What broadly speaking does ‘stabilisation’ refer to?
An umbrella term for ensuring that wine once inside the bottle is unable to form undesirable further reactions and faults.
What are the three main instability issues in wine relate to?
- Microbial spoilage.
- Oxidation.
- Tartrate instability.
Tartaric acid’s mono salt, which is less soluble is called what, and what does it do which is undesirable?
- Potassium Bitartrate.
- At low temperatures precipitates out to form small, clear, glass like crystals.
What is the most traditional way of stabilising wine to prevent tartrate instability? Describe the process.
- Cold stabilisation.
A process by which the wine is chilled to a temperature between -4°C to -8°C. At these temperatures the pottassium bitartrate precipitates out and sinks to the bottom, allowing the clean wine to be racked off.
Which process has superceded cold stabilisation? Describe it.
- Contact process.
Wine is chilled to around 0°C, it is seeded with a small amount of potassium bitartrate crystals, which acts a nuclei for further growth. The deposited crystals are removed, dried, ground and reused.
Name two other substances that can be used to make wine tartrate stable.
- Gum arabic (prevents crystals growing large enough to be visible), short lived, only 12 months protection.
- Metatartaric acid (prevents formation), short lived only 12 months.
What is oxidation?
The bonding of oxygen molecules with oxidisable compounds.
Which process has superceded cold stabilisation? Describe it.
- Contact process.
Wine is chilled to around 0°C, it is seeded with a small amount of potassium bitartrate crystals, which acts a nuclei for further growth. The deposited crystals are removed, dried, ground and reused.
Name two other substances that can be used to make wine tartrate stable.
- Gum arabic (prevents crystals growing large enough to be visible), short lived, only 12 months protection.
- Metatartaric acid (prevents formation), short lived only 12 months.
What is oxidation?
The bonding of oxygen molecules with oxidisable compounds.
Name factors that can affect oxidation.
- Presence of yeasts and acetic acid bacteria.
- Amount of dissolved oxygen in the wine or must.
- pH - The lower the pH, the more protective the SO2.
- SO2 concentration
- Temperature - the lower the temperature the slower the oxidation rate, but there will be more dissolved oxygen.
How can white wines be protected from oxidisation after fermentation?
- Tanks fully topped up.
- Free SO2 level should be at minimum 20g/l .
- Blanketing with inert gas.
What is reduction?
Situations where winemaking has been too anaerobic can cause the formation of hydrogen sulphide which can cause undesirable aromas.
How can reduction be removed?
Aerating the wine or adding copper sulphate.
What are the key factors that effect the growth of microorganisms in wine?
- pH - the lower the pH the more difficult it is for organisms to survive.
- Alcohol, inhibits most microbes if above 15%.
- Temperature - growth is usually faster between 20-35°C.
- SO2 - 20mg/l free will inhibit the growth of most microbes.
- Winery hygiene.
- O2 - essential for growth of acetic bacteria.
Explain how acetic acid spoils wine.
Acetic acid bacteria react with alcohol to cause the production of acetic acid. Acetic acid combines with alcohol to form ethyl acetate (nail polish remover).
When would sorbic acid be used?
To prevent re-fermentation in bottle when bottling procedures cannot be trusted.
What is a common test for protein stability?
- Immerse a test tube sample into 80°C water bath for 6 hours. Unstable protein can be seen as a milky deposit at the bottom.
What is the maximum permitted copper content in the EU?
1mg/l
What do higher levels of this risk?
Copper Casse - A reddish brown haze and deposit.
What is the most effective way of removing excess copper from wine?
- Potassium Ferrocyanide, known as ‘blue fining’. The treatment produces deposits of copper which can be filtered out.
What is the maximum permitted level of iron in the EU?
10mg/l
