15 Yeast and Fermentation Mexico 2012
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Transcript of 15 Yeast and Fermentation Mexico 2012
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Levadura y Fermentacin de Cerveza
by Chris [email protected]
Cerveza Mexico 2012
Mircoles 19 de Septiembre
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Pure Liquid Yeast CulturesBeer Testing
Fermentation Lab
Classes
Tasting Bar
White Labs
San Diego, CA
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Yeast is a single cell fungus
Similar cell structure to human cells
Feeds on sugar, just like human cells
Found everywhere, some yeast like ripe fruit
Same yeast species as bread, wine, distilledspirits, and laboratory yeast.
YEAST FACTS
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SCIENTIST CONTRIBUTIONS
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Typical Fermentation
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THE YEAST CELL
Fig 2.1 Yeast: The Practical Guide to Beer Fermentation, White 2010
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ENZYMES
Enzyme + substrate Enzyme-substrate complex Enzyme + product
Conditions which affect enzyme activitypH
temperature
ionic strength of the solution
substrate concentration
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THE SIGNIFICANCE OF YEAST:
FERMENTATION
Fig 2.3 Yeast: The Practical Guide to Beer Fermentation, White 2010
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Glycolysis
Acetaldehyde + CO2
Ethanol
2 NAD+
2 NADH
2 NADH
2 NAD+
Pyruvate
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FLAVORS ASSOCIATED WITH FERMENTATION
Alcohol
Higher (fusel ) alcohols
Esters
Diacetyl
Sulfur
Acetaldehyde
Phenolic compounds
Flavors may be positive or negative, depending on beer style
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HIGHER ( FUSEL ) ALCOHOLS
Flavor- alcoholic, spicy, vinous
Formation
Intermediates in amino acid metabolism
Produced during uptake of amino acids
Produced from glucose when yeast needs to make amino acids
Directly related to yeast growth
Control
Any conditions that stimulate yeast growth will stimulate fusel alcohol production
Aeration
Lipid (fat) content of the wort
Trub
Agitation
Temperature
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Glucose pyruvate acetaldehyde + CO2
Amino keto aldehydes + CO2
acids acidsNADH
NAD+
NADH
NAD+
Higher alcohol
Ethanol
YeastCell
HIGHER ALCOHOLS
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ACETALDEHYDE
Flavor - green apples, cut grass, avocado
Production
Formed during fermentation of glucose
Second last step in reaction
Green, or immature beer
Control
Healthy yeast
No CO2 back-pressure
Adequate conditioning time
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ESTERS
Flavors - fruity, banana, apples, perfumy, solvent, nail varnish
remover
Production
Reaction of alcohol group and acid group in the
yeast cellAlcohol part comes from ethanol and the fusel
alcohols
Acid part comes from various acids that are insidethe yeast (acyl CoA compounds)
Reaction is catalyzed by an enzyme (acyl-alcohol
transferase)
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Control
Ester synthesis not that simple.
No direct relationship between yeast growth andester synthesis.
The amount of ester formed will depend on :
The amount of the acid (Acyl CoA compounds)
The amount and activity of the enzyme
(Acyl-alcohol transferase)
The amount of the higher alcohol
Low temperature = low esters
Highly yeast strain dependentMore problematic in very strong beers
May be symptom of Acetobacter
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DIACETYL
Flavor -buttery, butterscotch
Produced
During yeast growth in fermentation
When the yeast has to make a specific amino acid -
valine
An intermediate in this pathway is the pre-cursor of diacetyl
The precursor leaks out of the cell
Outside the cell it is CHEMICALLY converted to diacetyl
The reaction is favored by high temperatures and low pH
It goes back into the cell as diacetyl
The yeast changes it into less flavor-active compounds
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DIACETYL
glucoseglucose pyruvate
acetolactate
acetolactate
diacetyl
diacetyl
acetoin
2,3-butanediol
pHtemperature
valine
NADH2
NAD+
acetoin
2,3-butanediol
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Control
1. Limit the amount of precursor ( alpha -
acetolactate)
Lower temperatures
Good yeast growth (healthy yeast)
Adequate aeration
2. Accelerate the conversion of precursor to
diacetyl
Diacetyl rest at elevated temperatures, even for ales
Good yeast growth (healthy yeast)
Lower end of fermentation pH (< pH 4.5)
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SULFUR COMPOUNDS
Flavor - sulfury, rotten eggs, burnt rubber, striking a match
Production
Intermediates in amino acid metabolism
When yeast needs to make sulfur containing amino acids
Control
Yeast strain (common in certain lagers)
Wort oxygen content (more is better)
Yeast pitching rate (higher is better)
Fermentation temperature
Yeast health
Pantothenic acid
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1. YEAST STRAIN
2. Yeast Health
a) Keep stress low
3. Yeast Mutation
4. Yeast Purity
5. Number of Generations
6. Pitching Rate
7. Oxygen Levels at Pitching
WHAT ELSE IMPACTS FLAVOR?
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Some Examples
German Wheat Beer
Belgian Tripple
American IPALager Beer
Different Yeast Strains
THE REAL SIGNIFICANCE OF YEAST:
FLAVOR
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THANK
YOU!