Stuck fermentation
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8.5.1 Sluggish/Stuck Fermentations
A vinifi cation problem of tremendous economic importance encountered
by winemakers is slow alcoholic fermentation rates, especially in the case
of fermentation of high-sugar musts (Alexandre and Charpentier, 1998;
Bisson, 1999; Bisson and Butzke, 2000). Premature cessation of yeast
growth and fermentation results in a wine with unfermented sugars and
an ethanol concentration lower than expected (Fleet and Heard, 1993).
The problem may manifest itself as sluggish activity during middle and
later phases of alcoholic fermentation, whereas in other cases, cessation
of fermentative activity may be abrupt. From a commercial standpoint,
sluggish or stuck wines are a problem due to their sweet taste, inferior
sensory quality, and the potential for microbial spoilage.
Causes of sluggish (slow) and stuck (stopped) fermentations include
nutritional defi ciency, inhibitory substances, processing diffi culties, as well
as bacterial antagonism (Section 6.6.2). Although various factors can
contribute to a sluggish or stuck fermentation, the exact cause(s) of a
particular occurrence cannot always be identifi ed.
8.5.1.1 Nutritional Defi ciency
Nitrogen defi ciency is well-known to limit yeast growth and slows fermentation
rates (Agenbach, 1977; Ingledew and Kunkee, 1985; Monteiro and
Bisson, 1992; Henschke and Jiranek, 1993; Jiranek et al., 1995b; Spayd
et al., 1995). Henick-Kling (1994) reported yeast may utilize 1,000 mg/L
of amino acids in agreement with Dittrich (1987). It has been estimated
that a minimal assimilable nitrogen concentration of 140 to 150 mg N/L
is necessary to complete fermentation (Agenbach 1977; Spayd et al., 1995).
However, it is clear that these values do not represent absolute minimal
required concentrations. For instance, Bisson and Butzke (2000) suggested
that the 140 mg N/L value does not take into account the high
sugar concentrations in many musts and made recommendations based
on must composition (Table 8.3). Furthermore, some winemakers have
not observed a relationship between nitrogen requirements and the sugar
concentrations in musts. For example, Wang et al. (2003) reported that
fermentations of a synthetic grape juice medium containing only 60 mg N/
L yeast assimilable nitrogen and 24% sugar achieved dryness. In addition,
it is known that nitrogen requirements vary with yeast strain (Ough et al.,
1991; Manginot et al., 1998; Julien et al., 2000).
Unlike bacteria, Saccharomyces can accumulate large intracellular concentrations
of amino acids. Depending upon the particular amino acid,
stage of growth, and activity of necessary transport enzymes, these amino
acids may be (a) directly incorporated into proteins, (b) degraded for
either their nitrogen or carbon components, or (c) stored in vacuoles or
cytoplasm for later use (Bisson, 1991).
Another “nutrient” important for yeast is the presence of oxygen.
Without some initial oxygen, fermentation can slow because O2 is needed
for sterol synthesis (Section 1.4.2). Owing to the activity of grape oxidases
(Section 7.5.1), the oxygen content of unsulfi ted musts can rapidly
decrease. Aeration or pumping-over musts are methods employed by winemakers
to supply additional oxygen to a defi cient must, and addition of
SO2 at crush can help limit the activity of oxidases.
8.5.1.2 Inhibitory Substances
It is well established that high sugar concentrations in grape musts are
inhibitory to yeast growth and therefore slow fermentation (Ough, 1966;
Lafon-Lafourcade, 1983; Casey et al., 1984). More recently, Erasmus et al.
(2004) noted lower maximum cell densities of Saccharomyces grown in
40% w/v sugar musts compared with populations grown in musts with only
Table 8.3. Estimated amounts of nitrogen required
by Saccharomyces in musts with different sugar
concentrations as recommended by Bisson and
Butzke (2000).
