Lecture 11:

Stuck Fermentations: Diagnosis and Rectification

In this lecture we will learn how to recognize an arrested fermentation, how to diagnose the cause of the problem and how to re-initiate fermentation

Normal Fermentation Profile

Glucose is consumed faster than fructose

Arrested fermentations will be high in fructose relative to glucose

Sugar Consumption

0

50

100

150

200

250

300

0 20 40 60 80 100 120 140 160 180 200Fermentation Time (hours)

Suga

r (g/

L)

Glucose

Fructose

Total Sugar

Problem Fermentation Profiles

Types of Problem Fermentations

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Time (hours)

Bri

x

Normal

Long Lag

Becoming Sluggish

Sluggish Throughout

Abrupt Arrest

Types of Sluggish Fermentations

• Long Lag

Types of Problem Fermentations

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Time (hours)

Bri

x

Normal

Long Lag

Becoming Sluggish

Sluggish Throughout

Abrupt Arrest

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

Poor yeast strain present

Presence of Inhibitors

• Sulfur dioxide concentration too high

• Sulfur dioxide added improperly

• Microbial activity resulting in inhibition

• 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 Sluggish Fermentations

• Long Lag

• Slow Rate Over Entire Course of Fermentation

Types of Problem Fermentations

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Time (hours)

Bri

x

Normal

Long Lag

Becoming Sluggish

Sluggish Throughout

Abrupt Arrest

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

Types of Sluggish Fermentations

• Long Lag

• Slow Rate Over Entire Course of Fermentation

• Rapid Rate Becoming Slow

Types of Problem Fermentations

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Time (hours)

Bri

x

Normal

Long Lag

Becoming Sluggish

Sluggish Throughout

Abrupt Arrest

Causes of a Decrease in Rate

• Poor ethanol tolerance

• Loss of viability

• Loss of fermentative capacity

• Nutrient (survival factor) limitation

• Poor strain

Types of Sluggish Fermentations

• Long Lag

• Slow Rate Over Entire Course of Fermentation

• Rapid Rate Becoming Slow

• Abrupt Stop

Types of Problem Fermentations

-5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Time (hours)

Bri

x

Normal

Long Lag

Becoming Sluggish

Sluggish Throughout

Abrupt Arrest

Causes of an Abrupt Stop

• Temperature shock

• Rapid build up of inhibitors: acetic/organic acids

• pH decreases too much

• Strain very ethanol sensitive

Most Common Causes of Stuck/Sluggish Fermentations

• Nutrient deficiency

• Temperature extreme

• Presence of a toxic substance

• Microbial incompatibility

• Deficient yeast strain

• 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

Fermentation Variables Impacting Progression and Rate

• Oxygenation

Oxygenation

• Oxygen is a micronutrient electron acceptor

• Oxygen is a survival factor

• Oxygen can lead to color changes (brown, pink, orange)

Fermentation Variables Impacting Progression and Rate

• Oxygenation

• Mixing: natural or assisted

Mixing

• Separates yeast from end products

• Brings yeast in contact with new nutrients

• Can facilitate skin extraction in reds and solids extraction in whites and reds

Fermentation Variables Impacting Progression and Rate

• Oxygenation

• Mixing: natural or assisted

• Type of Fermentation Vessel

Fermentation Vessel

• Stainless steel can be cooled• Stainless steel easier to sanitize• Stainless steel is inert• Wood: Can impart positive flavors and

aromas• Wood: Difficult to clean; impossible to

sterilize• Wood: Develops stable biofilm of microflora

Fermentation Variables Impacting Progression and Rate

• Oxygenation

• Mixing: natural or assisted

• Type of Fermentation Vessel

• Inoculation Practices

Inoculation Practices

• Spontaneous Fermentation

• Inoculated Fermentation

Fermentation Variables Impacting Progression and Rate

• Oxygenation

• Mixing: natural or assisted

• Type of Fermentation Vessel

• Inoculation Practices

• Temperature of fermentation

Temperature

• Affects presence and persistence of wild flora

• Affects fermentation and growth rates

• Extremes are inhibitory

• Impacts spontaneous chemical reactions

Fermentation Variables Impacting Progression and Rate

• Oxygenation

• Mixing: natural or assisted

• Type of Fermentation Vessel

• Inoculation Practices

• Temperature of fermentation

• Supplementation/Juice treatments

Supplementation

• Prevents nutritional deficiency

• May impact spectrum of yeast end products

• Residual nutrients encourage growth of spoilage organisms

• Unwanted byproducts may be made

Juice/Must Supplements

• Diammonium phosphate (0.96 g/L; 8 lbs/1000 gal)

• Yeast nutritional supplements (varies by producer)

• Yeast autolysates (3lb/1000 gal)

• Thiamin hydrochloride (0.005 lb/1000 gal)

Other Juice Treatments

• Fining

• Centrifugation

• Aeration

• Clarification: settling/filtration

Fermentation Variables Impacting Progression and Rate

• Oxygenation

• Mixing: natural or assisted

• Type of Fermentation Vessel

• Inoculation Practices

• Temperature of fermentation

• Supplementation/Juice treatments

• Lees contact

Grape Lees Contact

• Extraction of nutrients

• Extraction of grape characters

• Function as solids

Fermentation Variables Impacting Progression and Rate

• Oxygenation• Mixing: natural or assisted• Type of Fermentation Vessel• Inoculation Practices• Temperature of fermentation• Supplementation/Juice treatments• Lees contact• Presence of solids

Presence of Solids

• Natural (grape material) or added (bentonite, yeast hulls)

• Stimulate fermentation

• Stimulate growth

• Source of nutrients

• Removal of inhibitory components

Re-initiation of Stuck Fermentations

• Correct diagnosis of nature of the problem important

• If re-inoculating, make sure inoculum is adapted to conditions of stuck wine

• Serial re-inoculation

• May need to remove existing biomass

Serial Re-Inoculation

Fresh Juice

Arrested Wine

Serial Re-Inoculation

Transfer initial 50:50 blend to second tank when it is 2-4 Brix above the arrested Brix level of the stuck fermentation

Do not let any of the intermediate steps in the series go dry, transfer them at the equivalent or slightly higher Brix than the arrested wine

This concludes the section on the alcoholic fermentation.