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Selection and domestication of yeast for improved enological and sensorial properties in wine
production
MFS Project Report
Presented to the Faculty of the Graduate School
of Cornell University
In Partial Fulfillment of the Requirements for the Degree of
Master of Food Science, Specialization of Enology
by
Gabriela A. Polanco
Fall 2021
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© 2021 Gabriela A. Polanco
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ABSTRACT
Natural yeasts often provide interesting flavor and aroma compounds due to the impact of
the environment and terroir. However, their inability to efficiently ferment under harsh
conditions has inhibited their potential to be the sole pioneer within the winemaking concert.
With technological advancements, these indigenous yeast species are often used as starter
cultures to refine these strains with specific characteristics required by the fermentation industry.
The domestication of sophisticated commercial Saccharomyces cerevisiae yeast strains allows
for a more direct approach in selecting strains that possess innovative and useful properties while
also guaranteeing the preservation of the genetic biodiversity of the wine producing area. The
improvement of wine yeasts permits the process of yeast strain selection to remain an important
and fundamental step of the winemaking process. The potential of a particular strain is
highlighted in its ability to survive through fermentative conditions as well as successfully
maintaining desirable post-fermentative flavors.
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BIOGRAPHICAL SKETCH
Gabriela Polanco is from Orange County, California and was inspired to pursue wine
through family and friends. In 2018, she earned a B.S. in Plant Sciences from the University of
California, Santa Cruz. After graduation, she studied abroad with UC Davis for an ‘Introduction
to Winemaking’ course based in Burgundy, France where she had the opportunity to travel to
different winemaking regions in the country and experience the wine and winemaking styles of
those areas. After her month abroad, she returned to California and did a harvest internship in
Napa Valley with Cakebread Cellars. She learned hands-on the applications and step by step
process of making wine. She wanted to take her firsthand experience and apply it towards a more
in-depth knowledge of winemaking where she then attended Cornell University to obtain a
Masters in Food Science with a specialization in Enology and is a member of the Gibney lab.
Her higher education allowed her to take the material she had physically learned during her time
in Napa and propel her into a more thorough comprehension of wine through class and lab work.
Gabriela hopes to return to Napa Valley and pursue a career in the winemaking industry.
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ACKNOWLEDGEMENTS
To my advisor Patrick Gibney, thank you for always being more than willing to lend out advice,
support, and encouragement. I am forever grateful for your guidance as well as your kindness
and sense of humor. My completion of this degree would not have gone as smoothly without
your helping hand especially throughout a pandemic. Thank you for everything.
To my parents, Jackie and Juan Polanco, thank you for always believing in me and pushing me to
do my best, especially in moments when I couldn’t do that for myself. I am beyond thankful for
the sacrifices you have made for educating and preparing me for my future. I would not be where
I am today without you.
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TABLE OF CONTENTS
Page
ABSTRACT…………………………………………………………………………………. 3
BIOGRAPHICAL SKETCH……………………………………………………………….... 4
ACKNOWLEDGMENTS………………………………………………………………….... 5
TABLE OF CONTENTS…………………………………………………………………….. 6
LIST OF FIGURES………………………………………………………………………….. 8
LIST OF TABLES…………………………………………………………………………….9
PREFACE……………………………………………………………………………………..10
SECTIONS
I. INTRODUCTION…………………………………………………………….11
II. CONTRIBUTION OF YEAST TO NON-ENOLOGICAL ENVIRONMENTS..12
Development of S. cerevisiae within non-enological environments………….13
Natural yeast diversity and their effect in wine production…………………..15
III. COMMERCIAL Saccharomyces cerevisiae YEAST STRAINS: ENOLOGICAL
USE…………………………………………………………………………...17
IV. YEAST IN WINEMAKING…………………………………………………19
Ethanol tolerance……………………………………………………………..20
Osmotic tolerance…………………………………………………………….21
Oxidative tolerance…………………………………………………………...21
Thermal tolerance…………………………………………………………….22
Discussion of results………………………………………………………….22
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V. YEAST STRAIN CONTRIBUTION TO CHEMICAL COMPOSITION
DIVERSITY IN WINE……………………………………………………..25
VI. FLAVOR AND AROMA CHEMISTRY: SENSORIAL EFFECTS ON WINE..26
Higher alcohols……………………………………………………………..27
Acetates………………………………………………………………..........28
Ethyl esters of fatty acids and fatty acids…………………………………...29
Sensory characteristics of the wines………………………………………..30
VII. CONCLUSION……………………………………………………………..31
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LIST OF FIGURES
Figure 1
Section 3……………………………………………………………………. 18
Figure 2
Section 4.5 …………………………………………………………………. 23
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LIST OF TABLES
Table 1
Section 4.5 …………………………………..…………………………….. 24
Table 2
Section 4.5 …………………………………..…………………………….. 24
Table 3
Section 5 ...…………………………………..…………………………….. 26
Table 4
Section 6.4 ...………………………………..……………………………... 31
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PREFACE
My first hands on experience with the winemaking process was during my time in Napa
Valley while completing my first harvest. During that time, I remember being fascinated by yeast
and being told by my fellow coworkers that they are alive so to speak kindly to them and
possibly play them music to flourish. I also remember how fickle and sensitive they were, getting
the temperature right during rehydration to bring them to an active state without accidentally
killing them. I found it interesting that this singular component in this winemaking process was
one of, if not the most crucial aspect, but also possibly the most sensitive. From the memories of
this experience, I felt compelled to write this literature review based on the point in which yeast,
specifically Saccharomyces cerevisiae, acts as the main character of the winemaking process
both during and after fermentation.