Yeast-assimilable nitrogen
Must ripeness (єBrix) (mg N/L)
21 200
23 250
25 300
27 350
Fermentation Problems 125
126 8. Fermentation and Post-fermentation Processing
20% sugar. Nishino et al. (1985) attributed the slower fermentation rates
of high-sugar musts to increases in osmotic pressure on yeasts.
Besides ethanol, the presence of fungicides on grapes may also slow
fermentations. In support, Conner (1983) noted that of the 25 vineyard
agrochemicals examined, fi ve inhibited yeast. However, Pilone (1986)
observed that triadimenol (SummitTM), a fungicide with activity against
powdery mildew and black rot, did not affect alcoholic fermentation by
Montrachet UCD 522 or Pasteur Champagne UCD 595. Fungicides affect
Saccharomyces in various ways including altered sterol content (Doignon
and Rozиs, 1992).
8.5.1.3 Other Factors
At temperatures of approximately 29єC/85єF, fermentation becomes
sluggish, and complete cessation is possible at higher temperatures
(37єC/100єF) (Ough and Amerine, 1966). The lethal effects of high
temperature transients do not result from temperature alone; rather,
the inhibition is coupled to lower ethanol tolerance. Temperature and
ethanol tolerance of yeast varies with species/strain but also refl ects
intrinsic and extrinsic properties of the growth medium. Ough and
Amerine (1966) also warn that infections by Lactobacillus can also be of
concern at high temperatures. Finally, overclarifi cation of musts prior to
fermentation can cause problems, potentially due to decrease of nutrients
and removal of oxygen physically trapped by the insoluble solids (Section
7.4.2).
8.5.1.4 Restarting Fermentations
If faced with a sluggish or stuck fermentation, winemakers have several
potential solutions. Because populations of spoilage yeasts and bacteria
may fl ourish in pomace and lees, wines should be racked-off prior to
attempting the restart. For the same reason, free-run wine is favored over
press wine to prepare the restart medium. Although it is tempting to use
already fermenting juice/must for re-inoculation, Peynaud (1984)
cautioned against this procedure, pointing out that the potential alcohol
differences between fermenting wine and a fermentation that is stuck late
in the cycle may be suffi cient to “shock” the fermenting yeasts, thereby
creating an even sweeter stuck fermentation. Cavazza et al. (2004) concluded
that the major obstacles to regaining yeast activity were the concentrations
of SO2 and ethanol in the stuck wine. However, these authors
reported successful restart of fermentations by direct inoculation of WADY
at a concentration of 1 g/L.
Although other techniques exist, the following recommendation is
generally useful in restarting stuck fermentations (M. Bannister, personal
communication, 1995). The restart medium is prepared by removing 2.5%
of the total volume of stuck wine and mixing this volume with an equal
volume of water. Because the stuck wine may be microbiologically contaminated
(Section 6.6.2), it is recommended that the wine be sterile-fi ltered
(0.45 m). Yeast nutritional supplements containing diammonium phosphate
are then added at recommended levels. The winemaker may elect
to use one of several commercial yeast formulations that contain vitamins
and yeast hulls (ghosts), the latter known to be stimulatory to fermentation
(Section 8.2). Sugar levels are then adjusted to approximately 5% w/v and
the medium is warmed to 30єC/86єF prior to re-inoculation of yeast at a
rate of 2 to 4 lb/1000 gal. Yeast must be rehydrated according to the manufacturer’s
instructions (Section 8.3.2). Once inoculated, the medium is
slowly cooled to 20єC/68єF to 22єC/72єF over several hours. When the
sugar (єBalling) of the restart medium decreases by approximately half,
additional sterile-fi ltered stuck wine is added in increments of 20% v/v.
Subsequent incremental additions of 20% are made each time the sugar
concentration decreases by half or until all the stuck wine has been added.
This process may take several weeks during which it is important to prevent
MLF.
