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Native Fermentations:
Managing the Microbiota
Linda F. Bisson
Department of Viticulture and Enology
University of California, Davis, CA
Wine Flavor 101
July 28, 2015
Outline of Presentation
 Introduction
to Native Fermentations
– Issues in use of native microbiota
– The important variables
 Microbial
Diversity of Native Fermentations
 Optimizing the Microbiota
Native Fermentations:
 Rely
on microbial flora of grapes and
wineries to conduct fermentation
 No deliberate inoculation with commercial
strains
 May be inoculated from an existing native
fermentation
The Positive Impacts of Native Microbiota
 Complexity:
Unique flavors and aromas
 Enhancement of varietal character
– Excretion of hydrolases
– Degradation of aroma precursor molecules
 Slower
primary fermentation
– Reduced temperatures
– Less volatilization and entrainment in carbon
dioxide stream
– Stress characters produced by Saccharomyces
The Negative Impacts of Native Microbiota
Arrest of fermentation
 Spoilage character formation

–
–
–
–

Ethyl acetate
S-taints
Volatile phenols
Depends upon organisms present
Loss of varietal character
– Degradation of esters via esterases
– Release of too high of a concentration of aroma molecules so aging
of wine is impacted
Native Fermentations: Many Styles
100% native; no additions of any microbes
 Native yeast but with ML inoculation?
 Native Saccharomyces only or native nonSaccharomyces?
 Partial natives:

– At some point inoculated with a commercial strain of
Saccharomyces
– A blend of commercial and native fermentations
– Conducted in tanks previously used for commercial yeast

Manipulated microbiota natives
Manipulated Microbiota
 Sulfur
dioxide or other antimicrobial agent
used
 Temperature of pre-incubation/fermentation
 pH adjustment or selection
 Level of aeration
 Nutrient addition practices
 Inoculation with non-Saccharomyces yeasts
and bacteria
Source of Native Flora
 Vineyard
 Winery
 Early
Both are a source of microbes
in season grape microbiota is most
important but as microbes become
established on winery surfaces, winery
microbiota become more important
 And it all depends upon sanitation practices
Successful Natives: The Most Important Variables
 Condition
of the fruit
 Organisms on fruit/coming in with fruit
 Time of harvest
 Processing conditions
 Juice/Must amendments
 pH
 Sulfite addition
 Temperature
Condition of the Fruit
 Damaged
Clusters
– Rot amplifies acetic acid bacteria
– Mold metabolites may be present
 Uneven
Ripening/Raisining
– Leads to differences in flora on the surfaces of the fruit
 Presence
of Material Other than Grape
– Soil, leaves, bark: all contain microbes
– Some of these microbes can persist early in fermentation
 Contribute
to Winery Flora
Time of Harvest

During ripening the berry surface flora change
– More seepage from the berry providing nutrients
– Yeast population continues to increase in relative numbers postveraison

Aerobic basidiomycetes are replaced by the fermentative
ascomycetes
– Consume available oxygen rapidly
– Create localized anaerobic zones
– Produce toxic waste products

Aerobic bacilli and pseudomonads replaced by acetic and
lactic acid bacteria
– Produce toxins and toxic waste products
Processing Conditions

Berry Integrity
– Partially damaged berries encourage growth of surface flora
– Presence of juice will lead to growth and more rapid oxygen
consumption

Skin Contact
– Allows greater extraction of surface flora
– Allows leaching of berry biofilms

Oxygen Exposure
– Favors persistence of the aerobic microbes
– Leads to production of acetic acid

Temperature
Juice/Must Amendments
 Nutrient
Additions
– Which population is being fed?
– Partial or complete nutrients?
 Acidity
Adjustments
– Acids are nutrients for some microbes and allow
adaptation to sugars
 Water
Addition
– Reduces osmolarity
– High osmolarity favors Zygosaccharomyces
pH
 Very
low pH (less than 2.5) favors
Brettanomyces
 Many bacteria require pH values above 3.5 in
order to grow
 All berry fermentative yeasts can grow above
pH 2.5
Sulfite Addition
 Use
of antimicrobials impacts flora
 Sulfite impacts bacteria more than wild
yeasts
 Level of sulfite used varies from low to high
(25 ppm to 100 ppm)
 Effectiveness depends upon pH
Temperature
 Of
harvest, processing, fermentation
 In general warmer temperatures favor bacteria
 Cold soaks favor wild yeast populations
 Saccharomyces combines the tolerances of yeasts
with a preference for slightly warmer temperatures
than wild yeasts
 The combination of ethanol and higher temperature
leads to dominance of fermentations by
Saccharomyces
MICROBIAL DIVERSITY OF NATIVE
FERMENTATIONS
The Native Microbiota: Bacteria
 Species
present depends upon time of harvest
 Some vineyard specificity (Pediococcus) but in
general the bacteria found are fairly consistent
across vineyards and regions
 Variation in relative numbers depends upon
several factors
– Vineyard management practices
– Insect vectors
– pH of fruit
– Leakiness of fruit
The Desirable Bacteria
 Oenococcus,
but this is rare
 Lactics that are not able to dominate the
fermentation
– Fastidious nutrient requirements
– High sensitivity to sulfite
– Low pH intolerant
The Undesirable Bacteria
 Spoilage
lactic acid bacteria
– Off-characters
– Negative impacts on fermentation
 Spoilage
acetic acid bacteria
– Negative impacts on fermentation
 May
come from grapes, may come from
insects associated with grapes
The Native Microbiota: Yeasts
 Show
strong impacts of maturity
 Show differences by climate
 Show seasonal effects
 Show differences by varietal
 Show site effects
 More strongly influenced by insect trafficking
The Most Common Yeasts of the Berry
Surface
Yeast
Kloeckera apiculata/
Hanseniaspora uvarum
Candida species
Issatchenkia
Metschnikowia
Pichia species
Basidiomycetes
% of Total Isolates
65-80
5-10
5-10
5-10
10-20
10-20
Saccharomyces
 Not
often isolated from vineyards
 Depends upon practice of dumping yeast lees in
vineyard
 Depends upon level of rot
 Is present but in very low numbers, one out of
ten million yeast isolates
 Even at these low numbers will dominate wine
fermentations
Diversity of Saccharomyces
Well-established variation in genome of wild and
commercial strains
 Studies have shown the presence of recurring genotypes in
specific wineries: winery-specific strains
 Studies in vineyards have shown great diversity of
Saccharomyces but no vineyard-specific genotypes
 Some studies show some regionality of yeast genotypes,
others do not
 The amount of genetic variation of wine strains is extreme
with many only being able to be isolated in a single vintage

Management of Native Microbiota
 Sulfur
dioxide, sulfur dioxide, sulfur dioxide
 Judicious feeding of populations
 Need a microscope
 Analysis of species of microbiota present
(and SO2 sensitivity)
 Avoid cluster rot; high insect infestation of
fruit during processing
Conclusions
 Sulfur
dioxide trials of natives are key to
optimizing positive impacts
 Some vineyards will never yield optimal
native wines
 Microbial monitoring is advisable: organisms,
both good and bad, grow exponentially