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INTRODUCTION
Most of the yeast strains utilized in the food industry for the production of beer, wine and
bread are classified as Saccharomyces cerevisiae. This yeast is the most prominently known
yeast used within the winemaking process. It exists in diverse niches across the world and can be
found in natural habitats associated with fruits, tree soil, and insects. The coexistence of both
wild and commercial S. cerevisiae indicates its unique ability to be domesticated from temperate
forest soil to harsh oenological conditions (Buzzini et al. 2017). This adaptation has occurred
through mechanisms that contribute to the manipulated evolution of wine yeast genomes.
Saccharomyces cerevisiae strains naturally possess an ideal combination of traits that make
fermentation processes successful, however the refinement of commercial strains have achieved
genetic and molecular bounds that greatly improve enological traits (Rainieri and Pretorious,
2000). Genetic improvement of natural yeasts has been fundamental in providing strains with
novel and useful characteristics for winemaking.
Saccharomyces cerevisiae is a vital component in carrying out the alcoholic fermentation
process, which is essential to wine production. Yeast cells carry out vinification while sensing
and responding to stress conditions that challenge their osmotic, ethanol, thermal, and oxidative
tolerances. The enological parameters of yeasts are important to conduct a proper fermentation in
winemaking. The systems that sense stressors initiate rapid synthesis of protective molecules and
the activation of signal transduction pathways that induce secondary events such as the triggering
of pre-existing enzyme activities and the transcription of genes encoding factors that have
protective functions (Carrasco et al. 2000). The selection of a yeast strain that is suitable to
endure and thrive based on the conditions presented throughout the winemaking process is
instrumental towards production. The interaction between yeast strains and their potential
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stressors during alcoholic fermentation can contribute to the aroma, taste and color of the final
wine.
The purpose of this report is to discuss the domestication and genetic improvement of
natural yeast to commercial yeast and compare the performance of those yeast strains,
specifically of Saccharomyces cerevisiae. The potential impact of yeast strain variability from
their biogeographical origin affected by terroir to their technologically significant characteristics
under oenological conditions signifies the contributions in which each aspect of the viticultural
and winemaking process enhances an expression of wine quality and style. Saccharomyces
cerevisiae has evolved to ecological dynamics such as adaptation of vineyards and grape must, as
well as enhanced fermentative capacities under specific oenological circumstances. There is
value in understanding the genetic and phenotypic variation in these strains and the way their
genomic expression ultimately affects the wine. Different strains are capable of changing the
wine composition and subsequently its sensorial effects. This study will highlight that different
strains show different properties based on their individual physicochemical and ecological
properties.
CONTRIBUTION OF YEAST FROM NON-ENOLOGICAL ENVIRONMENTS
It is clear that the involvement of Saccharomyces cerevisiae yeast within the winemaking
process is vital if not the spotlight of production. Wine yeasts can only grow in wine must for a
short period every year, therefore they spend the rest of their lives in and around the vineyard or
in insects (Gallone et al. 2016). Their contribution to wine composition and quality begins in the
vineyard typically associated with the phyllosphere, grapes, and soil. Saccharomyces cerevisiae
strains isolated from non-enological environments have been found to possess interesting
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characteristics. However, these strains typically do not carry out the fermentation process as
efficiently because they are not vigorous or competitive in oenological conditions (Rainieri and
Pretorious 2000). Selecting a yeast strain for its sensorially elements is important, but if it is
unable to initiate or finish fermentation, that is a much greater issue at hand. This portion will
discuss the origins of S. cerevisiae within their natural environment and the inception of
development towards its improvement for a more predictable and efficient process.