A similar technique to restart fermentations was outlined by Bisson and
Butzke (2000). Here, the authors recommended racking and/or rough
fi ltering the stuck wine, adding 30 mg/L total SO2, and adjusting the
temperature to 20єC/68єF to 22єC/72єF. The re-initiation medium for
1000 L of stuck wine is prepared by mixing 15 L of water at 20єC/68єF to
22єC/72єF with about 3000 g grape juice concentrate (65єBrix) and 30 g
diammonium phosphate. Yeast (1 kg of Saccharomyces bayanus) is then rehydrated
in 5 L of water at 38єC/100єF to 41єC/106єF for 15 to 20 min. The
re-initiation medium is then gradually added to the yeast inoculum over
a period of 30 min and allowed to ferment until about half of the sugar
has been metabolized (only a few hours). At this time, the volume of the
re-initiation medium is doubled by adding an equal volume of the stuck
wine. The mixture should be aerated by either pumping-over or sparging
with sterile compressed air at a rate of 10% of the tank volume per minute.
Fermentation should be allowed to continue until one-half the sugar is
metabolized, at which time, the volume is again doubled by addition of
more stuck wine. Repeat the addition of stuck wine one more time to reach
a total volume of about 160 L (this procedure may require between 12 h
and 3 days to reach this stage). Add the fermenting medium (160 L) to
the remaining stuck wine (860 L) along with 20 g diammonium phosphate.
Bisson and Butzke (2000) warn not to allow the fermentation to reach
dryness or a єBalling lower than the stuck wine. For stuck wines greater
than 14.5% v/v ethanol, it may be necessary to reduce the ethanol content
before attempting to restart fermentation.
During harvest and crush, it may not be feasible to deal with stuck fermentations
immediately. In these cases, the wine should be stabilized
against further biological deterioration by racking and sulfi ting (30 to
40 mg/L) the free-run wine followed by storage at a low temperature until
time is available to attempt the restart. Higher inoculum levels of yeast
(8 to 10 lb/1000 gal) may be necessary using strains recommended by a
supplier.
8.5.2 Hydrogen Sulfide
During and toward the end of alcoholic fermentation, H2S may be released
(Eschenbruch et al., 1978; Hallinan et al., 1999; Spiropoulos et al., 2000).
Having an odor reminiscent of “rotten-eggs,” H2S has a very low sensory
threshold of only a few parts per billion (Henschke and Jiranek, 1991).
Hydrogen sulfi de can also act as a precursor for other reduced sulfur
compounds (mercaptans) that impart additional off-odors to wine
(Lambrechts and Pretorius, 2000). Yeast strains differ in their ability to
produce H2S as exemplifi ed by Montrachet, a strain known to produce
higher amounts of H2S (Guidici and Kunkee, 1994; Wang et al., 2003). As
noted by Sea et al. (1998), consistently low-sulfi de-producing strains of
yeast are not commercially available, even with hundreds of strains
examined.
Because nitrogen defi ciency is a well-known contributor to hydrogen
sulfi de production, one strategy that can reduce the risk of formation is
the addition of nutritional supplements to grape musts prior to and/or
during fermentation (Vos and Gray, 1979; Guidici and Kunkee, 1994;
Jiranek et al., 1995a; 1995b; Hallinan et al., 1999; Tamayo et al., 1999; Park
et al., 2000; Spiropoulos et al., 2000). Jiranek et al. (1995a; 1996) concluded
that excessive H2S produced under nitrogen defi ciency was due to
ongoing reduction of sulfi te by sulfi te reductase even though the nitrogencontaining
precursors that react with sulfi de, O-acetylserine (OAS) or Oacetylhomoserine
(OAH), were exhausted (Fig. 1.12).
