Maturation Flashcards
Explain maturation?
Once alcoholic fermentation and malolactic conversion (when allowed to occur) have been carried out, the wine can either be finished and packaged relatively quickly or it may be matured for a period of time before it is bottled.
Young, fruity or aromatic styles of wine that have been protected from oxygen throughout the winemaking process (such as many examples of New Zealand Sauvignon Blanc or Veneto Pinot Grigio) are likely to be kept protected from oxygen at this stage to best retain their primary aromas. They may be stored in vessels that are impermeable to oxygen, such as those made from stainless steel, and kept protected by use of SO2 and/or blanketing with inert gas. They are likely to be finished and packaged shortly after the end of fermentation.
Some winemakers making these styles of wines choose to bottle only when an order from a customer has been placed (hence the wine is stored in bulk), or they choose to ship the wine in bulk. Storing the wine in bulk means the winery has more flexibility when it comes to selling the wine through the year (depending on demands it can be packaged with different labels, sold in bulk or blended with other wines). Storage in bulk is often more efficient in terms of winery space than the same volume of wine kept in stacks of bottles, and it also means wine labels are less likely to get dirty or become ruined (e.g. by humidity).
By contrast, for many winemakers the period of maturation after fermentation is essential in determining the style and quality of wine. The key factors that may have an influence on the wine during maturation are oxygen, new wood and yeast lees; not all of these are always used, depending on the style of wine that is being aimed for.
Blending can have a significant impact on the style, quality and price of the wine, and although it may take place at any point in the winemaking process, it is often carried out at the end of maturation process.
Explain the role of oxygen in maturation?
Oxidation reactions lead to a number of changes in the aromas and phenolic compounds in wine, and therefore have a marked influence on the style and quality of the wine.
Generally, oxidation leads to a gradual reduction in many primary aromas and a development of tertiary aromas, such as dried fruits and nuts. If the primary aromas develop into pleasant tertiary aromas, and the wine has a structure suitable for extended ageing, this can maintain or enhance the quality of the wine. If the primary flavours fade and are not replaced by positive tertiary characteristics, this can result in a reduction in quality.
Additionally, oxidation can influence the colour of wines. With exposure to oxygen, white wines become darker, gradually becoming gold and then brown. Exposing a young red wine to oxygen can result in greater colour stability and intensity. In this reaction, anthocyanins bind with tannins, which, for example, protects anthocyanins from being adsorbed by yeast lees or bleached by SO2 additions. This is particularly important for light-coloured red wines such as Pinot Noir. After much more extensive ageing, red wines gradually become paler and browner.
For red wines, oxidation reactions also seem to result in the softening of tannins, which may be positive for quality. This area of science is not yet well understood but may at least in part be linked to anthocyanin and tannin bonding.
The speed of oxidation is influenced by the amount of oxygen exposure, the compounds in the wine and temperature, among other factors. Wine that is fully exposed to the air (and hence oxygen) will generally oxidise quicker than when oxygen exposure is more gradual (e.g. wine kept in a large oak vat). That said, red wines can 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). This is why red wines are often matured for longer than white wines, with 12–24 months pre-bottling maturation being common for red wines and 6–12 months more common for white wines. Temperature is also important with warm temperatures increasing the speed of oxidation reactions.
A high level of oxidation is a key part of the style of some wines (such as Oloroso Sherry), the oxidation evident from the brown colour of the wine, and lack of fresh fruit on the palate instead replaced by nuts and dried fruits. In this case, the wine is stored for a number of years in barrels that are not filled to the top, allowing oxygen within the headspace of the barrel (ullage). More commonly, for un-fortified red and white wines, the exposure to oxygen is limited and therefore has a much slower, subtler effect. Too much oxidation in these wines is usually considered a fault.
Explain wooden vessels and oxygen?
