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SO PRODUCTION BY WINE YEAST
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DURING ALCOHOLIC FERMENTATION
What is sulphur Sulphur dioxide is a molecule commonly known as SO2. It is used in many dried fruits (figs,
dioxide? raisins apricots, etc) as an antimicrobial agent and has been used in winemaking by the Ro-
mans, when they discovered that burning sulphur candles inside empty wine vessels keeps
them fresh and free from vinegar smell.
Figure 1: Molecular structure of sulphur dioxide.(SO )
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Why it is important in Sulphur dioxide is used during several steps of the winemaking process. It is added to prevent
wine? the unwanted developments of microorganisms, as an anti-oxidant, as an antioxidasic to inhibit
polyphenol oxidases (laccase and tyrosinase) and as a dissolvent.
However, sulphites can have a negative impact on wine sensory properties, can delay the onset
of malolactic fermentation, and can cause some health concerns in case of high concentrations
in the final wine. That’s why SO levels in wine are regulated. On wine bottles, "contains sul-
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phites" must be displayed on the label when found above 10 mg/L. Consequently, it is important
in the winemaking process to control and manage the SO content of wine in order to maintain
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the lowest possible concentration while preserving its interesting properties.
SO can be added in wines in several forms such as liquid gas, SO solution, potassium metabi-
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sulphite powder or effervescent tablets .
SO is not only and exogenous compounds, as it can also be produced by yeast as it will be
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discussed in this document.
The many forms Sulphur dioxide can be found in many forms in wines, and it will have an impact on the final
of SO concentration found in the product. It is important to understand the nature of the form that it
2 takes in the wine and the impact it has.
• Free SO : the active and most efficient form of the sulphites found in wines. This is the
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form that will be active as an antimicrobial agent, as well as an antioxidant. It is called free
because it is not bound or attached to any other compounds.
• Bound SO : When the SO is added to wine or must, a portion will be bound by
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sugars and by aldehydes (such as acetaldehyde) and by ketones. This form of SO2
is not active.
• Total SO : Free + Bound SO .
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• Molecular or Active SO : the molecular SO is the most active and efficient form of the free
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SO. This form of SO is more precise than the free SO in the degree of protection that it
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offers to the wine. It’s calculated with a formula taking in account pH, temperature, the %
of alcohol and the free SO . The pH of the wine is one of the main factor intervening in the
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balance molecular, free and total SO . Generally, a concentration between 0,35 mg/L and
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0,60 mg/L of molecular SO will allow for a proper protection of the wine.
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SO formation during
2 Saccharomyces cerevisiae wine yeast, whether selected or spontaneous, will produce SO .
fermentation 2
Wine yeasts are able to produce from a few mg/L of sulphites to more than 90 mg/L, depending
on the fermentation conditions and the yeast strain. It was reported by Delteil (1992) that 30%
of indigenous wine yeast from Côte Rôtie (France) were strong SO producers. Sulphur dioxide
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is an intermediate metabolite in the sulfate assimilation pathway (figure 2) leading to sulphur
amino acid synthesis. Under certain conditions, it may be synthesized in excess then excreted
into the medium. Furthermore, sulphites are precursors for the synthesis of sulphide, a highly
undesirable by-product. Although the sulfate assimilation pathway has been widely studied,
little is known about the parameters that influence sulphite production, and the molecular basis
responsible for the differences between yeast strains has not yet been completely identified.
The best strategies to avoid such situation is 1) to select a wine yeast that will produce very little
SO, 2) to know if your selected yeast has a high demand for nitrogen during fermentation and
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3) to properly manage alcoholic fermentation.
SO 2
4
SO 2
4
Amino acids
Homoserine SO SO
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HS HS
OAH 2 2
Homocysteine
Methionine
Cysteine
Figure 2 : Wine yeast sulfate assimilation pathway
THE RESULTS
The parameters influencing the production of sulphur
compounds by yeast are:
1. Temperature: it has been shown that at low temperature 40 16˚ C
(16 ° C), sulphur production is greater than at 28 ° C (Figure 3). 28˚ C
2. The wine yeast used: we know that the production of SO
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by wine yeast is genetically and environmentaly determined.
