The Effects of Varying Levels of
Grape Solids on Fermentation
Progress and Off-Character
Formation in French Colombard
Jonathan Cave
Ashley DeValentine
Max Reichwage
Lauren Schwarzburg
Introduction
Hypothesis
• Low solids levels may inhibit fermentation progress—stuck or
sluggish fermentations
• High solids will result in rapid fermentation completion
• Fermentations with high solids will be more likely to develop
negative aroma profiles, particularly H2S
• Lower solids during fermentations will yield more desirable
aromas
Effect of Grape Solids on
Fermentation: General Concepts
• Grape solids are associated with extraction of astringency,
oxidative enzyme activity, interference with fermentation
bouquet, and fermentation rate.
• White juices fermented with low solids at low temperature are
generally considered higher quality.
• Low solids tent to slow fermentation and increase potential for
stuck fermentations.
• Off-character formation is associated with high solids levels
during fermentation, particularly the formation of H2S.
Off-Character Production and
Nutrient Content
• Grape solids contain many yeast nutrients.
• Settling and racking prior to fermentation decreases total
nitrogen by 10 to 15 % (Zoecklein et al., 1995).
• Grape solids retain elemental sulfur and provide protein nitrogen
in the form of S-containing amino acids (Boulton et al., 1996).
• Free amino nitrogen (FAN) is linked to H2S formation (Vos and
Gray, 1979)
• Subsequent studies show FAN is not solely responsible.
High Solids and Off-Character
Production
• Fermentations of six white wine varieties, including French
Colombard, showed that wines prepared from clarified juice
were higher in quality and desirable aroma than other wines.
High solids samples showed absence of fermentation bouquet
and increases in off-character, particularly H2S (Singleton et al.,
1975).
• Study of several cultivars from Greece showed that increasing
quantities of grape solids led to increased concentrations of
volatile sulfur compounds in all wines except Muscat of
Hamburg (Karagiannis and Lanaridis, 2002).
• Possible variation amongst varietals.
Low Solids and Off-Character
Production
• Deficiency of nitrogen and/or vitamins in the must may be
directly related to solids level.
• Off-character associated with pantothenate deficiency (Boulton
et al., 1996).
• Grape solids contain esterases that act as catalysts for
hydrolysis of esters—removal of desirable and undesirable
esters in high solids samples (Bisson, 2010).
Grape Solids and Fermentation
Progress
•
Grape solids act as a source of nutrients for yeast (Boulton et al.,
1996).
•
Grape solids provide nucleation sites for carbon dioxide bubbles
carrying yeast—increased suspended viable yeast count results in
more rapid fermentation in high solids wines (Boulton et al., 1996).
•
Solutions supersaturated with carbon dioxide are inhibitory to yeast—
grape solids increase fermentation rate by decreasing CO2 saturation
(Boulton et al., 1996).
•
Effects of supersaturation rarely seen at fermentation pressures
(Boulton et al., 1996).
•
Effect of clarification may be related to variety (Houtman and du
Plessis, 1986).
Studies on Solids and
Fermentation Progress
• Clarification removes substances that are essential for yeast
proliferation, but the effect of clarification was partially
dependent on cultivar. Fermentation rate of French Colombard
was more strongly retarded than Chenin blanc or Riesling
(Houtman and du Plessis, 1986).
• Clarified juices containing less than 0.1 % solids fermented
rapidly if solids were finely ground. Stirring the must during
fermentation or addition or rentention of 0.5 % grape solids
resulted in rapid and complete fermentation (Groat and Ough,
1978).
Adsorption of Juice Components
to Grape Solids
•
Yeast growth and byproduct formation are impacted by adsorbed
nutrients.
•
Removal of solids amplifies existing nutrient deficiencies (Boulton et al.,
1996).
•
Nutrient adjustment, particularly N, significantly impacts the role of
solids during fermentation.
•
Interactions between solids, nitrogen level, fermentation temperature,
and yeast strain—addition of grape solids, even in very small
quantities, shorten fermentation time, but addition of N increased it
further in addition to improving quality (Tromp, 1984)
Materials &
Methods
Materials and Methods
• Varietal used: 2010 UC-Davis French
Colombard
• Pressed to 2mb pressure
• Yield: 505 gallons of juice
• Cold settled for 24 hours
• 7-10 gallon containers, varying levels of
solids.
