Overview of Problem Fermentations

1. Overview of Problem Fermentations Linda F. Bisson Department of Viticulture and Enology Issues in Fermentation Management, 2011

2. Problem Fermentations • Slow (sluggish) fermentation • Stuck (incomplete, arrested) fermentation • Off-character production • Hydrogen sulfide • Sulfur volatiles • Acetic acid • Undesired Esters

3. Stuck and Sluggish Fermentations

4. Stuck and Sluggish Fermentations • Characterized by failure of yeast to consume sugar • Multiple causes • Difficult to treat • Leads to reduced wine quality

5. Fermentation Profile 5 1 2 Brix 3 4 Time 1: lag time; 2: max fermentation rate; 3: transition point; 4: post-transition fermentation rate; 5: overall time to dryness

6. Fermentation Profile • 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

7. Fermentation Capacity Is a Function of: • Yeast Biomass Concentration • Fermentative Ability of Individual Cells

8. Causes of Stuck/Sluggish Fermentations • Nutrient limitation

9. 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

10. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Ionic imbalance

11. 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

12. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Nutrient imbalance • Substrate inhibition

13. Substrate Inhibition Transporters with a high substrate affinity can get “jammed” at high substrate concentrations G G F F F G

14. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Nutrient imbalance • Substrate inhibition • Ethanol toxicity

15. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Nutrient imbalance • Substrate inhibition • Ethanol toxicity • Presence of toxic substances

16. Presence of Toxic Substances • Toxins may arise from the metabolic activity of other microbes • Toxins may arise from metabolic activity of Saccharomyces • Toxins may have arisen in vineyard, but are not inhibitory until ethanol has accumulated

17. 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

18. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Nutrient imbalance • Substrate inhibition • Ethanol toxicity • Presence of toxic substances • Poor adaptation of strain

19. Poor Adaptation of Strain • Strain may not display ethanol tolerance • Strain may have high nitrogen/vitamin requirements • Strain may be a poor fermentor, but capable of dominating the fermentation • Temperature effects

20. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Nutrient imbalance • Substrate inhibition • Ethanol toxicity • Presence of toxic substances • Poor adaptation of strain • Low pH

21. 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

22. Causes of Stuck/Sluggish Fermentations • Nutrient limitation • Nutrient imbalance • Substrate inhibition • Ethanol toxicity • Presence of toxic substances • Poor adaptation of strain • Low pH • Temperature shock

23. Problem Fermentation Profiles

25. Types of Sluggish Fermentations • Long Lag

27. Causes of Long Lag • Poor health of starter culture • Presence of inhibitors • Poor grape quality • Mold infestation • Premature initiation of fermentation

28. 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

29. 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

30. Poor Grape Quality • Infected grapes: loss of micronutrients • Infected grapes: high microbial loads • Loss of free oxygen

31. Types of Sluggish Fermentations • Long Lag • Slow Rate Over Entire Course of Fermentation

33. 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

34. Types of Sluggish Fermentations • Long Lag • Slow Rate Over Entire Course of Fermentation • Rapid Rate Becoming Slow

36. Causes of a Decrease in Rate • Poor ethanol tolerance • Loss of viability • Loss of fermentative capacity • Nutrient (survival factor) limitation • Poor strain

37. Types of Sluggish Fermentations • Long Lag • Slow Rate Over Entire Course of Fermentation • Rapid Rate Becoming Slow • Abrupt Stop

39. Causes of an Abrupt Stop • Temperature shock • Rapid build up of inhibitors: acetic/organic acids • pH decreases too much • Strain very ethanol sensitive

40. 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

41. 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