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OVERVIEW OF
PROBLEM
FERMENTATIONS
Linda F. Bisson
Department of Viticulture and Enology, UCD
Types of Problem Fermentations
•Lack of fermentation progression
•Off-character production
PROBLEMS IN
FERMENTATION
PROGRESSION
Stuck and Sluggish Fermentations
• Characterized by failure of yeast to consume all available
sugar
• Multiple causes
• Can be difficult to treat as cells have arrested metabolism
• Leads to reduced wine quality
Typical Fermentation Profile
5
1
Brix
2
3
4
Time
1: lag time; 2: max fermentation rate; 3: transition
point; 4: post-transition fermentation rate; 5: overall
time to dryness
Stages of Fermentation
• Stage 1: Lag due to need to adapt then grow to maximize
population levels
• Stage 2: Fastest rate of fermentation: largest population
of cells supported by medium, absence of inhibitory
ethanol
• Stage 3: Transition: at this point ethanol has become
inhibitory to continued maximal fermentation; cells are
adjusting plasma membrane composition and internal
membranes and proteins to an ethanol tolerant form
• Stage 4: Maximal fermentation rate of ethanol-adapted
cells
• Stage 5: Complete metabolic consumption of sugar, cells
enter decline or death phase
Fermentation Progression Assessment
• Lag time
• Duration?
• Maximum fermentation rate
• Rate value?
• Duration?
• Transition point
• At what Brix level?
• How sharp?
• Post-transition fermentation rate
• Value relative to max fermentation rate?
• Length of time?
• Brix/ethanol/nitrogen level at which it occurs?
• Overall time to dryness
Fermentation Capacity Is a Function of:
• Yeast Biomass Concentration
• Fermentative Ability of Individual Cells
Fermentation Capacity Is a Function of:
• Yeast Biomass Concentration
• Fermentative Ability of Individual Cells
• Both are dependent upon:
• Adequate nutrition
• Absence of inhibitors
• Permissive growth conditions
• Permissive fermentation (energy generation) conditions
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
Nutrient Limitation
• Nitrogen: most often limiting
• Phosphate can also be limiting
• Depending upon circumstances, micronutrient
limitation may also be a problem
• Pitching yeast
• Deficiency in vineyard
• Microbial blooms prior to fermentation
• Loss during rehydration
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
• Ionic imbalance
Ionic Imbalance
Ratio of K+:H+
• Must be at least 25:1
• Needs to be adjusted early in fermentation
• Probably important in building an ethanol tolerant
membrane
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
• Nutrient imbalance
• Ethanol toxicity
Ethanol Toxicity
• Ethanol disrupts membrane component interactions
affecting transporter activity and ability to remove toxic
substances like free protons from the cell
• Cells adapt to ethanol by changing the composition of the
membrane and by changing the species of the proteins
present
• Adaptation requires the ability to make new proteins and
membrane components (lipids, sterols)
• If conditions do not enable development of ethanol
tolerance, fermentation rates will decrease
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
• Nutrient imbalance
• Ethanol toxicity
• Presence of toxic substances
Presence of Toxic Substances
• Toxins may arise from the metabolic activity of other
microbes
• Toxins may arise from sub-optimal metabolic activity of
Saccharomyces (such as acetic acid production)
• Toxins may have arisen in vineyard, but are not inhibitory
until ethanol has accumulated
The Most Common Toxins
• Acetic acid
• Higher organic acids (C2 – C4)
• Medium chain fatty acids/fatty acid esters
• Acetaldehyde
• Fungicide/Pesticide residues
• Higher alcohols
• Higher aldehydes
• Killer factors
• Sulfur dioxide
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
• Nutrient imbalance
• Ethanol toxicity
• Presence of toxic substances
• Poor adaptation of strain
Poor Adaptation of Strain
• Strain may not display sufficient ethanol tolerance
• Strain may have high nitrogen/vitamin requirements that
are not being met
• Strain may be a poor fermentor, but capable of
dominating the fermentation
• Rehydration/Implantation issues: Strain not rehydrated
properly, stored correctly or is being used past its
expiration date