However, nitrogen is not the only nutritional factor that infl uences H2S
evolution in grape musts as evidenced by Sea et al. (1998) who reported
poor correlations between H2S and must nitrogen concentrations. Metabolic
depletion of OAS and OAH could be the result of a lack of pantothenic
acid, a vitamin required for the synthesis of coenzyme A (CoA),
which is necessary for formation of these precursors (Fig. 1.12).
In agreement, pantothenic acid defi ciency is known to increase H2S pro
duction by Saccharomyces in synthetic media (Eschenbruch et al., 1978;
Slaughter and McKernan, 1988). Wainwright (1970) and others have noted
reduced H2S formation in synthetic media containing amounts less than
150 g/L.
Wang et al. (2003) reported that a complicated relationship exists
between nitrogen and pantothenic acid that affects H2S production (Fig.
8.2). Here, H2S production decreased with an increase of nitrogen but
only in the presence of 250 g/L pantothenic acid. If pantothenic acid was
present at 50 g/L or less, the amount of H2S evolved actually increased
with an increase in available nitrogen. This observation had not been
reported previously and casts doubt on the belief that addition of nitrogen
to grape musts will always reduce H2S problems (Tamayo et al., 1999).
Other factors are also known to impact H2S in wine. For instance,
Karagiannis and Lanaridis (1999) studied addition of sulfi te, must turbidity,
yeast strain, fermentation temperature, and lees contact on H2S formation.
Among other fi ndings, the authors noted that more H2S was present
if the wine was left on lees for 2 months. Although elemental sulfur used
in the vineyard can also be a source of H2S (Acree et al., 1972; Eschenbruch,
1974), very high concentrations on the treated grapes may be
required (Thomas et al., 1993).
10 50 250 10 50 250
60 mg/L YAN 250 mg/L YAN
Figure 8.2. Cumulative evolution of H2S by EC1118 (_) or UCD 522 ( ) during
fermentation of a synthetic grape juice containing variable concentrations of YAN and
pantothenic acid. Within a given yeast, means with different letters are signifi cantly
different at p 0.05 for EC1118 (letters a to e) and UCD 522 (letters u to z). Adapted
8.5.1 Sluggish/Stuck Fermentations
A vinifi cation problem of tremendous economic importance encountered
by winemakers is slow alcoholic fermentation rates, especially in the case
of fermentation of high-sugar musts (Alexandre and Charpentier, 1998;
Bisson, 1999; Bisson and Butzke, 2000). Premature cessation of yeast
growth and fermentation results in a wine with unfermented sugars and
an ethanol concentration lower than expected (Fleet and Heard, 1993).
The problem may manifest itself as sluggish activity during middle and
later phases of alcoholic fermentation, whereas in other cases, cessation
of fermentative activity may be abrupt. From a commercial standpoint,
sluggish or stuck wines are a problem due to their sweet taste, inferior
sensory quality, and the potential for microbial spoilage.
Causes of sluggish (slow) and stuck (stopped) fermentations include
nutritional defi ciency, inhibitory substances, processing diffi culties, as well
as bacterial antagonism (Section 6.6.2). Although various factors can
contribute to a sluggish or stuck fermentation, the exact cause(s) of a
particular occurrence cannot always be identifi ed.
8.5.1.1 Nutritional Defi ciency
Nitrogen defi ciency is well-known to limit yeast growth and slows fermentation
rates (Agenbach, 1977; Ingledew and Kunkee, 1985; Monteiro and
Bisson, 1992; Henschke and Jiranek, 1993; Jiranek et al., 1995b; Spayd
et al., 1995). Henick-Kling (1994) reported yeast may utilize 1,000 mg/L
of amino acids in agreement with Dittrich (1987). It has been estimated
that a minimal assimilable nitrogen concentration of 140 to 150 mg N/L
is necessary to complete fermentation (Agenbach 1977; Spayd et al., 1995).
