The Impact of Yeast on Wine Aroma and Flavor: The Good, the Bad

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The Sulfur Taints
Linda Bisson
Department of Viticulture and
Enology
Why Are Sulfur Taints a Problem?
Low thresholds of detection
 Chemical reactivity
 Difficulty in removal
 Difficulty in masking

S-Volatiles: Negative Impacts on
Flavor
Hydrogen Sulfide: Rotten egg
 Post-fermentation S-taints
 Sur lie Sulfide Taints

Sources of Sulfur Compounds

Sulfate reduction pathway
 Degradation of sulfur containing amino acids
 Inorganic sulfur
• Non-enzymatic
• Requires reducing conditions established by yeast

Degradation of S-containing pesticides/fungicides
HYDROGEN SULFIDE
Hydrogen Sulfide Formation:
The Old Story

Due to release of reduced sulfide from the
enzyme complex sulfite reductase
 Reduction of sulfate decoupled from amino
acid synthesis
 Sulfate reduction regulated by nitrogen
availability
 Lack of nitrogenous reduced sulfur acceptors
leads to excessive production of reduced
sulfate and release as H2S
 See strain variation
Hydrogen Sulfide Formation:
The New Story

Hydrogen sulfide plays an important population
signaling role
– Inhibits respiration: coordinated population
fermentation
– Inhibits respiration: inactivation of bacteria and other
yeasts
Hydrogen sulfide formation is protective against
stress
 Strain variation due to exposure to different
environmental conditions in combination with
the multiplicity of roles of H2S

Sulfate Reduction Pathway
SO4
SUL1, SUL2
SO4
MET3
Adenylylsulfate
MET14
Phosphoadenylylsulfate
Sulfite
Sulfide
Cysteine
Cystathionine
CYS3
MET16 (1,8,20,22)
MET10 (1,5?,8,20)
MET17/25/15
Homocysteine
CYS4
Methionine
MET6
Hydrogen Sulfide Formation

From degradation of S-containing amino
acids
– When present in excess to harvest nitrogen
– As Redox needs change: S-containing amino
acids needed to maintain redox status of cells

Also a stress response
– S-compounds needed for 1C transfers and
adaptation to ethanol and other stressors

Strain variation
Current Understanding of H2S
Formation
Nitrogen levels not well-correlated with
H2S formation, but generally see
increased H2S at lower nitrogen
 Under complex genetic control
 Tremendous strain variation in H2S
production

Factors Impacting H2S Formation

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Level of total nitrogen
Level of methionine relative to total nitrogen
Fermentation rate
Use of SO2
Vitamin deficiency
Presence of metal ions
Inorganic sulfur in vineyard
Use of pesticides/fungicides
Strain genetic background
Timing of Formation of H2S
Brix
H2S
Time
Timing of Formation of H2S
•Early (first 2-4 days): due to N
imbalance? Or signaling?
•Late (end of fermentation): due to
degradation of S-containing compounds
•Sur lie (post-fermentation aging): due
to autolysis
•H2S produced early can be driven off
by carbon dioxide during active phase
of fermentation
Elimination of Hydrogen Sulfide

Rely on volatility and fermentation gas or
inert gas sparging to remove
– Need to make sure it is gone and not just
converted to a non-volatile form


Use of volatiles stripping technologies
Precipitation via copper
– Emerging issue: health and environmental
concerns about copper


Use of fining agents
Use of strains not producing sulfides
Hydrogen Sulfide
Screen of native isolates to define nonproducers
 Screen of mutant library to define genes
involved in sulfide production
 Genetic crosses to identify the genes
altered in the native populations
 Assessment of allele swap in transferring
the non-producer phenotype to producer
strains

MET10-932
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Contains a change of amino acid 662 from
threonine to lysine
Does not affect protein structure
Does not affect activity
Prevents sulfide release
Eliminates sulfide production in several
strains including UCD522
Can be crossed into any commercial strain
background
Sulfate Reduction Pathway
SO4
SUL1, SUL2
SO4
MET3
Adenylylsulfate
MET14
Phosphoadenylylsulfate
Sulfite
Sulfide
Cysteine
Cystathionine
CYS3
MET16 (1,8,20,22)
MET10 (1,5?,8,20)
MET17/25/15
Homocysteine
CYS4
Methionine
MET6
HIGHER SULFIDES
Higher Sulfides


Emerge late in fermentation and during sur lie aging
Release of compounds during entry into stationary
phase by metabolically active yeast
 Come from degradation of sulfur containing
compounds by viable cells
– Biological
– Chemical
 From reaction of reduced sulfur intermediates with other cellular
metabolites?
 Formed chemically due to reduced conditions?