Development of Saccharomyces cerevisiae within non-enological environments
Humans have utilized Saccharomyces cerevisiae yeast to convert sugars into ethanol and
desirable flavor compounds to create food and beverages since prehistoric times. However, we
are unsure whether yeast diversity is shaped by selection and niche adaptation to societal needs
(domestication) or neutral divergence caused by geographic isolation and limited dispersal
(Gallone et al. 2016). Studies have used microsatellite-based research to rediscover the origin of
S. cerevisiae, which has been shown to date back as far as 7000 BC from China where it further
traveled towards Europe and later spread to the New World (Marsit and Dequin 2015). The
discovery that suggests this is that Chinese isolates harbor almost twice as much genetic
variation as isolates from the rest of the world (Liti 2015). Five distinct lineages were discovered
based on their technological and geographic origin (Gallone et al. 2016). Studies have explored
the genetic diversity of Saccharomyces cerevisiae strains. An approach often used to establish a
relationship between genotypic variation and phenotypes is mapping of quantitative trait loci
(QTL). This has allowed us to view the performance of yeast based on factors such as ethanol
resistance, acetic acid production, and fermentation (Tofalo et al. 2013). Genomic analyses have
been utilized to identify the genes impacting volatile formation (Bisson and Karpel 2010). The
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two main types of analyses performed are investigations of changes in expression patterns of a
single strain where the aroma compounds are produced and the second are comparisons of
mRNA profiles of strains with differing levels of aroma compound production under the same
growth (Bisson and Karpel 2010). Functional analyses based on the first genome sequence have
provided insight into the adaptation of yeast towards the harsh conditions that we now apply
towards S. cerevisiae within the fermentation process.
Recent studies have investigated S. cerevisiae populations to determine their ecological
origin by sequencing the genomes of various strains. Wild yeasts show higher rates of
spontaneous mutagenesis which can lead to the rapid creation of significant diversity across a
population (Bisson 2012). However, strains rapidly lose some phenotypes associated with growth
in the wild upon formulated strains (Bisson 2012). A study highlighted that Saccharomyces
cerevisiae endured hybridization between different domesticated subpopulations, such as wine
yeast and beer yeast, which are phenotypically separated from wild stocks due to human
selection (Gallone et al. 2016). Throughout the year, Saccharomyces cerevisiae primarily reside
in natural habitats such as insects as well as the vineyard itself (Mardit and Dequin 2015). Insects
are important in terms of natural yeast in a vineyard because they are likely the main vectors that
spread cultivable yeasts (Mardit and Dequin 2015). During nutrient-poor periods, wine yeasts
likely undergo sexual cycles and even hybridize with wild yeasts (Gallone et al. 2016). Though
Saccharomyces cerevisiae strains used in winemaking are genetically more homogenous, they
still comprise a great deal of variation.
This variation has been studied based on global-scale results, however, studies have
investigated the structure and gene flow based on smaller ecological levels. Analyses of vineyard
isolates have provided evidence for region specific subpopulations (Marsit and Dequin 2015).
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Between populations within a vineyard, there is evidence of gene flow across small distances,
suggesting some degree of connectivity between populations. There is a lot of variability within
environmental factors of a vineyard location and the interactions between each organism and its
environment influence rearrangement and the evolution of phenotypes (Tofalo et al. 2013). Scott
Labs has stated that majority of the strains that they sell under the Lallemand brand are isolated
from a natural environment in which they are isolated and and commercialized for enological
use. The importance of a geographical origin of wine yeast is established at a regional level
based on yeast environmental isolates that are useful to tailor strains to meet enological demand
and later, consumer demand.
Natural yeast diversity and their effect on wine production
After discussing the basic principles of genetic variation and evolutionary history of
Saccharomyces cerevisiae yeast, we will now be able to highlight the natural form of this yeast
and the diversity that later is utilized for industrial purposes. Originally, fermentation was
brought about by the natural yeasts attached to the surface of the ripe harvested berries. The
activity of yeast cells on the sugars of the grape juice resulted in the production of alcohol,
carbon dioxide, heat, and many other by-products. Natural yeast found on the skin of berries
comes in many different strains that will provide different attributes to the wine. The main yeast
used in winemaking is S. cerevisiae, however, there are many other yeasts that are possibly
present at the start of fermentation, they are just strongly inhibited by the high alcohol
concentrations found in wine (Linskens and Jackson, 1988). This wine yeast can be isolated from
vineyards and is most commonly isolated from heavily damaged grapes whether it be from
mechanical harvesting, weather, or rot (Bisson 2012). Upon the crushing that can appear from
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this damage, enriched and fermentative conditions are openly presented to this microbe (Bisson
2012). Various factors affect yeast flora present on grapes such as rainfall, temperature, soil type,
berry maturity, insects, mechanical damage, and geographical location.