All wine yeast, selected or spontaneous, will produce various
concentration of SO . (Figure 4). (mg/L)
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Many wine yeast were characterized based on their SO SO
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production in a synthetic media. Figure 4 (on next page)
illustrate the range of concentration produced by the dif- 10
ferent wine yeast from the lowest at 5 mg/L to the highest
at 90 mg/L. The concentration of SO produced are those
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that are intrinsically produced by the wine yeast since 0
there were no sulphur addition to the synthetic must. JN11 JN17 JN60 JN10
When selecting a yeast for winema-king, based on the
condition of the must, the level of SO2 added, then this
factor can be taken into consideration based on the wine- Figure 3: SO production by different wine
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making itine-rary chosen and the wine style desired. yeast strain at different temperature
THE RESULTS
SO2 produced (mg/L) this specific yeast. Oenological and sensory properties of the
®
0 10 20 30 40 50 60 70 80 90 100 Lalvin ICV oKay have been shown to be very positive to pro-
duce quality wines. Moreover, since this wine yeast produces
Y1 little or no SO during alcoholic fermentation, malolactic fermen-
Y2 2
Y3 tation is compatible when needed.
Y4
Y5 2012 Maccabeu – Initial SO – 55 mg/L 2012 Merlot – Initial SO – 45 mg/L
Y6 2 2
Y7 (INRA Pech-Rouge) (INRA Pech-Rouge)
Y8 70 40
Y9 60 35
Y10 50 30
Y11 ed (mg/L) ed (mg/L)25
Y12 40
Y13 duc duc 20
Y14 o 30 o
Y15 pr 2 pr2 15
Y16 20 10
Y17 10 5
Y18 otal SO otal SO
Y19 T 0 T 0
Y20 3 4 5 End of AF 3 4 5 End of AF
Y21 Days Days
Y22
Y23
Y24 2012 Syrah Rosé – Initial SO – 15 mg/L
2012 Maccabeu – Initial SO – 55 mg/L 2012 Merlot – Initial SO – 45 mg/L 2
Y25 2 2012 Maccabeu – Initial SO – 55 mg/L 2 2012 Merlot – Initial SO – 45 mg/L
Y26 (INRA Pech-Rouge) 2 (INRA Pech-Rouge) (INRA Pech-Rouge) 2
(INRA Pech-Rouge) 50 (INRA Pech-Rouge)
Y27 70 70 40 40
Y28 60 35 40 35
Y29 60 30
Y30 50 50 ed (mg/L) 30
Y31 ed (mg/L) ed (mg/L) ed (mg/L)25 30 ed (mg/L) 25
Y32 duc 40 40 duc 20 duc
Y33 o 30 duc o o duc 20
Y34 pr o 30 pr 15 pr2 20 o
Y35 2 20 pr2 2 pr 2 15
Y36 20 10 10 10
Y37 otal SO10 10 otal SO 5 otal SO 5
Y38 T 0 otal SO T 0 T 0 otal SO
Y39 T 0 T 0
3 4 5 End of AF
Y40 3 4 5 End of AF 3 4 5 End of AF
3 4 5 End of AF 3 4 5 End of AF
Y41 Days Days Days Days Days
Y42
Y43 2012 Syrah Rosé – Initial SO – 15 mg/L
Y44 2 2012 Syrah Rosé – Initial SO – 15 mg/L Lalvin ICV oKay® yeast
Y45 (INRA Pech-Rouge) 2
Y46 50 (INRA Pech-Rouge) Reference yeast
Y47 50
Y48 40
40
ed (mg/L)30 ed (mg/L)
Figure 4: Total SO production by wine yeast
2 duc 30
in synthetic media MS 300 o duc
pr 20 o
2 pr2 20
For example, if malolactic fermentation is desired, and knowing
10
the sensitivity of wine bacteria to SO , then a wine yeast produ-
2 otal SO 10
cing lower concentration of sulphite can be selected. Recent
T 0 otal SO
wine yeast selection has also been focused on finding a micro- T 0
3 4 5 End of AF
organism able to produce less or no SO . During a collaborative 3 4 5 End of AF
2 Days Days
work between Lallemand, Institut Coopératif du Vin (France)
and the SupAgro INRA (France), a natural wine yeast, Lalvin
® Lalvin ICV oKay® yeast
ICV oKay was obtained with a directed breeding strategy ap- Lalvin ICV oKay® yeast
proach that produces very low levels of SO , H S and acetalde-
2 2 Reference yeast
hyde (SO binding compound). This wine yeast has shown in all Reference yeast
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situations, a lower production of SO , as seen in Figure 5. In the
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different wines, Lalvin ICV oKay was in some instances, not Figure 5: Total SO production during alcoholic fermentation
producing any SO , as shown when there is no red column for 2
2 in three wines comparing Lalvin ICV oKay® with
the reference yeast
Bruno Blondin A WORD FROM OUR EXPERT
The exogenous supply of SO is not alone in determining a wine’s
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final SO content; in fact, wine yeasts also produce a significant
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amount of SO . In the yeast’s metabolism, SO is an intermedi-
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ate component in the synthesis of sulfur-containing amino acids
(methionine and cysteine). This pathway is specifically active
in the growth phase—once the very small quantities of sulfur
containing amino acids present in the grape must have been
exhausted—to meet the anabolic demand for protein synthesis.
