15 Yeast and Fermentation Mexico 2012

7/30/2019 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!

15 Yeast and Fermentation Mexico 2012

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