A study in India was conducted where they tested wild Saccharomyces cerevisiae yeast
flora present on six separate grape cultivars to reveal their significance in wine production. The
grapes were picked from different vineyards based in different districts all within Maharashtra,
India. The isolation of S. cerevisiae yeast strains was carried out by the spread plate method on
MGYP agar containing 0.025% tetracycline at 28°C for 48 hours. Nine isolates of the S.
cerevisiae yeast strain, AM262831, were revealed within the grapes (Chavan et al. 2009).
Chavan stated that although most studies indicate low concentrations of natural S. cerevisiae in
grape juice and must, there were reported high counts of the yeast in one of the districts within
Maharashtra, Anatolia, due to excessive use of sulfite in the vineyard. This further supports the
notion that different factors of viticultural management and environment can affect yeast flora.
The natural availability of yeast strains is unlikely to provide the desirable medley of
sensory aromatics and specific compounds one may strive to achieve in wine production. Wild
yeast has been found to have a complex and interesting sensorial impact, however they do not
ferment as efficiently as commercial yeast strains. Although Saccharomyces cerevisiae plays the
main role in fermentation, it is not necessarily widespread on grapes as it depends on a vector,
such as insects, for its dispersal. As previously stated, there are various other yeasts that grow on
grapes, it is mostly that S. cerevisiae is one of the main species that is able to survive under the
harsh condition of high ethanol, while the other microbial yeast species are inhibited severely as
they are not ethanol-tolerant, sensitive to SO2, and have more potential to produce undesirable
compounds (Rainieri and Pretorius 2000). Knowing that Saccharomyces cerevisiae was able to
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have the most fermentative success as well as it is in low abundance naturally, this began the use
of indigenous yeast as starter cultures to produce commercial yeast strains which has improved
the reproducibility, efficiency, survival, and possible sensorial effects of the winemaking process
and finished product.
COMMERCIAL Saccharomyces cerevisiae YEAST STRAINS: ENOLOGICAL USE
We briefly introduced the idea of domestication of Saccharomyces cerevisiae, we will
now be analyzing the evolutionary divergence of industrial yeasts that developed through
industrial application and geographical origin. As previously stated, the concentration of natural
yeast in vineyards is relatively low, therefore the domestication of wine yeast was necessary for
production. The choice to develop and utilize particular Saccharomyces cerevisiae yeast strains
for a specific industrial purpose, such as winemaking, has become both historical and scientific
(Steensels et al. 2014). The likely main purpose was to cultivate yeast strains that will help steer
the wine towards an ideal product. Selection of industrial yeast is able to provide certain
desirable characteristics whether it be sensory attributes or specific forms of resistance to
enological processes (Linskens and Jackson 1988). Saccharomyces cerevisiae was the most
obvious candidate for yeast starter cultures as it was the most ethanol-tolerant yeast wine species
(Rainieri and Pretorius 2000). Once starter cultures were developed, the ability to improve wine
yeasts was possible. They could be modified to accommodate the stress factors that are
encountered within fermentation. Evidence of this is to detect the differences amongst different
Saccharomyces cerevisiae strains as they endure different tolerances. A study was done to test
the genome of beer yeasts with wine yeasts through these different tolerances to see the
difference in their tolerances (Gallone et al. 2016). For the beer S. cerevisiae strain, it was
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evident that it was able to have a significantly higher capacity to metabolize maltotriose while
the wine S. cerevisiae strain did not (Figure 1). While under sulfite tolerance (Figure 1), wine
showed a superior performance as it likely reflects the high-sugar and high-alcohol environment
in winemaking while beer strains perform poorly in general stress conditions that are not usually
encountered in the brewing environment (Gallone et al. 2016). This further highlights that
beverage yeasts in general have been molded to improve and adapt under their respective
conditions. Methods such as induced mutation and selection, hybridisation, and gene cloning and
transformation were among the most widely used techniques for this improvement (Rainieri and
Pretorius 2000). Hybridisation was the most common method and occurred because most S.
cerevisiae strains in nature are homothallic, whereas commercial yeast strains are generally
heterothallic, which can provide more direct spore-cell mating to construct better fermentative
efficiency when cultivating wine strains (Raineri and Pretorius 2000). Homothallism is defined
as self breeding as it produces both types of mating nuclei to form a zygote while heterothallism
is defined as outbreeding as it only produces one type of mating nuclei and requires two
organisms to form a zygote (Ni et al. 2011). These techniques ultimately are directed towards the
construction of strains that will contain useful and innovative properties for winemaking, while
guaranteeing the preservation of the genetic biodiversity of wine production.
