1. Beer Basics Fermentation June 2008

2. Today’s Topics Fermentation Stages Introduction Yeast Types Ale Yeast
Lager Yeast
Yeast Attributes
Flocculation
Attenuation
Temperature
Pre-Fermentation
Wort Aeration
Yeast Pitching Rates
Yeast Propagation
Fermentation Stages
Lag Phase
Growth Phase
Low Kräusen
High Kräusen
Late Kräusen
Fermentation By-Products
Esters
Fusel Alcohols
Ketones
Diacetyl
2,3-Pentanedione
Fatty Acids
Sulfur Compounds

3. Introduction
As brewers we take malt and perform a mash to convert starches to sugars and dextrins; however, it is yeast that is responsible for converting sugars into alcohol and CO2. So, without yeast we would only have cloying sweet wort and no beer.

4. Yeast Types Ale Yeast Scientific name is Saccharomyces Cerevisiae
Typically ferments at higher temperatures, normally between 55 – 75 °F
Usually is a top fermenter
Cannot completely ferment Raffinose
Higher fermentation temperatures usually produce more esters

5. Yeast Types Lager Yeast
Scientific name is Saccharomyces Carlsbergensis
Typically ferments at lower temperatures, normally between 46 – 56 °F; however, some ferment at the low end of ale yeast, i.e. California Common yeast ferments in the range 55 – 60 °F.
Usually is a bottom fermenter
Ferments Raffinose
Cooler fermentation temperatures usually lead to cleaner beers, i.e. less esters and less fusel alcohols

6. Yeast Attributes Flocculation
Flocculation means yeast cells clumping together during fermentation. Flocculation is a yeast specific characteristic. Some yeast clump together quickly while others do not flocculate.
Top fermenting yeast that flocculate are typically carried to the surface by CO2 gas. Bottom fermenting yeast tend to flocculate after CO2 levels have diminished and they sink to the bottom.
Yeast mutation can change the flocculation trait of pure yeast strains.

7. Yeast Attributes Attenuation
Attenuation is the ability of yeast to metabolize wort sugars
Apparent attenuation is measured by the drop in specific gravity as yeast converts sugar into ethanol and CO2, without correction for alcohol. Real attenuation corrects for alcohol. For normal strength brews, apparent attenuation is approximately 1.2 times the real attenuation
All things being equal, lager yeast ferment more completely than ale yeast. Yeast appropriate for brewing can metabolize single and double sugars but the amount of fermentation of triple sugars depends on the yeast strain.
Yeast mutation can change the attenuation characteristic of pure yeast strains

8. Yeast Attributes Temperature
Ale Yeast usually ferments at a higher temperature compared to lager yeast.
There are variations between different yeast strains. For example, Dusseldorf alts typically are fermented around °F which is on the cool side of ale yeast fermentation temperatures.
On the other hand, California Commons are typically fermented around °F which is on the warm side of lager yeast fermentation temperatures.

9. Pre-Fermentation Wort Aeration
It is very important to add oxygen to the wort just prior to pitching yeast because the yeast require oxygen in the first phase of fermentation to build up metabolic energy.
Aeration can be achieved by adding oxygen directly or by adding air. In each case, aerating stones are used to assist in dissolving oxygen into the wort.
Normal gravity worts (12 °P or less) require 4-5 ppm of oxygen, while wort with 15 °P requires 8-10 ppm of oxygen.
The level of dissolved oxygen in wort goes down as temperature and gravity units go up.

10. Pre-Fermentation
Yeast Pitching Rates
According to George Fix, the appropriate level of yeast to pitch for ales is 750,000 yeast cells per milliliter of wort for each degree Plato of wort.
Mathematically,
Ale pitching rate = 750,000*(mL of Wort)*(°Plato ofWort)
The pitching rate for lagers is twice the amount shown for ales.

11. Pre-Fermentation
Yeast Pitching Rates
Example – How much yeast should you pitch to make 10 gallons of your favorite lager if the original gravity is 1.048?
The answer is:
=1,500,000*37,853*12
=681,354,000,000
or, around 700 billion yeast cells.

12. Pre-Fermentation
Yeast Propagation
Typically liquid yeast packs do not have enough yeast cells to carryout an effective fermentation.
As an alternative to pitching multiple packs of liquid yeast, you may consider making a yeast starter.

13. Fermentation Stages A normal fermentation proceeds in stages:
lag phase, growth phase, low kräusen,
high Kräusen, and late kräusen.
The stages may overlap and the timing and
duration of each stage depends on the type of
yeast, specifically, whether you have an ale or
lager yeast.

14. Lag Phase
During this stage of fermentation the yeast become acquainted with their environment and they access the level of oxygen, amino acids and sugars.
In this phase the yeast produce enzymes that allow amino acids and sugars to permeate the yeast cell wall.
The yeast compile food reserves and store the fuel in the form of glycogen, a carbohydrate.

15. Growth Phase
During the growth phase yeast begin to grow through cell budding.
Oxygen dissolved in the wort is used by the yeast to generate sterols. High levels of sterols are required for yeast cell walls to become permeable. Glycogen levels are reduced during sterol synthesis, however, glycogen increases during the main part of a normal fermentation.
Poor yeast growth is usually caused by low amino acid levels, or low levels of dissolved oxygen in the wort.
If an appropriate level of healthy yeast are pitched then signs of fermentation (i.e. CO2 formation) should begin within 8 hours of pitching ale yeast and 18 hours for lager yeast.