However, it is clear that these values do not represent absolute minimal
required concentrations. For instance, Bisson and Butzke (2000) suggested
that the 140 mg N/L value does not take into account the high
sugar concentrations in many musts and made recommendations based
on must composition (Table 8.3). Furthermore, some winemakers have
not observed a relationship between nitrogen requirements and the sugar
concentrations in musts. For example, Wang et al. (2003) reported that
fermentations of a synthetic grape juice medium containing only 60 mg N/
L yeast assimilable nitrogen and 24% sugar achieved dryness. In addition,
it is known that nitrogen requirements vary with yeast strain (Ough et al.,
1991; Manginot et al., 1998; Julien et al., 2000).
Unlike bacteria, Saccharomyces can accumulate large intracellular concentrations
of amino acids. Depending upon the particular amino acid,
stage of growth, and activity of necessary transport enzymes, these amino
acids may be (a) directly incorporated into proteins, (b) degraded for
either their nitrogen or carbon components, or (c) stored in vacuoles or
cytoplasm for later use (Bisson, 1991).
Another “nutrient” important for yeast is the presence of oxygen.
Without some initial oxygen, fermentation can slow because O2 is needed
for sterol synthesis (Section 1.4.2). Owing to the activity of grape oxidases
(Section 7.5.1), the oxygen content of unsulfi ted musts can rapidly
decrease. Aeration or pumping-over musts are methods employed by winemakers
to supply additional oxygen to a defi cient must, and addition of
SO2 at crush can help limit the activity of oxidases.
8.5.1.2 Inhibitory Substances
It is well established that high sugar concentrations in grape musts are
inhibitory to yeast growth and therefore slow fermentation (Ough, 1966;
Lafon-Lafourcade, 1983; Casey et al., 1984). More recently, Erasmus et al.
(2004) noted lower maximum cell densities of Saccharomyces grown in
40% w/v sugar musts compared with populations grown in musts with only
Table 8.3. Estimated amounts of nitrogen required
by Saccharomyces in musts with different sugar
concentrations as recommended by Bisson and
Butzke (2000).
Yeast-assimilable nitrogen
Must ripeness (єBrix) (mg N/L)
21 200
23 250
25 300
27 350
Fermentation Problems 125
126 8. Fermentation and Post-fermentation Processing
20% sugar. Nishino et al. (1985) attributed the slower fermentation rates
of high-sugar musts to increases in osmotic pressure on yeasts.
Besides ethanol, the presence of fungicides on grapes may also slow
fermentations. In support, Conner (1983) noted that of the 25 vineyard
agrochemicals examined, fi ve inhibited yeast. However, Pilone (1986)
observed that triadimenol (SummitTM), a fungicide with activity against
powdery mildew and black rot, did not affect alcoholic fermentation by
Montrachet UCD 522 or Pasteur Champagne UCD 595. Fungicides affect
Saccharomyces in various ways including altered sterol content (Doignon
and Rozиs, 1992).
8.5.1.3 Other Factors
At temperatures of approximately 29єC/85єF, fermentation becomes
sluggish, and complete cessation is possible at higher temperatures
(37єC/100єF) (Ough and Amerine, 1966). The lethal effects of high
temperature transients do not result from temperature alone; rather,
the inhibition is coupled to lower ethanol tolerance. Temperature and
ethanol tolerance of yeast varies with species/strain but also refl ects
intrinsic and extrinsic properties of the growth medium. Ough and
Amerine (1966) also warn that infections by Lactobacillus can also be of
concern at high temperatures. Finally, overclarifi cation of musts prior to
fermentation can cause problems, potentially due to decrease of nutrients
and removal of oxygen physically trapped by the insoluble solids (Section
7.4.2).