The exploitation of the existing natural diversity of wine yeasts through the use of
techniques to generate artificial diversity or genetic modification could potentially be labeled as
the domestication of natural yeast flora. Granted, there still exists wild S. cerevisiae
independently of domesticated strains, but in general the development of commercial yeast
strains originated from the domestication and alteration of wild yeast strains (Steensels 2014).
The genetic and phenotypic separation between wild and industrial Saccharomyces cerevisiae is
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likely due to human selection and manipulation (Gallone et al. 2106). The result of this
domestication has led to the selection of superior industrial yeast strains that are able to survive
under harsh conditions that natural yeast flora may not.
Figure 1: Observes different tolerances of Saccharomyces cerevisiae strains which are
represented by each point on each plot by measuring the growth of all strains from different
subpopulations. Looking specifically at D and E, domestication is evident.
YEAST IN WINEMAKING
The development of more robust strains while still capable of producing a high-quality end
product has been the main focus of the exploitation of wild yeast strains. Fermentation
conditions are often harsh for yeast cells as they are faced with, and must respond swiftly to,
fluctuations in oxygen, pH, osmolarity, ethanol concentration, nutrient supply, and temperature
(Steensels et al. 2014). Evolved strains showed higher fermentation rates and increased aroma
production than those of wild yeast (Steensels et al. 2014). This directed evolution was used to
fine-tune a specific genetic or phenotypic trait that was already present but not fully optimal. For
example, although the majority of the Lallemand brand sold through Scott Labs is isolated from
natural environments, many strains are developed through selective breeding. This process is
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done to breed out selective qualitative traits like H2S and SO2 production to avoid undesirable
flavors and aromas in wine. They also sell one strain that was developed through directed
evolution that is the first Saccharomyces cerevisiae yeast strain to significantly acidify must
during fermentation. Innovation like this is what is pursued while modifying yeast strains to
perform certain traits. However, the primary goal is for yeast to survive under the harsh
conditions of fermentation to be able to apply these characteristics. To further investigate the
parameters of fermentation, a study was done where ethanol, osmotic, oxidative, and thermal
tolerance were the main stress factors discussed in determining the variation amongst 14
different commercial Saccharomyces cerevisiae strains (Carrasco et al. 2001). The outcomes of
these tolerances are helpful when determining potential strains that could be most optimal for a
desired final product. Below I will discuss the results from the paper that conducted this study
and highlight some of the differences in wine strains (Carrasco et al. 2001).
Ethanol tolerance
Ethanol is the principal stress factor for yeast during alcoholic fermentation. This alcohol
primarily targets the cell membrane and is toxic to yeast metabolism and growth. Ethanol stress
affects various transport systems, including the general amino acid permease and glucose uptake
processes (Aguilera et al. 2005). In addition, it inhibits the activity of crucial glycolytic enzymes
and has been reported to damage mitochondrial DNA in yeast cells (Aguilera et al. 2005). To
properly observe the effect of ethanol stress, alcohol was exogenously added to the cells in a
single pulse. All 14 strains were significantly tolerant to 10% ethanol. However, solutions
containing 12% ethanol significantly affected most of the strains (Figure 2B), the most sensitive
being Fermiblanc arom SM102. The authors then used a 13.5% ethanol solution to test 9 of the
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14 strains that were least affected by the 12% ethanol solution; only two were unable to grow.
Finally, at 15% ethanol, only UCLM S377 was able to grow, indicating that this is the most
ethanol-resistant strain analyzed (Carrasco et al. 2000) (Table 1).
Osmotic tolerance
Osmotic stress is an adverse condition for yeast cells that occurs at the beginning of
vinification. Analyzing the way yeast strains react to this stressor can indicate their ability to start
growth and carry out this process. When testing for osmotic tolerance, the adverse situation was
introduced before plating and the results showed that the yeast strains were not very tolerant to
osmotic stress (Carrasco et al. 2000). The least affected were Lalvin T73 and Lalvin M69 (Figure
2A). With the exception of those two strains, the rest showed very similar results towards their
osmotic tolerance (Figure 2A).
Oxidative tolerance
Oxidative stress is another adverse effect that primarily affects yeast during biomass
production and drying. It was analyzed by a plate assay after the application of H2O2. The results
indicated that the wine yeasts most resistant to oxidative stress displayed a diameter of growth
inhibition of around 3.0 cm, with the exception of three strains that exceeded the others with a
value of around 3.5 cm. A diploid laboratory strain, W303, had a significant growth defect; the
diameter of the growth inhibition zone was 4.5 cm, meaning that it was most resistant to
oxidative stress.
