1. An Overview of Commercial Yeast Selection
Linda F. Bisson
Department of Viticulture and Enology
University of California, Davis, CA
Wine Flavor 101
July 28, 2015

2. Outline of Presentation
The Importance of Yeast
Desirable Traits of Yeast Strains
Selection of the “Right” Strain

3. The Importance of Yeast
Conversion of sugar to ethanol
Production of aromatic compounds
Effects on mouth feel
Modification of plant components
Consumption of nutrients and prevention of growth of other microorganisms
Creation of reductive environment impacting a range of subsequent chemical reactions and aging

4. Yeast Production of Aromatic Compounds
Synthesize positive characters
Synthesize negative characters
Modify existing grape characters
Intensify perception of existing characters
Reduce perception of existing characters
Alter wine matrix: change in chemical and sensory properties

5. Yeast Components Impacting Wine Flavor
Metabolites
Enzymes
Catalysts
Mannoproteins and Polysaccharides

6. Types of Yeast Impact on Aromatic Compounds
Primary roles
Production of flavor compounds de novo from nutrients
Liberation of grape flavor components from precursors
Secondary roles
Provide chemical reactants
Enzymatic modification of grape/oak flavors
Impact Redox status and buffering capacity

7. Major Classes of Yeast Flavor Compounds
Esters
Sulfur Compounds
Alcohols
Aldehydes
Acids
Carbonyl Compounds

8. Yeast Choice: Desirable Traits
Fermentation to dryness
Reasonable rate of fermentation
Predictable fermentation characteristics
Appropriate ethanol tolerance
Appropriate temperature tolerance
Killer factor resistance
Dominance of fermentation: wild Saccharomyces, non-Saccharomyces and bacteria

9. Yeast Choice: Desirable Traits
Little to no off-character production
Little to no inhibition of other desirable microbes
Production of “natural” sulfur dioxide
Production of desired aroma characters
Production of mouth feel characteristics
Enhancement of aging: enzymatic and matrix effects

10. Commercial Yeast Choices
Saccharomyces cerevisiae cerevisiae
Saccharomyces cerevisiae bayanus
Saccharomyces bayanus
Saccharomyces uvarum
Saccharomyces hybrid strains (native or constructed) often contain genetic information from Saccharomyces kudriavzevii
Non-Saccharomyces strains

11. Commercial Yeast Strain Diversity
Ethanol Tolerance: range from 12-18%
Relative Nitrogen Needs: low/medium/high (relative to other yeast strains)
Fermentation speed (medium/fast) (relative to other yeast strains)
Temperature range (10-32°C/50-90°F); but many yeast span a degree range within this overall range)

12. Commercial Yeast Strain Diversity
Stress tolerance:
Nutrient shock
Temperature shock
Microbial competitor sensitivity
Off-character production
Hydrogen sulfide
Acetic acid
Esters
Sulfur dioxide

13. Commercial Yeast Strain Diversity
Flavor and aroma production
Tropical esters
Floral esters
Thiol release
Glycoside release
Neutrality
Yeast signature (bread/toast)
Color stability (absorption?)
Mouthfeel effects

14. Commercial Yeast Impact
Is complicated . . .
Affected by juice/must composition
Impacted by nutrient additions
Influenced by other microbiota (including other strains of Saccharomyces)
Impacted by processing decisions: oxygen, temperature
Many effects are poorly understood biologically

15. Selection of the “Right” Strain
Make sure strain has the needed ethanol tolerance
Meet nitrogen and micronutrient needs
Rehydrate properly
Manage “bad” flora
Lactobacillus
Acetic acid bacteria
Strain trials are useful but be aware of impact of vintage variation

16. Strain Trials
Challenging to do due to juice/must/fermentation variation
Need replicates
Need to emulate production conditions
Need to confirm inoculated strain actually conducted the fermentation and not a robust house strain