The traditional, and still commonly used, method of gaining a slow, gradual exposure to oxygen is to store the wine in small wooden vessels. 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 also continues to pass through the gaps between the staves and the bung hole. (Note that there are different schools of thought on how wine in barrel is exposed to oxygen. Some state that oxygen only enters through the bung hole, while others suggest oxygen passes through the wood itself.) However, the times when the wine is most exposed to oxygen in this process is during any transfer of the wine, such as during racking, lees stirring or topping up in which the bung is removed.
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. This leads to a gradual concentration of the other components of the wine. Small vessels, such as barrels, have a large surface area to volume ratio, increasing the rate at which wine is lost. This means that barrels frequently need to be topped up with more wine to keep them full (and hence avoid ullage and excessive oxidation). The frequency of topping up leads to more oxygen exposure in these vessels than in much larger vessels. The amount of oxygen that enters the barrel relative to the volume of wine is also larger for small vessels compared to larger vessels, also leading to quicker (but not extreme) oxidation.
Explain micro-oxygenation?
However, maturing wine in barrels is costly for a number of reasons (see The Role of Wood in Maturation) and therefore micro-oxygenation is a technique that can provide a cheaper alternative. Micro-oxygenation involves bubbling oxygen through wine. The dose of oxygen is typically in units of mg/L per month. Micro-oxygenation is generally carried out in stainless steel tanks for a number of months post alcoholic fermentation. Initially, micro-oxygenation was generally used on inexpensive and mid-priced wines; however, a growing number of wineries producing premium and super-premium wines are using the technique.
Micro-oxygenation is thought to increase colour stability and intensity, soften tannins, improve texture and reduce the presence of any unripe, herbaceous flavours. Additional benefits are that it provides the effects of gentle exposure to oxygen more quickly than barrel ageing without needing expensive barrels, and the rate of oxygen exposure can be controlled much more tightly than it could be in a barrel. If used in conjunction with oak alternatives (such as chips or staves), it can also help to integrate the influence of the oak.
However, care must always be taken because the increasing oxygen levels in the wine can provide a more favourable environment for spoilage microbes such as acetic acid bacteria and Brettanomyces. This is a relatively new technique and therefore research into which grape varieties or types of wines benefit from micro-oxygenation and how wines made by micro- oxygenation age in bottle over the long term (and whether there is any difference compared to the same wine matured in barrel) is lacking.
Explain temperature and humidity in maturation?
Wines are usually matured in stable, cool temperatures to slow down the rate of oxidation and other reactions and lower the chance of microbial spoilage. White wines tend to be stored at 8–12°C (46–54°F), whereas red wines are stored a little warmer at 12–16°C (54–61°F) as they are less susceptible to the effects of oxidation. Stable, cool temperatures give a slower maturation than warm temperatures (e.g. 20°C / 68°F and above) due the effect of temperature on chemical reactions that happen between various compounds within the wine. However, warmer temperatures do not simply speed up the ageing. The temperature determines what reactions can occur and how quickly various reactions will occur. Unfortunately, many of the reactions that benefit from warm temperatures are undesirable in terms of wine quality.
Some wine is generally lost through the maturation process in wooden vessels. Low humidity and warm temperatures increase the rate of wine loss. This is undesirable as it reduces the volume of wine to sell; barrels need topping up more often (greater cost through labour requirement) and risk of oxidation is increased (harmful for quality). Conditions of low humidity (under approximately 70 per cent) 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, which in many instances may be seen as undesirable.
For these reasons, wines are usually matured in cool cellars of constant temperature and humidity. Temperature- and humidity-control systems are common in modern cellars. Many traditional cellars were built underground for this same reason.
Explain the role of wood in maturation?
AGE OF THE VESSEL
New wood contains various extractable compounds. Each time the barrel is used, the amount of extraction decreases because fewer extractable compounds remain. (A barrel loses about 50 per cent of its new oak flavours during the first year of use.) By the time the vessel is on its fourth usage, it will contribute very little at all (but still allow ingress of oxygen). The flavours of new oak may be too dominant or clash with the flavours of some wines (particularly aromatic grape varieties) and a number of winemakers choose to use entirely pre-used barrels. Where new oak is used, it is typically as a proportion of the blend with wine that has been stored in pre-used barrels.