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Thermal tolerance
Heat shock affects the yeast at a molecular level and can impact the viability of a yeast cell.
The analysis compared both control and heat-shocked cells after an incubation period of two
hours at 45°C. Based on the figure, it is difficult to examine the growth, however the author is
able to explain rather than show the final results. Two strains, UCLM S235 and Fermiblanc arom
SM 102, displayed no growth and were the most affected by this stress condition. Of the strains
that did exhibit growth, Fermichamp 67J, Fermivin crio 7303, and UCLM S377 showed the most
sensitivity. Vitilevure Pris Mouse, Lalvin T73, Lalvin M69 and CECT 12512 were the most
tolerant (Carrasco et al. 2000) (Figure 2A). Further experimentation was done when incubating
the plates at 50°C, in which there was a significant decrease in survival even for the heat tolerant
strains.
Discussion of results
This report presented an analysis of stress responses of 14 commercial wine Saccharomyces
cerevisiae yeast strains. The results indicated that most strains were tolerant to stress conditions.
UCLM S377 was most resistant to ethanol stress surviving at 15% ethanol, however that same
yeast strain was also the most sensitive to osmotic stress (Table 2). This suggests the variation
amongst yeast strains and their ability to respond to stress at a molecular level. The quality of
evidence within the study displayed in-depth analysis of each respective stressor. The evidence
dissects individual stress conditions and the ability of Saccharomyces cerevisiae yeast strains to
display mechanisms for sensing and responding to stress at the molecular level. From this
information, a winemaker would be able to select the yeast strain that is most compatible with
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the conditions of their wine. This evidence provides insight in the ability of a yeast to survive
through these stressors which presents the potential of a particular strain.
Figure 2: Analysis of tolerance to heat shock and osmotic tolerance (A) and 12% ethanol (B).
Osmotic shock was induced by transferring cells from exponential cultures to YPD liquid
medium containing 3 M KCl and the incubation continued for 3.5 hours. Ethanol stress analysis
was induced as dilutions of exponential cultures were spotted onto the YPD solid medium
containing ethanol concentrations between 10 and 15%. Dilutions indicated on the left of the
figures were prepared and 5 μl of each were spotted on YPD plates, normally incubated at 30°C
until colonies appeared, except for heat shock in which the experimental temperature was used.
Figures and data were adapted from Carrasca et al. 2001
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Table 1: The strains that carried out further testing after displaying growth on 10% and 12% ethanol.
This table indicates the strains that were able to grow on 13.5% and 15% ethanol. Five µl aliquots of
dilutions prepared from cultures of exponentially growing cells were spotted on YPD plates
containing these percentages of ethanol. The + and - indicates the ability to grow or not on the plates.
Figures were adapted from Carrasca et al. 2001.
Table 2: Summary of the results reported on stress resistance of the wine yeast strains under the
study cited. Shows which of the Saccharomyces cerevisiae strains tested had shown the most
resistant and the most sensitive results towards the stressors experimented under. Figures were
adapted from Carrasca et al. 2001.
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YEAST STRAIN CONTRIBUTION TO CHEMICAL COMPOSITION DIVERSITY IN
WINE
The need for Saccharomyces cerevisiae strains to better adapt to the fermentation process
is an ever-growing demand for new and improved wine yeast strains. In addition to survival and
tolerance for yeast to achieve their most primary role of fermentation, the desire to complete this
conversion without the development of off-chemical and sensory properties is also a main
objective within the winemaking process. The ability to obtain satisfactory enological properties
within various strains of Saccharomyces cerevisiae varies. However, with commercial S.
cerevisiae wine strains, their ability to successfully complete fermentation is higher than
indigenous strains due to their competitive nature. As previously stated, S. cerevisiae is one of
the main species that is able to survive under the harsh condition of high ethanol, while the other
microbial yeast species are inhibited severely as they are not ethanol-tolerant, sensitive to SO2,
and have more potential to produce undesirable compounds (Rainieri and Pretorius 2000).
Because of this, Saccharomyces cerevisiae strains are often selected based on the fermentative
conditions of the wine. A certain strain, based on their physicochemical properties, may produce
a more suitable chemical composition depending on what is desired for the final product. This
portion will be evaluating and comparing the fermented chemical composition of four different
strains of S. cerevisiae on Chardonnay grapes. The compounds analyzed were alcohol, reduced
sugar, total acidity, volatile acidity, and pH (Herjavec et al. 2003).
The harvested Chardonnay grapes were divided into five lots, each destemmed and
crushed. Free-run juice of each lot was sulfured with SO2 (100 mg/L) and settled for 24 hours.