Degradation of cellular components: autolysis
– Enzymatic
– Chemical
Common Volatile Sulfur
Compounds
Methanethiol: CH3-SH
 Ethanethiol: C2H5-SH
 Dimethyl sulfide: CH3-S-CH3
 Dimethyl disulfide: CH3-S-S-CH3
 Dimethyl trisulfide: CH3-S-S-S-CH3
 Diethyl sulfide: C2H5-S-C2H5
 Diethyl disulfide: C2H5-S-S-C2H5

Common Volatile Sulfur
Compound Ranges in Wine
Hydrogen sulfide: Trace to 80 ug/L
 Methanethiol: Trace
 Ethanethiol: 1.9 -18.7 ug/L
 Dimethyl sulfide: 1.4 - 474 ug/L
 Dimethyl disulfide: Trace to 1.6 ug/L
 Dimethyl trisulfide: 0.09 - 0.25 ug/L
 Diethyl sulfide: 4.1 - 31.8 ug/L
 Diethyl disulfide: Trace - 85 ug/L

Ehrlich Pathway S-Compounds
Ehrlich Pathway: source of fusel oils
 Removal of N from amino acid compounds
 Generates aldehyde
 Aldehyde reduced to alcohol
 In fermentation see high concentrations of
methionine-derived “fusel” compounds:
Methionol (100-6,300 ug/L) and Methional
(generally trace, but reaction products are
more aromatic)

Sources of Higher Sulfides
S-Containing Amino Acids
 S-Containing Vitamins and Co-factors
 Glutathione (Cysteine-containing tripeptide
involved in redox buffering)

Management of S-Taints
Diagnosis of Taint
 Taint Prevention
 Taint Mitigation

DIAGNOSIS OF SULFUR TAINTS
Correct Diagnosis of Fault Is Important
Is it an S-containing compound?
 When did taint first appear?
 What factors are associated with
appearance of the taint?

Is It a sulfur-containing
compound?
Be familiar with the characteristic offodors of sulfur compounds
 Other classes of off-odors can be
reminiscent of S-compounds
 Thresholds of detection are so low
compound may be difficult to detect
chemically

When did taint first appear?
Provides important clues as to the
reason taint is occurring
 If know why it is being made can take
steps to prevent formation

What factors are associated
with appearance of the taint?
Always found with a specific vineyard?
 Associated with unsound fruit?
 Associated with specific processing?

– Inert gas blanketing
– Type of vessel

Associated with specific fermentation
conditions?
TAINT PREVENTION
Preventing S-Taint Formation
Vineyard
 Wine chemistry
 Yeast strain selection
 Fermentation management

Preventing S-Taint Formation

Vineyard
– Judicious use of elemental sulfur
– Eliminate reliance on S-containing
pesticides
– Address excesses of metal ions
– Address vine stress: nutritional or
otherwise
Wine chemistry
 Yeast strain selection
 Fermentation management

Preventing S-Taint Formation
Vineyard
 Wine chemistry

– Minimize use of ‘enabling’ practices
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Temperature
Solids content
“Reductive” aging conditions
Role of the ‘lie’ in ‘sur lie’
Yeast strain selection
 Fermentation management

Preventing S-Taint Formation
Vineyard
 Wine chemistry
 Yeast strain selection

– Match strain to winemaking conditions
– Meet the nutritional needs of the specific strain

Fermentation management
Preventing S-Taint Formation
Vineyard
 Wine chemistry
 Yeast strain selection
 Fermentation management

–
–
–
–
Provide adequate nutrition
Keep cells suspended
Mix to prevent reductive stratification of tanks
Remove from lees/yeast residue at first sign of
trouble post-fermentation
TAINT MITIGATION
Elimination of Hydrogen Sulfide
•
Rely on volatility and fermentation gas or
inert gas sparging to remove
– Need to make sure it is gone and not just
converted to a non-volatile form
•
•
Use of volatiles stripping technologies
Precipitation via copper
– Emerging issue: health and environmental
concerns about copper
•
•
Use of fining agents
Use of strains not producing sulfides
Treatments for Higher Sulfides
Removal by reduction of disulfide bond
with ascorbate and copper binding
 Sulfide trapping by quinones
O
OH
O + RSH
OH
SR
 Removal by charcoal fining

Conclusions
Sulfur taints can be controlled
 Need to use correct strain
 Need to minimize use of compounds in
vineyard and winemaking techniques that
amplify S-compound formation
 Many factors leading to S-taint
appearance are still not well understood

Sulfur Compound Flight #1:
Taints produced late in fermentation
Glass 1: Control wine
 Glass 2: Hydrogen Sulfide
 Glass 3: Dimethyl sulfide (2 ug/L)
 Glass 4: Dimethyl sulfide (20 ug/L)
 Glass 5: Dimethyl sulfide (60 ug/L)
 Glass 6: Diethyl sulfide (20 ug/L)

Sulfur Compound Flight #2
Spiked Compounds
Glass 1: Control Wine (Cabernet
Sauvignon)
 Glass 2: Diethyl disulfide (40 ug/L)
 Glass 3: Dimethyl trisulfide (0.2 ug/L)
 Glass 4: Ethanthiol (15 ug/L)
 Glass 5: Methionol (500 ug/L)
 Glass 6: Methional (50 ug/L)

Sulfur Compound Flight #2
Spiked Compounds
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G 1: Control Wine (Cabernet Sauvignon)
G 2: Dimethyl sulfide: low concentrations
enhance varietal character
G 3: Dimethyl sulfide: cabbage, cooked corn,
asparagus, canned
vegetable
G 4: Dimethyl trisulfide: meaty, fishy, clams, green,
onion, garlic, cabbage
G 5: Diethyl sulfide:
garlic, onion
G 6: Diethyl disulfide: overripe onion, greasy,
garlic, burnt rubber,
manure
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