Winemaking Flashcards
Impact of oxygen in wine
Oxygen is responsible for a number of reactions between the compounds in grape must or wine and for this reason can have a significant effect on wine style and quality. Although oxygen itself is not very reactive with many compounds in the must and wine, the reactions it does take part in create products that then go on to react with many must and wine compounds. These reactions are oxidation reactions. The timing and amount of oxigen exposure is key, making the difference between a positive or negative effect.
Oxygen is generally threatening for the production of fresh, fruity wines. Many of the aroma compounds that give these wines their fruity style, example thiols in Sauvignon Blanc, break down in the presence of oxygen and this can lead to a loss of fruitiness.
Oxygen may contribute to 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 also turn darker.
How phenolic compounds react to oxygen
Phenolic compounds in wine have anti-oxidative effect, which means that they can absorb more oxygen before such effects are perceptible
How is called the practice of minimising oxygen exposure during winemaking process?
Reductive or protective winemaking
How can the level of oxygen be limited
- Avoiding ullage in vessels. 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.
- Use of inert gases. 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 container where the wine does not reach the top to prevent oxygen coming into contact with the wine.
- addition of sulfur dioxide, which has strongly anti-oxidative effects
- Use of impermeable containers. Stainless steel and thick concrete vessels are impermeable to oxygen, whereas wooden vessels allow gentle ingress of oxygen. The use of glass bottles with screwcap can also minimise exposure to oxygen during storage in bottle.
- Cool, constant temperatures. Cool temperatures slow the rate of oxidation reactions: hence the reason for maturing wines in relatively cool cellars or picking grapes early in the morning so that the fruit is not warm.
Positive exposure to Oxygen
Controlled exposure to oxygen can be positive for many wines. Oxygen is required at the start of fermentation of all wines to promote growth of a healthy yeast population and, in some cases, lack of enough oxygen in winemaking or storage can lead to reductive off-flavours. In the production of some white wines, exposing the must to oxygen before fermentation is thought to lead to greater oxidation stability in the wine, and to result in increased ageing potential. In red wines, oxygen is essential in the reactions between anthocyanins and tannins that leads to greater colour stability, anthocyanins.
Exposire to oxygen overtime also leads to changes in the aromas/flavour of wine. These can give a greater range and diversity of characteristics: Fresh fruits become dried fruits and notes such as honey, caramel, coffee, leather and mushroom can develop. A high level of oxidation is vital in some wine styles, such as Oloroso Sherry, Madeira and Tawny Port, but less extreme oxidation also contributes to the complexity of many matured white and red wines.
How to increase oxygen exposure
- Use of cap management techniques in red wine fermentation that spray or spash the must or wine
- Use of small wooden barrerls that can only contain a small volume of wine relative to the amount of oxygen that enters through the bung holes and staves
- Increasing the number 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 iner gases in the headspace
- Use of techniques that involve pumping oxygen through the must or wine (eg. micro oxygenation)
Sulfur dioxide properties
Sulfur dioxide is a preservative that is almost universally used in winemaking.
1. Anti oxidant - SO2 only reacts with oxygen itself very slowly it reduces the effects of oxidation by reacting with the products of oxidations, so they cannot oxidise further compounds in the wine. It also inhibits oxidative enzymes.
2. Antimicrobial - it inhibits the development of microbes such as yeast and bacteria. Different species of yeast and bacteria can vary in their tolerance to SO2
Level of sorting in the winery depends on:
A judgement has to be made as to the level of sorting that is justifiable in relation to the return expected from the sale of the wine.
1.Ripeness and health of the fruit arriving at the winery
2. Final wine quality and price
3. Whether any sorting has been carried out in the vineyard
4. Physical state of the grapes ( if grapes arrive in large containers, the bottom grapes will have crushed and released juice
In poor years and cool climates a greater level of sorting may simily be required to remove under-ripe grape.
Sorting for quality wines
- Removing unwanted grapes/bunches before picking or during hand-harvesting
- Sorting by hand on a table or a moving or vibrating belt
- Optical sorting, which is a high tech, high cost option that uses digital imaging and software technology to scan individual grapes.