The must of each lot was racked and divided into 15 L glass bottles for the following commercial
S. cerevisiae fermentation treatments: Uvaferm-CM, Uvaferm-CS2, Lalvin-71 B and
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Lalvin-2056 (Herjavec et al. 2003). Fermentation with commercial strains began one day after
inoculation. The chemical analysis occurred after the first racking in November. Ethanol, total,
and volatile acidity were determined using methods such as analysis through a gas
chromatograph (Herjavec et al. 2003). Results (Table 3) show no significant differences among
the physicochemical properties found. The samples fermented by Lalvin-71 B had a significantly
lower content of alcohol and total acidity as tartaric acid. According to the study, Lalvin-71 B
can cause a decrease in malic acid of up to 30%, this could potentially also be linked to the malic
acid degradation by malolactic fermentation of the Chardonnay (Herjavec et al. 2013). Although
the chemical composition of these strains displayed similar results, they differ in the
concentration of volatile compounds. In the next section, we will be comparing the volatile
compounds present amongst the Saccharomyces cerevisiae strains and the effect they have on the
flavor and aroma chemistry of the finished wine.
Table 3: Chemical composition of wines based on different commercial Saccharomyces
cerevisiae wine yeast treatments. The chart exhibits that there were no essential differences
among the strains in respect to reducing sugar and volatile acidity. As for alcohol and total
acidity, Lalvin-81 B strain had significantly lower content. Figures were adapted from Herjavec
et al. 2013.
FLAVOR AND AROMA CHEMISTRY : SENSORIAL EFFECTS ON WINE
The flavor and aromas that are experienced in wine stem from a complex system of
interactions among various compounds. The impression of both aroma and taste components
derive from volatile compounds which evolve as a result of fermentation. Saccharomyces
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cerevisiae is responsible for primary alcoholic fermentation of sugar, but producing ethanol can
also lead to a number of by-products. Yeast-derived metabolic products such as higher alcohols,
acetates, fatty acid ethyl esters all contribute to the fermentative bouquet of wines (Lambrechts
and Pretorius, 2000). These aroma compounds are linked to the metabolism of yeast cells. The
formation of metabolites can contribute to specific flavors. Many of these metabolites derive
from nitrogen, sulfur, and sugar metabolism (Bisson and Karpel 2010). They act as precursors
and result in flavor active components such as ethanol, ester, fusel alcohols/acids/aldehydes, and
other volatiles (Bisson and Karpel 2010). The principal flavor characteristics detected are
variable and yeast strain specific. Winemakers are able to intentionally select a yeast strain that is
able to unveil their desired chemical and flavor complexity. Employing a specific wine yeast
strain for certain advantageous and particular characteristics ensures a reproducible product and
allows for a predictable control of fermentation and quality. This portion will discuss the effects
of commercial Saccharomyces cerevisiae yeast strains on volatile composition and the sensory
characteristics that are identified from the produced compounds.
Higher alcohols
The most important flavor and aroma compounds formed from amino acids are higher
alcohols and their produced esters and volatile acids. The process by which these amino acids are
catabolized into higher alcohols derived via the Ehrlich pathway which impacts the synthesis of
other aroma compounds both directly and indirectly (Styger et al. 2011). The important factor of
higher alcohols is that they are substrates for acetate ester production. Small amounts of higher
alcohols have a positive effect on wine quality, however excessive amounts, typically above 300
mg/L, can cause an undesirable aroma in wine (Herjavec et al. 2003). Below 300 mg/L, higher
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alcohols contribute to the desired complexity of a wine. As previously mentioned in the last
section, we will further discuss the analysis of the four Saccharomyces cerevisiae yeast strains in
the Chardonnay and their volatile compound production after fermentation.
The volatile compound evaluation of the Chardonnay took place in April after the wines
were bottled and stored under cellar conditions. The concentrations of individual higher alcohols
differed depending on the yeast strain utilized to carry out fermentation. Lalvin-71 B had a
considerably lower higher alcohol concentration than the others (Table 4) with a result of 264
mg/L (Herjavec et al. 2003). It was also the only strain used that was below 300 mg/L. Both the
Uvaferm strains had significantly higher content of isoamyl alcohol, while Lalvin-71 B had
significantly lower of this alcohol as well as isobutanol (Herjavec et al. 2003). 2-Phenyl ethanol
has a solvent characteristic but acts as a substrate that can be esterified into 2-phenylethyl acetate
which has rose, fruity, and honey characteristics (Cordente et al. 2021). Lalvin-2056 had a
significantly lower concentration of 2-PE in comparison to its counterparts. Based on the overall
higher alcohol concentrations of these yeast strains, it is likely that Lalvin-71 B would have the
best results as it is within a desirable range to avoid any off aromas (Table 4).
