14.1 The Role of Oxygen in Maturation

Question 1

In maturation what is the impact of o2 and oxidation on the aromas of wine?

Answer 1

Positive impact (improves quality)

• if tertiary aromas are desirable and wine has suitable structure for ageing:
• gradual reduction in many primary aromas
• development of tertiary aromas (dried fruit, nuts)

Negative impact (decreases quality)
-fading of primary aromas, no dev or tertiary

Question 2

In maturation comment on the effects of o2 and oxidation on white wine

Answer 2

• white wines become darker, gradually becoming gold and then brown

Question 3

In maturation comment on the effects of o2 and oxidation on red wine

Answer 3

Oxidation over time makes red wines turn brown.

Controlled exp of a red wine to o2 stabilises colour as

• anthocyanins bind with tannins
• prevents the anthocyanins being bleached by so2
• prevents the anthocyanins being absorbed by yeast lees
• quite NB for light reds like PN
• tannins soften

Phenolics (colour compounds) in reds have antioxidant properties which is why reds are matured longer than whites.

Question 4

How does oxidation affects red wines in general?

Answer 4

• Softens tannins - may be linked to anthocyanin and tannin bonding

Question 5

Why can red wines withstand a higher level of oxygen exposure than white wines before any signs of oxidation are apparent?

Answer 5

due to their higher content of anti-oxidative phenolic compounds (e.g. tannins).

Question 6

On average, how long do white and red wines spend maturing pre-bottling?

Answer 6

white 6 - 12 mth

red 12–24 mth

Question 7

What affect does temperature have on oxidation rates?

Answer 7

warm temperatures increase the speed of oxidation reactions

Question 8

Name a wine in which a high level of oxidation is a key part of its style. Describe the wine.

Answer 8

Oloroso Sherry - brown colour - lack of fresh fruit - nuts and dried fruits

Question 9

How are wines that require a high level of oxidation matured?

Answer 9

• the wine is stored for a number of years in barrels that are not filled to the top - allows oxygen within the headspace of the barrel (ullage)

Question 10

What is ullage?

Answer 10

allowing oxygen within the headspace of the barrel

Question 11

What is the most commonly used method of gaining a slow, gradual exposure to oxygen?

Answer 11

• store the wine in small wooden vessels

Question 12

Describe how wooden vessels slowly oxidize wine.

Answer 12

• Some oxygen is released from the pores in the vessel within the first month that the vessel is filled with wine. - A very small amount of oxygen continues to pass through the gaps between the staves and the bung hole.

Question 13

What are the two main schools of thought related to how wine in barrel is exposed to oxygen?

Answer 13

1. oxygen only enters through the bung hole
2. while others suggest oxygen passes through the wood itself

Question 14

At what point in maturation is a wine most exposed to oxygen?

Answer 14

During any transfer of the wine in which the bung is removed. Examples: - racking - lees stirring - topping up

Question 15

Describe how some wine is generally lost during the maturation process in wooden vessels.

Answer 15

• Water and alcohol evaporate very slowly through the wood pores/slats and when / if the barrel is opened.

Question 16

What effect does the size of the wooden vessel have on evaporation? What is required and how does that effect the end wine?

Answer 16

• Small vessels: large surface area to volume ratio, increasing the rate at which wine is lost
• Need to be topped up with more wine to keep them full (to avoid ullage) - topping up frequency leads to more oxygen exposure in these vessels than in much larger vessels.

Question 17

What effect does the size of the wooden vessel have on oxidation?

Answer 17

Smaller vessels: - larger the amount of oxygen enters the barrel relative to the volume of wine - leads to quicker (but not extreme) oxidation

Question 18

Explain micro-oxygenation and how it is used in wine maturation.

Answer 18

• a cheaper alternative to barrel aging
• stainless steel tanks -
• involves dose of oxygen (typically in units of mg/L) per month bubbling through wine for a number of months post alcoholic fermentation

– Initially used for inexpensive and mid-priced wines - Now, used for all price point wines

• *Benefits:**
• increase colour stability and intensity
• soften tannins - improve texture
• reduce unripe, herbaceous flavours
• provides the effects of gentle exposure to oxygen more quickly than barrel ageing without needing expensive barrels
• the rate of oxygen exposure can be controlled much more tightly than it could be in a barrel

Question 19

Why must care be taken when carrying out micro-oxygenation?

Answer 19

• presence of o2 provides favourable environment for spoilage microbes (acetic acid bacteria and Brettanomyces)
• this is a new process and research is lacking:
• on its influence per grape variety is lacking.
• how wines made by micro-oxygenation age in bottle over the long term -difference compared to the same wine matured in barrel

Question 20

Summarise the effects of humidity and temperature on wine storage

Answer 20

Reds stored slightly warmer than whites as they are less reactive to the effects oxidation.

Low humidity - More water than alcohol is evaporated at lower humidity, so in wine abv levels will slowly rise.

Low Hum & High Temp - Loss of wine due to evaporation leads to more top ups which exposes the wine again to o2 (ROTSO)

Higher temps cause undesirable chemical reactions in wine and speed up the ageing

In essence - best storage in cellars with controlled consistent temperature and humidity.