13. General Winemaking Options

Question 1

Why is oxygen contact generally limited for white wines, particularly fresh, fruity wines?

Answer 1

• Many of the aroma compounds that give these wines their fruity style, for example the thiols found in Sauvignon Blanc, break down in the presence of oxygen, and this can lead to a loss of fruitiness.
• The products of oxidation reactions may contribute unwanted aromas to the wine; for example, acetaldehyde (from the oxidation of ethanol) can give a nutty, apple aroma.
• The colour of white wines can turn darker, becoming gold and then brown with increased oxidation

Question 2

Why can red wines tolerate higher levels of oxygen contact than white wines?

Answer 2

Phenolic compounds in red wines have an anti-oxidative effect, which means that they can absorb more oxygen before such effects are perceptible.

Question 3

What can a winemaker do to limit the effect of oxygen on the must or wine?

Answer 3

• Avoid ullage in vessels
• Use of inert (chemically inactive) gases
• Addition of sulfar dioxide
• Use of impearmable containers (stainless steel or concrete instead of wood).
• Maintain cool, constant temperatures

Question 4

What is ullage, and how can it be avoided?

Answer 4

Ullage is the headspace of air between the wine and the top of the container. It can be avoided by ensuring vessels are filled up to the top. In vessels that are not completely airtight, such as those made of wood, there may be a gradual loss of liquid through evaporation. Therefore, these vessels should be topped up regularly with more wine to avoid ullage.

Question 5

What inert gases are used to limit oxygen contact of must and wine? Outline the process.

Answer 5

Gases such as nitrogen, carbon dioxide and argon can be used to flush out oxygen from vessels, pipes and machinery (such as presses) because these gases do not react with compounds in the wine. Inert gases can also be used to fill the empty headspace of any containers where the wine does not reach the top to prevent oxygen coming into contact with the wine.

Question 6

Why might a winemaker want to expose must and wine to a small amount of oxygen?

Answer 6

• Oxygen is required at the start of fermentation to ensure a healthy population of yeast; a lack of oxygen can lead to reductive flavours.
• In white wines, some oxygen exposure is thought to lead to higher oxygen stability in wine, leading to better ageing potential.
• In red wines, oxygen is essential in the reaction between anthocyanins and tannins that leads to greater colour stability
• Exposure to oxygen over time also leads to changes in the aromas/flavours of wine, desirable depending on the style of wine being produced.

Question 7

How can oxygen exposure of the wine and must be increased?

Answer 7

• Use of cap management techniques
• Use of small wooden barrels that can only contain a small volume of wine relative to the amount of oxygen that enters through the bung holes and stave
• Increase the amount of rackings or amount of lees stirring during ageing; any procedures that require the bung of a barrel or lid of a vessel to be removed and the wine to be moved will increase oxygen exposure
• Allowing ullage in wine containers without the use of inert gases in the headspace
• Use of techniques that involve pumping oxygen through the must (e.g. hyperoxidation) or
  wine (e.g. micro-oxygenation)

Question 8

What are the risks of exposing must/wine to oxygen?

Answer 8

Oxygen can favour the growth of spoilage organisms, such as acetic acid bacteria and Brettanomyces, especially if other conditions are favourable (e.g. grape must or wine with residual sugar). Therefore, wines exposed to oxygen must be carefully monitored for these microbes and their associated faults.

Question 9

What are the two properties of sulfar dioxide?

Answer 9

• Anti-oxidant – SO2 only reacts with oxygen itself very slowly; it reduces the effects of oxidation by reacting with the products of oxidation reactions, so they cannot oxidise further compounds in the wine. It also inhibits oxidative enzymes.
• Anti-microbial – It inhibits the development of microbes such as yeast and bacteria. Different species of yeast and bacteria can vary in their tolerance to SO2.

Question 10

In what forms can sulfar dioxide be applied to must/wine?

Answer 10

SO2 can be applied in various forms: gas, liquid or solid, as sulfur dioxide, potassium metabisulfite or potassium bisulfite.

Question 11

How much sulfar dioxide is naturally produced during fermentation?

Answer 11

Around 10mg/l or less

Question 12

What are the permitted SO2 levels in Australia?

Answer 12

250mg/l; for wines with less than 35g/l RS
300mg.l; for wines with more than 35g/l RS

Question 13

What are the permitted levels of SO2 in the EU?

Answer 13

Red wine; 150mg/l (100 for organic)
White wine; 200mg/l (150 for organic)
Sparkling; 235g/l (205 for organic)
Botrytis/late harvest; 300mg/l (270 for organic).

These levels can increase depending on RS

Question 14

What are the permitted SO2 levels in the USA?

Answer 14

350mg/l

SO2 additions are not permitted for organic wines

Question 15

Is sulfar dioxide toxic to humans?

Answer 15

The concentrations of SO2 found in wine are far below toxic levels; however, even at these very low levels some people can experience an allergic reaction. If a wine contains over 10 mg/L of SO2, the label must state that the wine contains sulfites.

Question 16

What is the difference between bound SO2 and free SO2?

Answer 16

When SO2 is added to must or wine, it dissolves and some of it reacts with compounds in the liquid. This is bound SO2 and is ineffective against against oxidation and microbes.
The proportion that is not bound is called ‘free SO2’. The vast majority of the free SO2 exists in
a relatively inactive form and a small proportion exists as molecular SO2, which is the most effective against oxidation and microbes.

Question 17

What factors influence the effectiveness of SO2?

Answer 17

• The pH level of the must or wine has a key effect on the efficacy of SO2 in that a greater proportion of free SO2 is in the molecular form at lower pH levels. This means that a greater amount of SO2 needs to be added to musts and wines with relatively high pH to protect them from oxidation and microbes.
• The timing and size of SO2 additions also influences the effectiveness of the added SO2. Adding a larger amount when the grapes are crushed, at the end of malolactic conversion and at bottling is considered as more effective than adding smaller amounts throughout the winemaking process.