Nonetheless, certain yeasts can produce amounts of SO that
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exceed their needs or absorption capacities and are therefore
excreted into the medium. Depending on the strain, the produc-
tion varies between a few milligrams and more than 100 mg/L.
Bruno Blondin has a pH.D. in Food Science from Université Until now, little was known about the molecular bases responsible
de Montpellier (France) and is a professor and Scientific for the differences in production among yeast strains. However, a
Director at the Supagro (INRA Montpellier). Burno is the new study reBillycently identified the MET2 and SKP2 genes as
expert on yeast metabolism. He is the author of 42 pub- responsible for controlling the production of SO , and also of H S
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lications, and 2 patents. He is also an expert at the OIV and acetaldehyde (SO binding compounds). QTL mapping—a
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(Organisation Internationale de la Vigne et du Vin) and genetic study that identifies regions of the genome involved in this
he is a member of the scientific council of the IFV (Institut phenotype—has identified new alleles of these genes in a very
Français de la Vigne et du Vin). His research activities with low SO -producing strain.
the microbiology groupe of the UMR Oenological Science 2
INRA-Montpellier Supagro-UM1 are focused on functional These two alleles have especially powerful control over SO2
genetics of wine yeast, namely the genetics of sulphur production as they intervene at two key steps of sulphur
compounds produced during fermentation. His work has metabolism. First, by limiting its synthesis, the SKP2 gene allele
resulted in understanding the origin and production of controls, in post translation, the efficacy of an enzyme involved
sulphur compounds, and help better control their forma- in the synthesis of SO . Second, the MET2 gene allele increases
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tion. He is responsible for microbiology and biotechnology its incorporation by enhancing the synthesis of carbon precursors.
courses for the Viticulture/Enology diploma at Montpellier
Supagro.
A QUICK SUMMARY
The best strategy for SO management is the keep the lowest efficient level of SO while respecting legal, health and chimerical
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requirements. Knowing the production of SO by wine yeast is part of the strategy of proper management of SO in wine.
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The production of SO by wine yeast is not only regulated by must or fermentation conditions, or by stress factors, but is rather
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mainly an intrinsic characteristics, genetically determined, that varies from one wine yeast to another. With extensive research to
understand and characterize selected wine yeast, we can show the different levels of SO that a wine yeast can produced. When
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this factor is important in the wine to be fermented, whether for malolactic compatibility, wine style or market need, it can become a
®
criteria for the wine yeast to use in a particular vinification. The new wine Lalvin ICV oKay is a good choice for alcoholic fermenta-
®
tion when SO production is a concern as it produces little or no SO , H S or acetaldehyde. Lalvin ICV oKay is an innovative yeast
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selection (Patent pending PTC/IB220131050623) particularly interesting on white and rosé wines, ensuring low levels of volatile
acidity and promoting aromatic esters. It brings freshness and balance in the mouth.
Lallemand Australia Pty Ltd | Jason Amos | 23-25 Erudina Ave | Edwardstown, 5039, SA |
Ph: +61 8 8276 1200 | jamos@lallemand.com | www.lallemandwine.com
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