The Chemistry of Winemaking

Report
The Chemistry of Winemaking
April 9, 2013
Debbie Knutzon, Synapse Wines
Basic Wine Chemistry Analyses
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pH
Titratable Acidity
Free and Total SO2
Volatile Acidity
% Alcohol
Malolactic Fermentation Assessment
pH
-log [H+]
Normal levels in wine
white: 3.1 – 3.6
red: 3.3 - 3.9
Why measure pH?
• pH affects:
– Effectiveness of SO2
– Rate of browning
– Microbiological growth
– Related to TA but not directly
– Effect on color
Titratable Acidity (TA)
• Measurement is an acid-base titration that
represents the total amount of available
protons from all the acids in the wine
• Expressed as grams tartaric acid per 100 ml
(or per liter)
• Not a direct correlation with pH due to
buffering capacity of the grape/wine
TA
• TA has a big impact on flavor /perception of
wine
• “Typical” range: 5-8 g/L
- Higher end: wine tastes sharp or tart but will age
more gracefully
- Lower end: wine tastes dull, flabby and will not
age well
Measuring TA
• Titrate a known amount of wine (degassed)
with 0.1N NaOH to a known endpoint. By
measuring the amount of base (NaOH) added,
the amount of acidity in starting wine can be
calculated.
• Phenolphthalein vs. pH meter
Sulfur Dioxide (SO2)
Uses:
– Prevent chemical and enzymatic oxidation
– Protect against microbial spoilage
– Functionality derives only from free forms
Sulfur Dioxide (SO2)
• Total vs. Free SO2
Free SO2 exists as either
molecular or bisulfite
Uses:
–Prevent chemical and enzymatic
oxidation
• bisulfite
–Protect against microbial spoilage
• molecular
Amount of SO2 in molecular form
depends on the pH of wine
pH
3.0
3.5
3.8
4.0
minimum ppm free SO2
0.8 molecular 0.5 molecular
13 ppm
8 ppm
40 ppm
25 ppm
79 ppm
49 ppm
125 ppm
78 ppm
Measurement of SO2
Ripper
Free or total
yes
Relative ease
easy
Cost
less
Ease of end-point
white
easy
red
hard
Aeration-Oxidation
yes
medium
more
easy
easy
Addition of SO2
Potassium metabisulfite (KMBS)
• KMBS contains only 57% sulfur dioxide, so
need a correction factor of 1.75
•
can find online calculators or printable
tables
Volatile Acidity (VA)
• Indicator of microbial spoilage (acetic acid)
• Acetic acid produced by Acetobacter (duh!)
• Conversion of both glucose and ethanol
• Lactic acid bacteria can convert glucose to
acetic
• Brettanomyces can produce acetic acid
Prevention: SO2 and no head space
VA measured by steam distillation
% Alcohol
• Ebulliometer
–simple and accurate
–Based on ethanol’s depression of
boiling point
Malolactic Fermentation
• Bacterial fermentation that converts malic
acid to lactic acid
• Generally a qualitative determination using
paper chromatography to follow the depletion
of malic acid from a sample
• Useful for determining when to add KMBS at
cessation of fermentation
“Old-School” References
The Complete Handbook of Winemaking
The American Wine Society
published by GW Kent
Knowing and Making Wine
Emile Paynaud
John Wiley & Sons
Winery Technology & Operations
Dr. Yair Margalit
Wine Appreciation Guild

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