Solids Levels
Sample
Solids Added Back
(gallons of lees)
Percentage of
Original Solids
A
0
0%
B
0.22
25%
C
0.45
50%
D
0.70
75%
E
0.89
100%
F
1.34
150%
G
1.78
200%
Materials and Methods
• Rhone 4600 yeast selected, inoculated
at 35g/hL
• Desirable for short lag phase and low need
for nutrients
• Ferments easily at low temperatures
(13.5C)
Materials and Methods
• Fermentation carried out at 62F
• Brix and temperature monitored throughout
• Racked and bottled for analysis
• Analysis by duo trio test and preference test
Results and
Discussion
Fermentation Profiles: All Samples
25
25
20
20
Sample A Brix
Sample B Brix
15
15
Sample C Brix
10
10
Temperature (C)
Corrected Brix
Sample D Brix
Sample E Brix
Sample F Brix
Sample G Brix
Sample A Temp.
Sample C Temp.
Sample D Temp.
5
5
Sample E Temp.
Sample F Temp.
Sample G Temp.
Sample B Temp.
0
0
0
-5
50
100
150
200
Time (hours)
250
300
350
400
-5
Fermentation Profile Discussion
• The low solids samples did not seem to inhibit the
fermentation progression, i.e. no stuck or sluggish
fermentations, which contradicts the hypothesis
• Yeast must have had sufficient nutrients
• The higher the solids levels, the greater the rate of
fermentation which agrees with the hypothesis
• More solids = more nutrients
• Greater amount of nucleation sites for CO2 thus increasing
yeast viability
Final Solids Concentrations
Table 1: Mass of solids and calculated concentrations from
centrifuged samples
Sample
Percentage of
Original Solids
Mass of solids
extracted
from the
centrifuge (g)
Calculated
Concentration
(g/mL)
A
0%
0.426
0.00473
B
25%
0.745
0.00828
C
50%
0.672
0.00747
D
75%
0.951
0.0106
E
100%
1.029
0.01143
F
150%
1.188
0.01320
G
200%
1.737
0.01930
Lack of proportionality between
the initial solids added and the
final solids obtained. Why?
•
Approximate volumes of lees
were added
•
Different viable yeast counts in
each sample effect the final
mass of solids obtained
•
Different keg shaking
techniques
•
Small sample size
Sensory Notes
Sample
Percentage of
Original Solids
Aromas & Flavors Noted
A
0%
Skunky, reductive character
B
25%
Skunky, reductive character
C
50%
Clean, very slight skunkiness
D
75%
Clean, Pinot Grigio, tart
E
100%
Most complex, tart
F
150%
Wet, rotten walnut
G
200%
Wet, rotten walnut
Preference Test Results
Table 5: Ranking of the seven different wine samples
Rank
Sample
Points
1
C
61
2
G
55
3 (tie)
E
50
3 (tie)
F
50
5
D
49
6
A
31
7
B
19
• C (75% of original lees) was most preferred, however samples D through G
were all fairly close
• A and B are clearly the least preferred wines
Duo-Trio Results
Table 2: Number of responses from the fifteen sensory panelists for the different sample
Sample 1
Responses
Reference
C
D
A
F
B
G
E
E
B
A
F
B
D
E
Sample 2
Responses
12
8
2
9
4
11
5
C
D
G
C
F
G
A
3
7
13
6
11
4
10
Correct
Responses
12
8
13
6
11
11
10
Table 3: Percentage of correct duo trio responses for the seven different trials
Samples
C vs. E
D vs. B
A vs. G
F vs. C
B vs. F
G vs. D
E vs. A
Percentage of correct
responses
80%
53%
87%
40%
73%
73%
67%
Duo-trio Statistical Analysis
Table 4: Summary table of the duo-trio test, showing the z-test values and their
respective probability values along with statistical significance at α=0.05
Samples
z-test
Probability
Statistically significant
difference?
C vs. E
2.32
0.0094
Yes
D vs. B
0.26
0.3632
No
A vs. G
2.84
0.0022
Yes
F vs. C
-0.77
n/a
No
B vs. F
1.81
0.0322
Yes
G vs. D
1.81
0.0322
Yes
E vs. A
1.29
0.0885
No
Conclusion
Variability of solid levels in the fermenting wine did
not cause sluggish or stuck fermentation at the levels
investigated here, though the anticipated trend of
increasing fermentation rate in correlation with
increasing amounts of solids was observed. It was
further determined that variability of solid level in
white wine causes a discernable difference in the
resulting aroma profile. However, a direct correlation
between solid levels and preference cannot be found
due to the complex nature of the wine when
examined as a solvent matrix.
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Solids Level Tasting
•
•
•
•
•
Glass 1: Control: Low solids Chardonnay
Glass 2: Inocel
Glass 3: Fermoclean
Glass 4: Fermocasein
Glass 5: Control: Low solids French
Colombard
• Glass 6: 100% solids French Colombard