Overview of Problem Fermentations Linda F Bisson Department
Overview of Problem Fermentations Linda F. Bisson Department of Viticulture and Enology Issues in Fermentation Management, 2011
Problem Fermentations n Slow (sluggish) fermentation n Stuck (incomplete, arrested) fermentation n Off-character production – Hydrogen sulfide – Sulfur volatiles – Acetic acid – Undesired Esters
Stuck and Sluggish Fermentations
Stuck and Sluggish Fermentations n Characterized by failure of yeast to consume sugar n Multiple causes n Difficult to treat n Leads to reduced wine quality
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
Fermentation Profile n Lag time – Duration? n Maximum fermentation rate – Rate value? – Duration? n Transition point – At what Brix level? – How sharp? n Post-transition fermentation rate – – – n 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: n Yeast Biomass Concentration n Fermentative Ability of Individual Cells
Causes of Stuck/Sluggish Fermentations n Nutrient limitation
Nutrient Limitation Nitrogen: most often limiting n Phosphate can also be limiting n Depending upon circumstances, micronutrient limitation may also be a problem n – – Pitching yeast Deficiency in vineyard Microbial blooms prior to fermentation Loss during rehydration
Causes of Stuck/Sluggish Fermentations n Nutrient limitation n 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 n Nutrient limitation n Nutrient imbalance n Substrate inhibition
Substrate Inhibition Transporters with a high substrate affinity can get “jammed” at high substrate concentrations G G F F FG
Causes of Stuck/Sluggish Fermentations n Nutrient limitation n Nutrient imbalance n Substrate inhibition n Ethanol toxicity
Causes of Stuck/Sluggish Fermentations n Nutrient limitation n Nutrient imbalance n Substrate inhibition n Ethanol toxicity n Presence of toxic substances
Presence of Toxic Substances n Toxins may arise from the metabolic activity of other microbes n Toxins may arise from metabolic activity of Saccharomyces n Toxins may have arisen in vineyard, but are not inhibitory until ethanol has accumulated
The Most Common Toxins n n n n n Acetic acid Higher organic acids (C 2 – C 4) Medium chain fatty acids/fatty acid esters Acetaldehyde Fungicide/Pesticide residues Higher alcohols Higher aldehydes Killer factors Sulfur dioxide
Causes of Stuck/Sluggish Fermentations n Nutrient limitation n Nutrient imbalance n Substrate inhibition n Ethanol toxicity n Presence of toxic substances n Poor adaptation of strain
Poor Adaptation of Strain n Strain may not display ethanol tolerance n Strain may have high nitrogen/vitamin requirements n Strain may be a poor fermentor, but capable of dominating the fermentation n Temperature effects
Causes of Stuck/Sluggish Fermentations n Nutrient limitation n Nutrient imbalance n Substrate inhibition n Ethanol toxicity n Presence of toxic substances n Poor adaptation of strain n Low p. H
p. H n p. H is reduced by metabolism of Saccharomyces n Low p. H musts (below p. H 3. 0) may drop to an inhibitory level (p. H 2. 7) n Dependent upon K+ concentration
Causes of Stuck/Sluggish Fermentations n n n n Nutrient limitation Nutrient imbalance Substrate inhibition Ethanol toxicity Presence of toxic substances Poor adaptation of strain Low p. H Temperature shock
Problem Fermentation Profiles
Types of Sluggish Fermentations n Long Lag
Causes of Long Lag n Poor health of starter culture n Presence of inhibitors n Poor grape quality – Mold infestation – Premature initiation of fermentation
Poor Health of Starter Culture n Active Dry Yeast: Past expiration date Not hydrated properly Not stored properly n Natural Fermentation: Yeast numbers low Inhibitory microbes present Poor yeast strain present
Presence of Inhibitors n Sulfur dioxide concentration too high n Sulfur dioxide added improperly n Microbial activity resulting in inhibition n Pesticide/fungicide residues on grapes at harvest n Temperature of must/juice too high/low
Poor Grape Quality n Infected grapes: loss of micronutrients n Infected grapes: high microbial loads n Loss of free oxygen
Types of Sluggish Fermentations n Long Lag n Slow Rate Over Entire Course of Fermentation
Causes of Slow Rate Over Entire Time Course n Failure to reach maximum cell density n Nutrient (growth factor) limitation n Strain a poor choice for conditions n Inhibitory fermentation conditions: temperature, p. H, ionic imbalances
Types of Sluggish Fermentations n Long Lag n Slow Rate Over Entire Course of Fermentation n Rapid Rate Becoming Slow
Causes of a Decrease in Rate n Poor ethanol tolerance n Loss of viability n Loss of fermentative capacity n Nutrient (survival factor) limitation n Poor strain
Types of Sluggish Fermentations n Long Lag n Slow Rate Over Entire Course of Fermentation n Rapid Rate Becoming Slow n Abrupt Stop
Causes of an Abrupt Stop n Temperature shock n Rapid build up of inhibitors: acetic/organic acids n p. H decreases too much n Strain very ethanol sensitive
Most Common Causes of Stuck/Sluggish Fermentations n Temperature extreme n Ethanol intolerance n Nutrient deficiency n Deficient yeast strain n Microbial incompatibility n Presence of an inhibitory substance n Poor fermentation management decisions
Why are stuck fermentations difficult to treat? n n n 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
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