belgium and is generally used to create a dry, cidery and slightly sour tasting beer. B. lambicus is used in spontaneous fermentation, a technique common when attempting to make a dark Lambic beer. After adding the main ingredients, the creator leaves the concoction exposed in efforts to create a spontaneous fermentation, brought on by the popular yeasts present in the air surrounding Belgium.

Fermentation is the enzymatic degradation of carbohydrates (Tortora 2010). During the creation of beer, the fermentation results in the creation of alcohol and carbon dioxide. Thus far we have discussed three types of fermentation; top, bottom and spontaneous. Each process produces a different flavored beer.
  • Top fermentation creates a sweeter, fruitier ale beer. Given the top prefix because of the foam present during the process. The water, starch and yeast mixture is kept at a warmer temperature for best results.
  • Bottom fermentation creates a more robust, dark and bitter lager beer. The concoction is kept at a colder temperature.
  • Spontaneous fermentation is a technique popular for Lambic beers, those that have a more bitter dry flavoring. After the mixture has cooled, the container is left in the open air for a spontaneous reaction.

There are three phases in beer fermentation, lag phase, exponential growth and the stationary phase.
  1. Lag Phase beings 3-15 hours after mixing the ingredients. During this phase, the yeast acclimate to their environment and begin to take in minerals and amino acids from the starch and water mixture. These nutrients will be used throughout the fermentation process in order to keep the yeast full, happy and productive. There is also a rapid absorption of oxygen taking place, in order to promote the growth of the yeast (White, 2000).
  2. Exponential growth is the second phase of beer fermentation. During this period of 1-4 days, the yeast is busy consuming the sugars present in the mixture. With top-fermenting beers, this phase brings foam to the top of the barrel, a positive indicator of beertastic growth. The cell count is increasing rapidly and the fermentation activity is at its highest.
  3. The Stationary phase is the maturing stage of the beer. At this point the yeast growth has slowed and the beer flavor and aroma compounds are mostly complete.

Once the three phases are complete, most breweries will cool the beer down in order to get the yeast to flake and gather at the bottom of the tank to be removed.

Literature cited:
  • Denny, M. 2009. Froth!: the science of beer. John Hopkins University Press, Balitmore.
  • Forget, C., Rabin, D. 1998. The dicitonary of beer and brewing. Fitzroy Dearborn, Chicago.
  • Legras, J. Bread, Beer and Wine: Saccharomyces cerevisae diversity reflects human history. Published online in Molecular Ecology 16, 2091-2102 (2007).
  • Totora, G. 2010. Microbiology: An Introduction. Pearson Benjamin Cummings, San Francisco.
  • Sigler, K. Net effect of wort osmotic pressure on fermentation course, yeast vitality, beer flavor and haze. Published online in Applied Microbial Biotechnology 82, 1027-1035 (2009).
  • Verbelen, P. Impact of pitching rate on yeast fermentation performance and beer flavour. Published online in Applied Microbial Biotechnology 82, 155-167 (2009).
  • White, C. Fermentation Time Line. White Labs : Pure Brewers Yeast. White Lab Inc. Jan 2000. Web. 2 Dec 2010.
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Tours and Tasting

Author: Alicia Naylor
Date: 9 December 2010