8.5.1.4 Restarting Fermentations
If faced with a sluggish or stuck fermentation, winemakers have several
potential solutions. Because populations of spoilage yeasts and bacteria
may fl ourish in pomace and lees, wines should be racked-off prior to
attempting the restart. For the same reason, free-run wine is favored over
press wine to prepare the restart medium. Although it is tempting to use
already fermenting juice/must for re-inoculation, Peynaud (1984)
cautioned against this procedure, pointing out that the potential alcohol
differences between fermenting wine and a fermentation that is stuck late
in the cycle may be suffi cient to “shock” the fermenting yeasts, thereby
creating an even sweeter stuck fermentation. Cavazza et al. (2004) concluded
that the major obstacles to regaining yeast activity were the concentrations
of SO2 and ethanol in the stuck wine. However, these authors
reported successful restart of fermentations by direct inoculation of WADY
at a concentration of 1 g/L.
Although other techniques exist, the following recommendation is
generally useful in restarting stuck fermentations (M. Bannister, personal
communication, 1995). The restart medium is prepared by removing 2.5%
of the total volume of stuck wine and mixing this volume with an equal
volume of water. Because the stuck wine may be microbiologically contaminated
(Section 6.6.2), it is recommended that the wine be sterile-fi ltered
(0.45 m). Yeast nutritional supplements containing diammonium phosphate
are then added at recommended levels. The winemaker may elect
to use one of several commercial yeast formulations that contain vitamins
and yeast hulls (ghosts), the latter known to be stimulatory to fermentation
(Section 8.2). Sugar levels are then adjusted to approximately 5% w/v and
the medium is warmed to 30єC/86єF prior to re-inoculation of yeast at a
rate of 2 to 4 lb/1000 gal. Yeast must be rehydrated according to the manufacturer’s
instructions (Section 8.3.2). Once inoculated, the medium is
slowly cooled to 20єC/68єF to 22єC/72єF over several hours. When the
sugar (єBalling) of the restart medium decreases by approximately half,
additional sterile-fi ltered stuck wine is added in increments of 20% v/v.
Subsequent incremental additions of 20% are made each time the sugar
concentration decreases by half or until all the stuck wine has been added.
This process may take several weeks during which it is important to prevent
MLF.
A similar technique to restart fermentations was outlined by Bisson and
Butzke (2000). Here, the authors recommended racking and/or rough
fi ltering the stuck wine, adding 30 mg/L total SO2, and adjusting the
temperature to 20єC/68єF to 22єC/72єF. The re-initiation medium for
1000 L of stuck wine is prepared by mixing 15 L of water at 20єC/68єF to
22єC/72єF with about 3000 g grape juice concentrate (65єBrix) and 30 g
diammonium phosphate. Yeast (1 kg of Saccharomyces bayanus) is then rehydrated
in 5 L of water at 38єC/100єF to 41єC/106єF for 15 to 20 min. The
re-initiation medium is then gradually added to the yeast inoculum over
a period of 30 min and allowed to ferment until about half of the sugar
has been metabolized (only a few hours). At this time, the volume of the
re-initiation medium is doubled by adding an equal volume of the stuck
wine. The mixture should be aerated by either pumping-over or sparging
with sterile compressed air at a rate of 10% of the tank volume per minute.
Fermentation should be allowed to continue until one-half the sugar is
metabolized, at which time, the volume is again doubled by addition of
more stuck wine. Repeat the addition of stuck wine one more time to reach
a total volume of about 160 L (this procedure may require between 12 h
and 3 days to reach this stage). Add the fermenting medium (160 L) to
the remaining stuck wine (860 L) along with 20 g diammonium phosphate.
Bisson and Butzke (2000) warn not to allow the fermentation to reach
dryness or a єBalling lower than the stuck wine. For stuck wines greater
than 14.5% v/v ethanol, it may be necessary to reduce the ethanol content
before attempting to restart fermentation.
During harvest and crush, it may not be feasible to deal with stuck fermentations
immediately. In these cases, the wine should be stabilized
against further biological deterioration by racking and sulfi ting (30 to
40 mg/L) the free-run wine followed by storage at a low temperature until
time is available to attempt the restart. Higher inoculum levels of yeast
(8 to 10 lb/1000 gal) may be necessary using strains recommended by a
supplier.
