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

Problem Fermentation Profiles

Types of Sluggish Fermentations Long Lag

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

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

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

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

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

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.