The Chemistry of Winemaking
April 9, 2013
Debbie Knutzon, Synapse Wines

Basic Wine Chemistry Analyses
• pH
• Titratable Acidity
• Free and Total SO
2
• 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 SO
2
– 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

2
Uses:
– Prevent chemical and enzymatic oxidation
– Protect against microbial spoilage
– Functionality derives only from free forms

Sulfur Dioxide (SO
2
)
• Total vs. Free SO
2

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
79 ppm
125 ppm
25 ppm
49 ppm
78 ppm

Measurement of SO2
Cost
Ease of end-point white red
Ripper Aeration-Oxidation
Free or total yes
Relative ease easy less easy hard 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

– 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