4.1 The Role of Oxygen in Maturation Flashcards
Generally, what effect does oxidation have on wine flavors?
- gradual reduction in many primary aromas
- development of tertiary aromas (dried fruit, nuts)
Describe the conditions required for oxidation to enhance a wine’s quality.
- primary aromas develop into pleasant tertiary aromas
- wine has a structure suitable for extended ageing
Describe the negative effects of oxidation on wine.
- primary flavours fade
- not replaced by positive tertiary characteristics
How does oxidation affects the colour of white wines?
- white wines become darker, gradually becoming gold and then brown
What happens when light-coloured red wines are exposed to oxygen?
- can result in greater colour stability and intensity
- anthocyanins bind with tannins (protects anthocyanins from being adsorbed by yeast lees or bleached by SO2 additions)
How does oxidation affects the colour of red wines?
After much more extensive ageing, red wines gradually become paler and browner.
How does oxidation affects red wines in general?
- Softens tannins
- may be linked to anthocyanin and tannin bonding
Why can red wines withstand a higher level of oxygen exposure than white wines before any signs of oxidation are apparent?
due to their higher content of anti-oxidative phenolic compounds (e.g. tannins)
On average, how long do red wines spend maturing pre-bottling?
12–24 months
On average, how long do white wines spend maturing pre-bottling?
6–12 months
What affect does temperature have on oxidation rates?
warm temperatures increase the speed of oxidation reactions
Name a wine in which a high level of oxidation is a key part of its style. Describe the wine.
Oloroso Sherry
- brown colour
- lack of fresh fruit
- nuts and dried fruits
How are wines that require a high level of oxidation matured?
- 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)
What is ullage?
allowing oxygen within the headspace of the barrel
What is the most commonly used method of gaining a slow, gradual exposure to oxygen?
- store the wine in small wooden vessels
Describe how wooden vessels slowly oxidize wine.
- 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.
What are the two main schools of thought related to how wine in barrel is exposed to oxygen?
- oxygen only enters through the bung hole
2. while others suggest oxygen passes through the wood itself
At what point in winemaking is a wine most exposed to oxygen?
During any transfer of the wine in which the bung is removed. Examples:
- racking
- lees stirring
- topping up
Describe how some wine is generally lost during the maturation process in wooden vessels.
- Water and alcohol in liquid form impregnate the wood
- Within the staves of wood the water and alcohol turn to vapour (evaporate) and diffuse, along the concentration gradient, to the air outside the vessel
- leads to a gradual concentration of the other components of the wine
What effect does the size of the wooden vessel have on evaporation? What is required and how does that effect the end wine?
- 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.
What effect does the size of the wooden vessel have on oxidation?
Smaller vessels:
- larger the amount of oxygen enters the barrel relative to the volume of wine
- leads to quicker (but not extreme) oxidation
What is micro-oxygenation? What’s the typical unit used for measuring micro-oxygenation?
- a cheaper alternative to barrel aging
- involves bubbling oxygen through wine
- dose of oxygen typically in units of mg/L per month
In what type of vessel is micro-oxygenation typically carried out? For how long?
- stainless steel tanks
- for a number of months post alcoholic fermentation
Initially, on what types of wines was micro-oxygenation typically carried out? Now, what types of wines is it used for?
- Initially used for inexpensive and mid-priced wines
- Now, used for premium and super-premium wines
What are the benefits of micro-oxygenation?
- 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
Why must care be taken when carrying out micro-oxygenation?
- more favourable environment for spoilage microbes (acetic acid bacteria and Brettanomyces)
What 3 things do we not know for sure when it comes to micro-oxygenation?
- relatively new technique; not much research on
1. which grape varieties or types of wines benefit from micro-oxygenation
2. how wines made by micro-oxygenation age in bottle over the long term
3. difference compared to the same wine matured in barrel
At what temperature are white wines typically stored?
8–12°C (46–54°F)
At what temperature are red wines typically stored?
12–16°C (54–61°F)
Why are red wines typically stored at slightly warmer temperatures than white wines?
they are less susceptible to the effects of oxidation
Why do stable, cool temperatures give a slower maturation than warm temperatures?
due the effect of temperature on chemical reactions that happen between various compounds within the wine
Does the temperature itself directly speed up the aging?
No, temperature determines what reactions can occur and how quickly various reactions will occur.
How does humidity affect aging?
- low humidity and warm temperatures increase the rate of wine loss
- reduces the volume of wine to sell
- barrels need topping up more often (greater cost through labour requirement)
- risk of oxidation is increased (harmful for quality)
What is the effect of extremely low humidity (under approximately 70 per cent) on wine?
- Cause water to be lost at a greater rate than alcohol and, over time, this can lead to a higher alcohol concentration in the wine
Why were many traditional cellars built underground?
To keep a cool, constant humidity and temperature
In modern cellars, how are temperature and humidity controlled?
Temperature- and humidity-control systems