• Temperature shock effects during rehydration and pitching
into the tank
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
• Nutrient imbalance
• Ethanol toxicity
• Presence of toxic substances
• Poor adaptation of strain
• Low pH
pH
• pH is reduced by metabolism of Saccharomyces
• Low pH musts (below pH 3.0) may drop to an inhibitory
level (pH 2.7)
• Dependent upon K+ concentration
Causes of Stuck/Sluggish Fermentations
• Nutrient limitation
• Nutrient imbalance
• Ethanol toxicity
• Presence of toxic substances
• Poor adaptation of strain
• Low pH
• Temperature shock
Temperature Shock
• Superheating or supercooling
• Will impact cell membrane functionality and ability to
tolerate ethanol
• Main issue is a ratcheting down of maximal level of
ethanol tolerance
Problem Fermentation Profiles
Types of Problem Fermentations
30
Normal
Long Lag
25
Becoming Sluggish
Sluggish Throughout
Brix
20
Abrupt Arrest
15
10
5
0
0
50
100
150
200
-5
Time (hours)
250
300
350
Types of Problem Fermentations
30
Normal
Long Lag
25
Becoming Sluggish
Sluggish Throughout
Brix
20
Abrupt Arrest
15
10
5
0
0
50
100
150
200
-5
Time (hours)
250
300
350
Causes of Long Lag
• Poor health of starter culture
• Presence of inhibitors
• Poor grape quality
• Mold infestation
• Premature initiation of fermentation
Poor Health of Starter Culture
• Active Dry Yeast:
• Past expiration date
• Not hydrated properly
• Not stored properly
• Natural Fermentation:
• Yeast numbers low
• Inhibitory microbes present (heavy rot of fruit)
• Poor yeast strain present (high N requirements, lack of ethanol
tolerance)
Presence of Inhibitors
• Sulfur dioxide concentration too high
• Sulfur dioxide added improperly
• Microbial competition for nutrients
• Microbial activity generating inhibitory compounds
• Enzymatic competition for oxygen
• Pesticide/fungicide residues on grapes at harvest
• Temperature of must/juice too high/low
Poor Grape Quality
• Infected grapes: loss of micronutrients
• Infected grapes: high microbial loads
• Loss of free oxygen
Types of Problem Fermentations
30
Normal
Long Lag
25
Becoming Sluggish
Sluggish Throughout
Brix
20
Abrupt Arrest
15
10
5
0
0
50
100
150
200
-5
Time (hours)
250
300
350
Causes of Slow Rate Over Entire Time
Course
• Failure to reach maximum cell density
• Nutrient (growth factor) limitation
• Strain a poor choice for conditions
• Inhibitory fermentation conditions:
temperature, pH, ionic
imbalances, organic acids
Types of Problem Fermentations
30
Normal
Long Lag
25
Becoming Sluggish
Sluggish Throughout
Brix
20
Abrupt Arrest
15
10
5
0
0
50
100
150
200
-5
Time (hours)
250
300
350
Causes of a Decrease in Rate
• Poor ethanol tolerance
• Loss of viability
• Loss of fermentative capacity
• Nutrient (survival factor) limitation
• Poor strain
Types of Problem Fermentations
30
Normal
Long Lag
25
Becoming Sluggish
Sluggish Throughout
Brix
20
Abrupt Arrest
15
10
5
0
0
50
100
150
200
-5
Time (hours)
250
300
350
Causes of an Abrupt Stop
• Temperature shock
• Rapid build up of inhibitors: acetic/organic acids: generally
caused by addition of active ML culture
• pH decreases too much
• Strain very ethanol sensitive
Most Common Causes of Stuck/Sluggish
Fermentations
• Temperature extreme
• Ethanol intolerance
• Nutrient deficiency
• Deficient yeast strain
• Microbial incompatibility
• Presence of an inhibitory substance
• Poor fermentation management decisions
Why are stuck fermentations difficult to
treat?
• Cells adapt to adverse conditions by reducing
fermentation capacity
• Biological adaptation difficult to reverse
• Diagnosis of cause of fermentation problem
difficult
• Conditions that cause stuck fermentations are
also conducive to cell death
• New inocula respond to cell death by arresting
activities
Best Advice
• Pay attention to strain selection: match strain to
fermentation conditions
• Provide adequate nutrient levels
• For active dry yeast: follow packet instructions for
rehydration and adaptation to juice
• For native fermentations: be aware of microbiota issues,
heed historical observations for nutrient requirements and
ethanol tolerance
• Temperature Shock: should be avoided but if it happens
consider racking and re-inoculation with unshocked yeast
• Adjust juices of marginal pH (too high or too low)
• Limit activity of inhibitory microbes (wild lactics)