Destemming definition
Hand harvested grapes for most white wines and many red wines are destemmed on arrival at the winery. (Machine-harvested grapes are already destemmed because the grapes are shaken from their stems during harvesting). Destemmers generally work by a series of blades within a rotating drum that remove the grapes from the stems.
Stems in wine
Stems contain tannins, which can be extracted if the stem are left in contact with the wine. These tannins are not wanted in white wines and are additional to skin/seed tannings in red wines, so can be desirable in some wines and not in others. If stems are not ripe, they can convey unwanted green flavours a bitter tannins to wine.
Wines production methods where grapes are not destemmed
- Red wine fermentations that use some whole bunches ( Pinot Noir in Burgundy)
- Carbonic macerationg (eg Gamay in Beaujolais)
- Whole bunch pressing for some white wines ( eg common for high quality sparkling wines)
Crushing grapes definition
Is the application of sufficient pressure to the grapes to break the skins and release the juice, making it available for fermentation. It happens at the beginning of the winemaking process. It is important that the pressure applied is gentle enough not to crush the seeds, which would add bitterness. Traditionally, crushing was done by the pressure of human feet.
Define must
The mixture of grape juice, pulp, skins and seeds that comes from the crusher is commonly termed “must”. For white wines, must may also refer to the grape juice that is fermented (pressing and clarification means pulp, skins and seeds have been removed.
Pressing
In white winemaking, the grapes are almost always pressed to extract the juice from the grapes and to separate the skins from the juice before fermentation. In red winemaking, the grapes are typically crushed before fermentation and pressed after the desired number of days on the skins or at the end of fermentation.
Pneumatic pressing Advantages
The advantages of the pneumatic press are that:
1. it can be programmed to exert different amounts of pressure (light pressure for less extraction, harder pressure for greater extraction, which can provide different blending components if needed
2. it can be flushed with inert gas before use to protect the juice or wine from oxidation.
Pneumatic presses are common in wine regions around the world in medium to large-scale wineries that can afford the initial investment.
Define pneumatic press
Also called “air bag presses” Are made up of a cylindrical cage with a bladder that runs down the side or middle of it. Grapes are loaded into the tank on one side of the bladder and the other side of the cage is filled with air and as the bladder inflates, the grapes are gradually pushed against grates on the side of the cage separating juice and skins.
Basket press definition
Basket press are a more traditional form of press, but still in use. They are also called vertical presses or champagne presses. A “basket” is filled with grapes and pressure is applied from above. The juice or wine runs through gapes or holes in the sides of the basked and is collected by a tray at the bottom of the press. A pipe transfers the juice or wine to another vessel.
Basket presses are not sealed vessels, and therefore cannot be flused with inert gases to avoid oxygen exposure.
Basked press extra info
Some winemakers believe basked press to be gentler than pneumatic presses. However, they generally hold a smaller press load, are much more labor intensive and therefore most suited to small wineries making premium wines.
Horizontal screw press
Horizontal screw press: Is similar to a basked press mounted horizontally above a rectangular draining tray. It is less glentle than many other types of press and therefore less popular.
What does it mean batch processing
A volume of grapes is loaded into the press, they are pressed, the skins are removed, the press may be cleaned and the next batch is then loaded. Pneumatic, basket, horizontal press are batch processing
Continuous press
Allows grapes to be continually loaded into the press as it works b using a screw mechanism: This allows grapes to be continually loaded into the press as it work by using a screw mechanism: this allows for quicker pressing of large volumes of grapes. It’s less gentle and therefore best suited to producing high volumes of inexpensive wines.
Define pomace
The solid remains of the grapes left after pressing is called pomace
Must adjustments
The general aim is to create a more balanced wine, specially if there has been a compromise in achieving optimum ripeness of sugar, acids, tannins and flavours. Generally for white wines is made after clarification.
1. Enrichment
2. Reducing alcohol
3. Acidification
4. Deacidification
5. Adding tannins
