13.1. Winemaking Options: Oxygen and Sulfur Dioxide

Question 1

How does oxygen react with wine?

Answer 1

• oxygen itself is not very reactive with many compounds in the must and wine
• oxygen reactions create products that then go on to react with many must and wine compounds

Question 2

Oxygen can be considered a threat in the production of what types of wine?

Answer 2

In the production of fresh, fruity wines

Question 3

Name 3 reasons why oxygen can be considered a threat in the production of fresh, fruity, white wine.

Answer 3

1. Many of the aroma compounds that give these wines their fruity style break down in the presence of oxygen, loss of fruitiness
   - Example: thiols found in Sauvignon Blanc
2. The products of oxidation reactions may contribute to unwanted aromas to the wine
   - Example: acetaldehyde (from the oxidation of ethanol) can give a nutty, apple aroma
3. The colour of white wines can darken, becoming gold and then brown with increased oxidation

Question 4

Why do white wines require higher levels of protection from oxidation?

Answer 4

• The colour of white wines can darken, becoming gold and then brown with increased oxidation
• Phenolic compounds in red wines have an anti-oxidative effect, which means that they can absorb more oxygen before such effects are perceptible.

Question 5

What is reductive winemaking?

Answer 5

• AKA Protective Winemaking

- The practice of minimising oxygen exposure during the winemaking process

Question 6

Name 5 ways the effects of oxygen on the must or wine can be limited.

Answer 6

1. avoiding ullage in vessels
2. use of ‘inert’ gases (co2, argon, nitrogen) to flush out o2
3. addition of sulfur dioxide
4. use of impermeable containers
5. cool, constant temperatures

Question 7

What is ullage and why is it important when it comes to protecting wines from oxidation during winemaking?

Answer 7

• The headspace of air between the wine and the top of the container
• Can be avoided by ensuring vessels are filled up to the top.
• In not airtight vessels (e.g. wood), gradual loss of liquid through evaporation; must be topped up regularly

Question 8

How are inert gases used to protect wines from oxidation? Name 3 gases that are used.

Answer 8

• Used to flush out oxygen from vessels, pipes and machinery (such as presses)
• 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
• Do not react with compounds in the wine
• Examples: Nitrogen, Carbon Dioxide, Argon

Question 9

What types of fermentation vessels are impermeable to oxygen? What types of packaging?

Answer 9

• stainless steel
• thick concrete
• glass bottles with screwcap

Question 10

Why are cool, constant temperatures important in preventing oxidation?

Answer 10

• slow the rate of oxidation reactions

Question 11

Why is some oxygen needed in winemaking?

Answer 11

• required at the start of fermentation of all wines to promote growth of a healthy yeast population
• in some cases, lack of enough oxygen in winemaking or storage can lead to reductive off-flavours

Question 12

Why is oxygen beneficial in the production of some white wines?

Answer 12

Exposure to oxygen before fermentation is thought to lead to greater oxidation stability in the wine, increasing age potential

Question 13

Why is oxygen beneficial in the production of red wines?

Answer 13

Oxygen is essential in the reaction between anthocyanins and tannins that leads to greater colour stability

Question 14

How does exposure to oxygen benefit a wine over time? Give examples.

Answer 14

1) leads to changes in the aromas/flavours of wine, which can give a greater range and diversity of characteristics / add to complexity
Examples:
- fresh fruits become dried fruits
- notes such as honey, caramel, coffee, leather and mushroom can develop

2) Red wine - essential for reaction between anthocyanins and tannins resulting in enhanced colour stability
3) Promotes healthy growth of yeast in fermentation (starts off the fermentation)
4) must that has been exposed to o2 (hyperoxidation) may dev oxidation stability and enhanced ageing potential

Question 15

In what types of wines is a high level of oxidation vital?

Answer 15

• Oloroso Sherry, Madeira and Tawny Port

Question 16

Name 5 ways that oxygen exposure can be deliberately increased.

Answer 16

1. use of cap management techniques in red wine fermentation that spray or splash the must/wine
2. use of small wooden barrels that can only contain a small volume of wine relative to the ingress of oxygen through the bung holes and staves
3. increasing the number of rackings or amount of lees stirring during ageing (requires lid removal)
4. allowing ullage in wine containers without the use of inert gases
5. use of techniques that involve pumping oxygen through the must (e.g. hyperoxidation) or wine(micro-oxygenation)

Question 17

What might occur with too much oxygen exposure?

Answer 17

• Oxygen can favour the growth of spoilage microbes, such as acetic acid bacteria and Brettanomyces, especially if other conditions are favourable (e.g. grape must or wine with residual sugar).

Question 18

What is sulfur dioxide? In what two ways does it protect wine?

Answer 18

A preservative that is almost universally used in winemaking as an:

• Anti-oxidant: reduces the effects of oxidation by reacting with the products of oxidation reactions, so they cannot oxidise further compounds in the wine.
• Anti-microbial: inhibits the development of microbes such as yeast and bacteria.

Question 19

In what forms can SO2 be applied? What is the name for each?

Answer 19

• Gas: sulfur dioxide,
• Liquid: potassium metabisulfite
• Solid: potassium bisulfite

Question 20

How much SO2 is naturally produced during fermentation?

Answer 20

A small amount of SO2 (10 mg/l or less)

Question 21

A label must state that the wine contains sulfites if that wine contains how much SO2?

Answer 21

More than 10 mg/l

Question 22

What is the maximum amount of SO2 permitted in the EU for red wines? White wines?

Answer 22

Red Wines: 160 mg/l

White Wines: 210 mg/l

Question 23

Why must winemakers declare that the wine contain sulphites?

Answer 23

• SO2 is a toxic substance

- Even at low levels, some people can experience an allergic reaction

Question 24

When is SO2 added?

Answer 24

• soon after the grapes are picked and/or reach the winery
• at bottling
• sometimes during the winemaking process

Question 25

What happens when SO2 is added to must or wine? What is bound SO2 and free SO2?

Answer 25

• it dissolves
• bound SO2: the SO2 that reacts with compounds in the liquid, ineffective against oxidation and microbes
• free SO2: portion that does not react
  – most free SO2 is inactive
  – a small proportion exists as molecular SO2, which is the most effective against oxidation and microbes
  -
  Note at low PH levels a greater proportion of free so2 is molecular and able to perform its function. Opposite for high PH, thus for high PH more top up will be needed.

Question 26

What is the relationship between pH and SO2? What does that mean for wines with a relatively high pH?

Answer 26

• At lower pH levels, a greater proportion of free SO2 is in the molecular form
• A greater amount of SO2 needs to be added to musts and wines with high pH

Question 27

At what 3 points of the winemaking process is adding SO2 most effective?

Answer 27

1. when the grapes are crushed
2. at the end of malolactic conversion
3. at bottling

Question 28

When it comes to SO2 additions, what is the aim of quality-conscious winemakers?

Answer 28

Limit additions of SO2:

• legal restrictions
• can dull wine aromas/flavours
• can sometimes cause the wine to taste harsh

Question 29

What winery best practices can limit the amount of harmful microbes in the wine and the winery?

Answer 29

• Good winery hygiene

- Effective grape sorting

Question 30

What winery best practices can reduce the amount of SO2 needed?

Answer 30

• Limiting oxygen exposure

- Keeping grapes/must/wine at cool temperatures