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Brewing Science
Yeast
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.
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The Role of Yeast in Brewing
• Unwittingly, ancient brewers domesticated yeast.
– Selected yeast that made good beer.
• Deduced that yeast was important to make beer.
– Collect the creamy foam or sediment from one brew.
– Use it to pitch the next brew.
Did not know what yeast was.
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The Role of Yeast in Brewing
• 1680 Antonie van Leeuwenhoek
– Observed yeast in beer.
• 1837 - Cagniard Latour
– Microbe is responsible for alcoholic fermentation.
• 1839 -Justus von Liebig and Friedrich Wohler
– Alcohol is produced by a chemical process in which dead and
decaying yeast participated.
– Satired Latour’s theory in Annalen der Chemie . . .
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….small animal which sips sugar through its snout, and
excretes alcohol from its gut and carbonic acid from its
urinary organ.
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The Role of Yeast in Brewing
• 1866 - Louis Pasteur
– Yeast was responsible for alcoholic
fermentation.
• 1883 - Emil Christian Hansen
– Developed pure culture technique
– Isolated pure cultures of brewing yeasts
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Types of Brewing Yeasts
• Two types of brewing yeasts, originally classified
on flocculation behavior…
• Top-fermenting
– Ale yeast
– Weiss yeast
• Bottom-fermenting
– Lager yeast
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Top-Fermenting Yeast
Bottom-Fermenting Yeast
Weiss
Ale
Lager
Lab
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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
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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
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Taxonomy
• Ale and Weiss yeasts - Saccharomyces cerevisiae
– Polyploid and probably aneuploid.
– Non-mating
– Sporulates poorly and poor spore viability
• Lager yeast - Saccharomyces pastorianus
–
–
–
–
S. cerevisiae
S. carlsbergensis
S. uvarum
Sporulates very poorly - poor spore viability
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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.
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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
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Fermentation
• Yeast growth
• Alcohol and CO2
• Flavor compounds
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Lagering
• Carbonation
• Off-flavor reduction
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The Brewing Process
Malt Mill
Mash Tun
Cereal
Cooker
Brink
Hops
Aeration
Fermentation
Lauter Tun
Brew
Kettle
Hot Wort
Receiver
Lagering
Wort
Cooler
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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.
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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.
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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.
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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.
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Yeast Starter
Starter Recipe from https://www.wyeastlab.com/hb_makingastarter.cfm
The optimal media for cell growth and health require using a malt based media
(DME) fortified with nutrients. Gravity should be kept near 1.040 and cultures should
be grown at 70°F.
Recipe
0.5 cup DME (100g, 3.5oz), ½ tsp Wyeast Nutrient, 1qt.(1L) H2O
Mix DME, nutrient, and water; Boil 20 minutes to sterilize.
Pour into a sanitized flask or jar with loose lid or foil.
Allow to cool to 70°F.
Shake well and add yeast culture.
Timing of Starter:
Because starters are inoculated at high cell densities, growth is usually maximized
within 24-36 hours. The gravity of the starter should always be checked prior to
inoculation into wort to assure proper cell growth . Cultures should be used
immediately, or refrigerated for up to 1 week before using. Cell viability will decrease
rapidly if culture are left at ambient temperatures for extended time.
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.
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Fermentation Stages
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.
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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.
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Fermentation Stages
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.
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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.
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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.
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Balanced Growth
• Yeast growth affects beer flavor.
– Need balance between yeast growth and beer flavor.
• The brewer needs...
– Desired flavor profile in desired time.
– Sufficient yeast crop for subsequent fermentations.
• Oxygen is growth limiting nutrient.
– Control point
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Yeast Metabolism During Fermentation
Sugars
Oxygen
Membranes
Glucose
CO2
Ethanol
Acetaldehyde
Energy
Pyruvate
TCA
Cycle
Unsaturated Fatty Acids
Sterols
Amino Acids
Esters
Higher
Alcohols
VDK
Organic Acids
Amino Acids
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Sulfur
Volatiles
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
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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
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Higher Alcohols
• Formed by the decarboxylation and reduction of
a-keto acids.
– From amino acid anabolism and catabolism.
Alcohol
Isoamyl
Amino Acid
Leucine
a-keto acid
Amyl
Isoleucine
a-keto-3-methylvalerate
Isobutanol
Valine
a-keto-isovalerate
Propanol
Threonine
a-keto-butyrate
a-keto-isocaproate
Alcoholic, solventy, and fruity flavor notes
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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.
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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.
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Esters
• Closely linked to lipid metabolism - growth.
• Reaction of an alcohol and fatty acid intermediate
• Acetate esters
– Ethyl acetate
– Isoamyl acetate
– Phenethyl acetate
solventy, fruity, sweet
banana
roses, honey, apple
• Fatty acid esters
– Ethyl caproate
– Ethyl caprylate
– Isoamyl decanoate
apple, aniseed
apple
tropical fruits
Fruity flavor notes
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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.
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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.
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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.
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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.
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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.
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Vicinal Diketones
Threonine
Pentanedione
a-ketobutyrate
a-acetohydroxybutyrate
pyruvate
a-acetolactate
Diacetyl
Buttery, butterscotch flavor
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Isoleucine
Valine
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
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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
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