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