Transcript No Slide Title - the University of California, Davis

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
Outline of Presentation
 The
Importance of Yeast
 Desirable Traits of Yeast Strains
 Selection of the “Right” Strain
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
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
Yeast Components Impacting Wine
Flavor
 Metabolites
 Enzymes
 Catalysts
 Mannoproteins
and Polysaccharides
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
Major Classes of Yeast Flavor
Compounds
 Esters
 Sulfur
Compounds
 Alcohols
 Aldehydes
 Acids
 Carbonyl Compounds
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
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
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
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 10-20 degree range
within this overall range)
Commercial Yeast Strain Diversity

Stress tolerance:
– Nutrient shock
– Temperature shock
– Microbial competitor sensitivity
 Off-character
production
– Hydrogen sulfide
– Acetic acid
– Esters
– Sulfur dioxide
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
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
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
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