16. Low Kräusen Phase
At this point, the yeast have depleted all the oxygen dissolved in the wort. Consequently, from this point forward the process is anaerobic.
Yeast metabolism of amino acids and sugars are in full force.
Fusel alcohols and diacetyl may be produced during this phase. At this stage, lower fermentation temperatures will inhibit the production of fusel alcohols and diacetyl.

17. High Kräusen Phase
For ales, most of the sugars have been metabolized by this phase. However, lager yeast will metabolize most sugars during the high kräusen phase.
During fermentation pyruvic acid is converted to acetaldehyde, which in turn, is reduced to ethyl alcohol.
Normally, primary fermentation lasts 3-5 days for ale yeast and 6-8 days for lager yeast.

18. Late Kräusen Phase
Lager yeast begin to metabolize some of the fermentation by-products that were produced during the low kräusen phase. In particular, diacetyl levels can be reduced by performing a diacetyl rest.
Most simple sugars have been converted after primary fermentation. During secondary fermentation, yeast slowly convert the more complex triple sugars.
Secondary fermentation typically last about 1-3 days for ale yeast, but may last up to a month for lagers.

19. Fermentation By-Products
Esters
Chemically, esters are formed when an alcohol combines with an organic acid
They typically impart a fruity aroma and flavor to beer.
There are many esters associated with beer fermentation. Two common esters associated with brewing are Ethyl acetate, and Isoamyl acetate. Ethyl acetate produces a fruity character and can be detected at 33 ppm, while Isoamyl acetate is responsible for the banana characteristic in German Wheat Beers. It can be detected at 3 ppm.

20. Fermentation By-Products
Esters
Ester formation is positively correlated to wort gravity, yeast growth, and fermentation temperature. That is, higher gravity worts, rapid yeast growth, and higher fermentation temperatures increase ester production.
Ester formation is reduced by high yeast pitching rates because the yeast will not grow as fast. Also, wort with insufficient oxygen levels favor ester formation.

21. Fermentation By-Products
Fusel Alcohols
These alcohols have a more complex molecular structure than ethyl alcohol.
They provide an initial sweetness followed by a harsh after taste.
Formed by the metabolism of amino acids, so over modification during malting or mashing can lead to higher fusel alcohol levels.
They increase with fermentation temperature, level of amino acids, and wort gravity.
Wild yeast can produce very high levels of fusel alcohols

22. Fermentation By-Products
Fusel Alcohols
Some yeast strains produce phenolic alcohols that typically have a medicinal flavor; however, the clove like character of German Wheat beers is produced from the phenolic 4-vinyl-guaiacol
Wild yeast can produce phenolic alcohols with very unpleaseant flavors

23. Fermentation By-Products
Ketones
The two important ketones in brewing are
diacetyl and 2,3-pentanedione. In the
literature, these two ketones are classified
together as the vicinal diketone level in beer.

24. Ketones Diacetyl has a very low flavor threshold, .10 mg/L (ppm)
In fresh beer, low levels of diacetyl may impart a caramel flavor, however, over time it will take on a butter or butterscotch characteristic
Early in a normal fermentation, during the aerobic stage, yeast will produce diacetyl. Later in the anaerobic fermentation stage, yeast reduce diacetyl to levels below the flavor threshold.

25. Ketones
Diacetyl
Gram-positive lactic acid bacteria can produce large amounts of diacetyl
Mutant yeast cells can lose their ability to reduce diacetyl, leading to elevated levels of diacetyl
Wort that does not contain sufficient levels of the amino acid valine can lead to higher levels of diacetyl. Fortunately, most all-malt worts contain an over abundance of amino acids.

26. Ketones
Diacetyl
Higher temperatures early in the fermentation lead to higher levels of diacetyl and higher temperatures later in the fermentation lead to a greater reduction in diacetyl.
For lager yeast that typically ferment at lower temperatures, some brewers perform a diacetyl rest during the latter fermenatation stage called the late krausen phase. To perform a diacetyl rest, slowly raise the fermentation temperature to around 60°F and hold this temperature for two days and then slowly lower the temperature back to the
original fermentation temperature. Here slowly means no more that 5 °F per day, otherwise you may shock the yeast.

27. Ketones 2,3-pentanedione Has a flavor threshold of 1 mg/L (ppm)
Produces a flavor similar to honey
Found in some Belgium ales where honey flavors are appropriate for the style
Wort that does not contain sufficient levels of the amino acid leucine can lead to higher levels of 2,3-pentanedione. Fortunately, most all-malt worts contain an over abundance of amino acids.

28. Fatty Acids Tend to add a soapy flavor to beer
They are produced when yeast break down amino acids
They are suppressed by lower fermentation temperatures
Usually the yeast will convert fatty acids to aldehydes then into alcohols

29. Sulfur Compounds
Hydrogen sulfide production during fermentation can lead to flavors reminiscent of rotten eggs; however, during normal fermentation, hydrogen sulfide is reduced during the fermentation process. The flavor threshold for hydrogen sulfide is ppm.
Gram-negative bacteria, like Escherichia coli can produce large amounts of sulfer compounds
For ales, higher fermentation temperatures tend to suppress sulfur compounds