Impact of Yeast Strain Diversity
Lucy Joseph
Department of Viticulture and Enology
UC Davis
Outline of Yeast Diversity
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Overview of biological strain diversity
Commercial yeast strains
Choosing a yeast strain
Native yeast strains
Interaction with other yeast and fungi and
bacteria
What is a Strain?
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Strains are the same species of organism
They have measureable differences
These differences are consistent and inherited
Strains are often a clonal population (budding)
Differences may be genetic or due to the
expression of genetic material
How do Differences Arise?
• Genetic change on the chromosome either by
sequence or position
• Genetic difference in organelle DNA, example:
mitochondrial or ribosomal DNA
• Genetic mutation in or loss or gain of extrachromosomal DNA
• Extra copies of chromosomes, anueploid/polyploid
• Hybrids between Saccharomyces species
• Inherited transcriptional state, i.e. epigenetic
• Inherited protein conformational state i.e. prion
Selection for Differences
Natural variation occurs
Unfavorable variants
selected against
New Strain
Favorable variants more likely
to survive and reproduce
Domesticated vs. Wild Saccharomyces
cerevisiae
• Wild Saccharomyces are found in the vineyard
and in oak trees and soil near vineyards
• Domesticated Saccharomyces are found in the
winery and for sale from inoculant companies
• Domesticated yeast are typically better suited for
making wine
• They are selected both naturally and purposefully
to survive and thrive in the winery environment
Definition of Domesticated
‘species bred in captivity and thereby modified
from its wild ancestors in ways making it more
useful to humans who control its reproduction
and (in the case of animals) its food supply’
Diamond J (2002)
Evolution, consequences and future of plant and animal domestication
Nature, 418, 700–707
Wine Yeast and Other Domesticated Yeast
LeGras et.al. Molecular
Ecology (2007) 16:2091-2102.
Characteristics Selected For in Wine
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Acid Tolerance
Temperature range
Sulfite resistance – Vineyard and Winery
Fermentation speed
Alcohol tolerance
Nitrogen requirement
Competitiveness
What traits do you want in a yeast?
What traits do you want in a yeast?
Characteristics not Selected for in Nature
• Sensory properties
– Low VA, SO2, H2S
– Positive ester production
– Enhanced mouthfeel
• Those important in production
– Low foam production
– Flocculation
– ML compatibility
What traits do you want in a yeast?
What traits do you want in a yeast?
Choosing a Yeast Strain
Consider yeast as a tool for wine making
Factors to Consider:
• Grape varietal
• Brix and alcohol
• Fermentation temperature
• Additions
• Production capacity
• Wine making style – ML, aging, micro-ox, etc.
Before Inoculation
Before Inoculation
Testing May Affect Your Yeast Choice…
• If you might add nitrogen or use a yeast that
doesn’t require high nitrogen
• If other nutrients are lacking
• If you should consider a yeast that tolerates
high alcohol
• If you are concerned about H2S or SO2
production
Other factors for your
considerations…
Native Fermentations
• Yeasts other than Saccharomyces
can occur and complete fermentations
• Commercial strains can persist in the winery
• Native strains may have greater potential for change
(instability)
• Potential for strains with undesirable characteristics
to dominate and persist
• Multiple strain fermentations are more likely to
occur
Stuck Fermentation
• Once a fermentation sticks it can be hard to
un-stick
• Some strains do better than others at this
• Native fermentations should be monitored
more closely
Sensory Defects
• These may occur more often with native
fermentations
• Monitor closely
• Consider inoculation at first signs of trouble
• The point of inoculation is to get rapid and
consistent fermentations
Mixed Fermentations
• Pro:
– Wild yeasts can add complexity to wines
– More than one strain of Saccharomyces could
add complexity and unique character
– Diverse population may handle stress better
• Con:
– Competition between yeasts and strains can
select for those with undesirable traits
– Strains that grow well do not always produce the
best wine
Other Microbe Interactions
• Some yeasts are not compatible with ML
conversion
– Possibly due to more complete utilization of
nutrients leaving less for Oenococcus
• High SO2 can inhibit ML
• Low pH can also prevent ML
Other Microbe Interactions
• Specialty wines like botrytized have specific
issues, botrytized wines will have far more
diverse flora making fermentation more of a
challenge
• Sparkling and sherry wines have another yeast
fermentation during the second stage of
production which must be compatible with
the first yeast
Summary
By Analogy….
Domesticated Organisms Retain
Much Diversity
Strains are Bred for Specific Conditions
• No single strain does well under all conditions
• No single strain serves all purposes
Some Strains are Just More Pleasing to
the Senses
Wild Strains Have the Most Diversity