Transcript Brew Day Presentation

Beer Basics
Fermentation
June 2008
Today’s Topics
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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
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Diacetyl
2,3-Pentanedione
Fatty Acids
Sulfur Compounds
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.
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
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
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.
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
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 60-65 °F which is on the cool side of ale yeast
fermentation temperatures.
• On the other hand, California Commons are typically
fermented around 55-60 °F which is on the warm side of
lager yeast fermentation temperatures.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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
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 10-35 ppm.
• Gram-negative bacteria, like Escherichia coli can
produce large amounts of sulfer compounds
• For ales, higher fermentation temperatures tend to
suppress sulfur compounds