8.5.2 Hydrogen Sulfide
During and toward the end of alcoholic fermentation, H2S may be released
(Eschenbruch et al., 1978; Hallinan et al., 1999; Spiropoulos et al., 2000).
Having an odor reminiscent of “rotten-eggs,” H2S has a very low sensory
threshold of only a few parts per billion (Henschke and Jiranek, 1991).
Hydrogen sulfi de can also act as a precursor for other reduced sulfur
compounds (mercaptans) that impart additional off-odors to wine
(Lambrechts and Pretorius, 2000). Yeast strains differ in their ability to
produce H2S as exemplifi ed by Montrachet, a strain known to produce
higher amounts of H2S (Guidici and Kunkee, 1994; Wang et al., 2003). As
noted by Sea et al. (1998), consistently low-sulfi de-producing strains of
yeast are not commercially available, even with hundreds of strains
examined.
Because nitrogen defi ciency is a well-known contributor to hydrogen
sulfi de production, one strategy that can reduce the risk of formation is
the addition of nutritional supplements to grape musts prior to and/or
during fermentation (Vos and Gray, 1979; Guidici and Kunkee, 1994;
Jiranek et al., 1995a; 1995b; Hallinan et al., 1999; Tamayo et al., 1999; Park
et al., 2000; Spiropoulos et al., 2000). Jiranek et al. (1995a; 1996) concluded
that excessive H2S produced under nitrogen defi ciency was due to
ongoing reduction of sulfi te by sulfi te reductase even though the nitrogencontaining
precursors that react with sulfi de, O-acetylserine (OAS) or Oacetylhomoserine
(OAH), were exhausted (Fig. 1.12).
However, nitrogen is not the only nutritional factor that infl uences H2S
evolution in grape musts as evidenced by Sea et al. (1998) who reported
poor correlations between H2S and must nitrogen concentrations. Metabolic
depletion of OAS and OAH could be the result of a lack of pantothenic
acid, a vitamin required for the synthesis of coenzyme A (CoA),
which is necessary for formation of these precursors (Fig. 1.12).
In agreement, pantothenic acid defi ciency is known to increase H2S pro
duction by Saccharomyces in synthetic media (Eschenbruch et al., 1978;
Slaughter and McKernan, 1988). Wainwright (1970) and others have noted
reduced H2S formation in synthetic media containing amounts less than
150 g/L.
Wang et al. (2003) reported that a complicated relationship exists
between nitrogen and pantothenic acid that affects H2S production (Fig.
8.2). Here, H2S production decreased with an increase of nitrogen but
only in the presence of 250 g/L pantothenic acid. If pantothenic acid was
present at 50 g/L or less, the amount of H2S evolved actually increased
with an increase in available nitrogen. This observation had not been
reported previously and casts doubt on the belief that addition of nitrogen
to grape musts will always reduce H2S problems (Tamayo et al., 1999).
Other factors are also known to impact H2S in wine. For instance,
Karagiannis and Lanaridis (1999) studied addition of sulfi te, must turbidity,
yeast strain, fermentation temperature, and lees contact on H2S formation.
Among other fi ndings, the authors noted that more H2S was present
if the wine was left on lees for 2 months. Although elemental sulfur used
in the vineyard can also be a source of H2S (Acree et al., 1972; Eschenbruch,
1974), very high concentrations on the treated grapes may be
required (Thomas et al., 1993).
10 50 250 10 50 250
60 mg/L YAN 250 mg/L YAN
Figure 8.2. Cumulative evolution of H2S by EC1118 (_) or UCD 522 ( ) during
fermentation of a synthetic grape juice containing variable concentrations of YAN and
pantothenic acid. Within a given yeast, means with different letters are signifi cantly
different at p 0.05 for EC1118 (letters a to e) and UCD 522 (letters u to z). Adapted