Acetates
As previously stated, higher alcohols are substrates for acetate ester production. This is a
reaction catalysed by yeast alcohol acetyltransferases (Cordente et al. 2021). Acetate esters are
typically associated with the desirable “fruity” and “floral” aromas in wine. An example of this
conversion is 2-Phenyl ethanol to 2-phenylethyl acetate which leads to rose, fruity, and honey
characteristics. Another popular conversion is isoamyl alcohol to isoamyl acetate which
influences the wine to have banana and general fruity aromas and flavors. Acetate esters are
29
important contributors to the aroma of young wines and decrease throughout the length of
storage time.
Looking at the results (Table 4), the most influential acetate in terms of concentration is
ethyl acetate. It is the main ester occurring in wine and has a significant influence on quality at
very low concentrations of 50 mg/L (Herjavec et al. 2003). Each yeast strain fell below 50 mg/L
for ethyl acetate. This is likely to have a positive effect on the wine and will provide those fruity
and floral notes. In terms of 2-phenyl ethyl acetate and 2-phenyl ethanol, there seems to be an
established positive correlation between their resulting concentrations (Herjavec et al. 2003). As
for isoamyl acetate, the results showed some varying correlations in accordance with isoamyl
alcohol concentrations. One would assume that higher concentrations of the alcohol would lead
to higher concentrations of the acetate, similar to what is displayed for phenyl ethyl acetate.
However, this does not seem to be the case.
Ethyl esters of fatty acids and fatty acids
Fatty acids produced by yeasts are intermediates in the biosynthesis of long-chain fatty
acid metabolism. These, along with their ethyl esters are natural components of wine and can act
as potential inhibitors of alcoholic fermentation. This inhibition of yeast growth is directly
proportional to their solubility which is dependent on ethanol concentrations during fermentation
(Lambrechts and Pretorius 2000). Thus the content of fatty acids and their ethyl esters depend on
the wine strain used, highlighting the importance of ethanol tolerance within a strain as
previously mentioned.
Based on the concentrations (Table 4), there were no significant differences amongst the
fatty acids ethyl esters in the different fermentation treatments. However, Lalvin-71 B did have a
30
slightly higher concentration of butyric and caproic acids, ethyl butyrate, ethyl caproate, and
ethyl caprate (Herjavec et al. 2003). This is likely due to growing conditions such as pH,
temperature, and nutrients.
Sensory characteristics of the wines
The study had a panel of nine judges that performed a sensory evaluation of the wines by
ranking them as well as applying a statistical method for a second ranking. Panelists did not
decipher any significant differences in terms of quality but were able to detect differences of
aroma and flavor. The results of both rankings indicated that the fermentation performed by the
Lalvin-71 B strain resulted in wines of better aroma and flavor (Herjavec et al. 2003). These
wines were described as having a clear and stronger bouquet, likely due to the considerably
lower total concentrations of higher alcohols and the slightly higher concentrations of fatty acids
ethyl esters (Herjavec et al. 2003). This balance between alcohol and acidity led to a more
balanced product that was received more positively. The yeast strain that was ranked the lowest
was Lalvin-A-2506. Panelists described it as having no prominent qualities or characteristics.
Through this experiment we are able to highlight the varying influence that different
Saccharomyces cerevisiae strains have on the composition and sensory properties of a wine.
31
Table 4: Concentration of volatile compounds in Chardonnay wines (mg/L) and the differing
compositions depending on the Saccharomyces cerevisiae yeast strain used. Analyses of many of
the compounds were made on gas chromatography. Figures were adapted from Herjavec et al.
2013.
CONCLUSION
Proper selection of Saccharomyces cerevisiae yeast strains remains a crucial component
of both the technical and sensorial elements of the winemaking process. The natural availability
of yeast strains possessing an ideal combination of enological characteristics was highly
improbable. These indigenous strains lacked competitiveness and tolerance to fermentative
conditions. The domestication of this wine yeast allowed for the refinement of strains that were
utilized to properly and efficiently execute fermentation while also limiting the potential for
off-aroma. The recent developments of expanding traditional genetic techniques have
empowered S. cerevisiae strains to be more stable, vigorous, and efficient in their chemical and
32
sensorial compositions. Further research into yeast strains will only contribute to the
diversification of yeast strains that can contribute to the wine industry. Ultimately, the genetic
analysis is crucial to better understand and improve the genetic and phenotypic basis of yeast
evolution towards winemaking that will be useful to tailor strains to meet enological and
consumer demand.
33
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