SIZE OF THE VESSEL
Small vessels, such as barriques (225 l), hold a relatively small volume of liquid compared to the surface area of the vessel. This means that any extraction from the wood and exposure to oxygen is greater in small vessels than in large vessels, such as foudres.
TYPE OF WOOD
The most common type of wood used for winery vessels is oak. It can easily be shaped into a barrel and, importantly, makes containers that are watertight. Oak is also prized for its positive effects on the aroma/flavour and structure of the wine. Different species of oak have different characteristics, but it is possible for the same species of oak to show different characteristics depending on where it is grown. Most winery oak vessels are made from European oak (typically French, but also Hungarian, Russian or Slavonian) or American oak. Both European and American oak have significant levels of vanillin, which contributes aromas of vanilla and is usually a key part of the aroma/flavour of oaked wines. However, one of the key differences is that American oak has been found to contain much higher levels of lactones, which give aromas of coconut. In general, American oak tends to impart a greater intensity of aromas/flavours than European oak, which is often said to be subtler. By contrast, European oak tends to impart more tannin.
The production process for barrels made from European oak is more expensive than that for American oak and this is part of the reason why French oak vessels are more expensive to buy than American oak vessels. The cost of American oak barrels tends to range from €300–€600, whereas that for French oak barrels tends to range from €600–€1200. European oak must be split to create staves, whereas American oak can be sawn. The ability to saw American oak means more vessels can be made from the same amount of oak. American oak also grows significantly quicker than European oak.
Another variable related to the type of wood is the tightness of the grain. A more tightly grained wood is the result of slower growth of the tree. For example, oak trees grown in continental climates, particularly Russia and Hungary, tend to grow slowly and therefore have the tightest grains. Wood with a tight grain slows down the extraction of compounds compared with coarser grained wood, and can alter what particular compounds are most extracted. For example, Hungarian oak, which tends to have a tight grain, is gaining popularity in New York State as an option that allows very gradual extraction of oak aroma compounds and tannins for their red wines (many of which are medium in alcohol, body, tannins and flavour intensity) so that they are not dominated by oak-derived characteristics.
Although oak is by far the most commonly used wood in wine production, other types of wood such as chestnut, cherry and acacia, are occasionally used.
PRODUCTION OF THE VESSEL
The way in which the vessel is made has a vital influence on how it will affect the wine, at least for new or nearly new vessels. Before the vessel is constructed, the wood needs to be seasoned. This typically takes place outside and lasts 2–3 years. This lowers the humidity levels in the wood, reduces bitter flavours and increases some aroma compounds, such as those that give flavours of cloves. The barrel production process involves heating the staves so that they can be bent into shape. This heating process also transforms the tannins and aroma compounds in the oak. The temperature and length of heat exposure is referred to as the level of toasting, and barrels are typically categorised as either light-, medium- or heavy-toasted. Toasting contributes notes of spice, caramel, roasted nuts, char and smoke. Many of these characteristics are more pronounced at heavy levels of toasting. Many cooperages produce their vessels to a house style (including how long they season the wood, exactly how long and how intensely or gently they heat the wood for the different toast levels), and it is common for wineries to use barrels from a range of coopers to increase blending options.
LENGTH OF TIME IN WOOD
The level and type of compounds extracted from wood vessels will also depend on the length of time that the wine is aged in the vessel. Very broadly, the longer the wine is aged in a wooden vessel the greater the extraction of compounds from the wood and the greater the exposure to oxygen. However, the rate at which various aroma compounds and tannins are extracted from the oak is not constant throughout the ageing process. Indeed some compounds are extracted more or less quickly at different periods of time and therefore longer ageing in wood can lead to a greater diversity of wood-derived aroma compounds. Extraction from wood is also highly dependent on other factors, for example the size of the vessel, type and age of wood, and environment in the cellar etc.
