Oxygen Flashcards
What is the role of oxygen at the start of fermentation
It ensures the yeast can begin fermenting, and also ensures the yeast is healthy.
What do the terms ‘protective’ or ‘reductive’ mean with regards to winemaking
They mean that contact with O is minimised and carefully controlled.
Name 3 reductive / protective measures:
1) avoiding ullage
2) use of SO2
3) Airtight containers
4) use of inert gases to flush out O2.
5) cool temperatures which slow the effects of O2.
Define ullage
Ullage is the gap between a wine and the top of a container. In barrels, this will naturally increase as the wood is porous, and the water in the wine will evaporate over time. Ullage can be controlled by regularly topping up the wine.
What inert gases might be used to flush out O2. What are the pros and cons of each
CO2. It is heavier than O2 so sinks to the bottom of a vessel. It is cheap and plentiful. You only need a volume equal to the volume of the vat to displace all the air. It is potentially dangerous as it gathers at the bottom of a container, but is odourless and colourless. There are frequent deaths through winey workers going in to a vat to clean it and being asphyxiated by CO2. Other risks: it dissolves in wine - meaning the wine isn’t protected from O2 and also has a slight spritz.
Argon: It is heavy like CO2, and isn’t dangerous, so is preferable to CO2, although it is much more expensive, and food-grade gas isn’t as easy to get. Good for Sparging.
Nitrogen: It is as dense as air so lots more of it is required than Argon or CO2 (3x as much) , which incurs a cost. It does not dissolve in wine completely, but a slight amount of dissolution means there can be a light froth in the glass (not desirable). But it still protects from O2. A mix of N and CO2 is good as it stops the wine tasting flat.
How is acetaldehyde formed, and what does it do to a wine
Acetaldehyde is the result of ethanol + oxygen. IT is also known as Ethanal.
It is usually formed during fermentation, when the yeast has transformed some of the sugar into ethanol.
In small quantities, it can be a positive, bringing fruity notes.
In quantities around 100/120mg/L it is pungent and negative, bringing notes of bruised apple, sherry, nuts and oxidation.
What does SO2 do to acetaldehyde?
It binds to it, which can counter the negative elements of the compound. BUT this means the SO2 isn’t able to bind to O2.
What are the benefits of oxygen in winemaking
1) Crucial for reaction between anthocyanins and tannins which result in colour stability
2) crucial for some styles of wine like olorosso, fino Sherry, tawny port, madeira)
3) keeps yeast healthy
4) removes reductive faults
5) can stabilise a must improving its ability to age
How do you increase O2 exposure
1) cap management
2) micro oxygenation
3) small barrels
4) number of rackings. more rackings mean more O2 exposure
5) allowing ullage
6) Not using inert gas to flush containers
7) hyper oxidisation
What is the difference between micro oxygenation and hyperoxidation?
Micro oxygenation :
This is the process of gently bubbling small amounts of O2 through a wine to expose the must to oxygen. This is thought to improves the colour stability as well as ensure the yeast has enough O2 present in the liquid to start fermentation. Yeast needs O2 to reproduce, so there is a large enough population of them to properly start fermentation. It can also amplify the process of extracting wood aromas from oak staves or chips in a wine. Micro-oxygenation also softens the harsh polyphenols (tannins) in a wine, much like would happen during maturation. the harsh polyphenols drop out of the wine as a black deposit.
Hyperoxidation
Hyperoxidation is the process of exposing a must to oxygen pre fermentation to purposefully oxidise the more volatile elements. One of the results of this process is white wines will brown and red wines will go much darker. These effects, however, will be reversed during the fermentation process. The end result is the wine has lost a lot of the more unstable elements of the wine which might have oxidised during the maturation process at which point their effects would be detrimental.
What is the benefit of O2 presence in a wine post bottling
It reduces reductive taint.
What is sparging
bubbling gas through the wine to removing dissolved oxygen. Nitrogen is often used, although it removes CO2 which can make the one taste flat, so a mix of CO2 and N is good.
Benefit is it’s a very effective way of clearing the wine of O2, but drawback is it removes other things as well as O2 - including volatiles elements, which can include flavours.