COST OF MATURATION IN WOOD
Barrel maturation is expensive for several reasons and therefore is typically only used for wines that are mid-priced, premium or super-premium. First,
barrels are expensive to buy. Small barrels, such as barriques, do not hold much liquid, which means that hundreds of these vessels may be necessary, even in modest-sized wineries. Second, monitoring the wine in each separate barrel and performing any winemaking operations, such as lees stirring or racking, is labour intensive. Spoilage microbes such as Brettanomyces like to live in wood and therefore meticulous cleaning and sanitation is also needed. Finally, barrel maturation is a slow process, and therefore it is not uncommon to store the wine for 1–2 years or even longer. In most cases the wine is only sold when it is packaged and reaches the market, therefore return on investment is slow.
OAK ALTERNATIVES
For these reasons, producers of inexpensive and mid-priced wines may choose to use alternatives to barrels when the flavours of oak are desired. Alternatives include oak chips and oak staves. Oak staves can be attached to the inside of the stainless steel or concrete vessel, or they can float in the wine. Oak chips are generally placed in a permeable sack, which is left to soak in the wine. For both options it is possible to purchase different species of wood and different seasoning and toasting levels. They are much cheaper than purchasing a barrel, can be added to a large tank of wine (therefore less labour intensive) and their large surface area means they are quick to have an effect. The amount needed for the desired stylistic effect can also be finely tuned. Some winemakers use micro-oxygenation alongside oak alternatives to replicate the gentle oxidation of barrel maturation as this helps integrate the oak flavours.
It is difficult to define the influence of oak alternatives on wine style and quality compared to that of oak vessels. The precise effects depend, among other factors, on the source, size and toasting level of the wood, and whether micro-oxygenation is used alongside the addition of oak alternatives.
Explain the role of lees in still wine maturation?
Lees describes the sediment that settles at the bottom of a wine vessel. It is made up of dead yeast, dying yeast and bacteria, grape fragments, precipitated tannins, nutrients and other insoluble compounds. The sediment that forms quickly after the end of fermentation (within the first 24 hours) is called gross lees. This is made up of the larger, heavier particles. Smaller particles may settle more slowly, and when they gradually form a sediment they are known as fine lees. The lees may be removed by racking. The first racking after fermentation removes the gross lees, and then subsequent periodic racking helps to manage the levels of fine lees. Some winemakers choose to keep the wine in contact with the lees. This has several effects.
After fermentation, the yeast cells will very slowly die and break down (autolysis), releasing compounds that contribute flavours, body and texture to the wine. Some of these compounds bind with phenolic compounds in the grapes, reducing colour and softening tannins. Compounds from the lees also bind with certain extractable components of the wood, such as wood tannins and flavours, and can therefore reduce astringency and modify the flavours from the wood. The precise aromas/flavours that lees contribute are hard to define. A range of compounds are released from the yeast, and these compounds can react with aroma compounds already in the wine. In white wines, where the effects of lees are generally more significant, aromas from lees are variously described as yoghurt-like, dough-like, biscuity or toasty, however, these can be very subtle.
Lees ageing also helps in the stabilisation of the wine against unstable proteins that can cause hazes. Generally, the effects of lees ageing are more significant in white wines, and the effect of gross lees is more significant than that of fine lees.
The lees also help to protect the wine from oxygen, helping to maintain a slow, controlled oxidation during maturation and lowering the need to use SO2 during this time. However, if the layer of lees is too thick, particularly a problem for gross lees, it can produce volatile, reductive sulfur compounds. At certain concentrations, some of these compounds can add complexity (e.g. giving aromas of struck match and smoke). However, if not controlled, these compounds may contribute unpleasant aromas, such as the smell of rotten eggs that comes from high levels of hydrogen sulphide, a fault termed reduction. Lees provide nutrients for microbes,
and therefore can assist the growth of lactic acid bacteria for malolactic conversion, but also encourage the development of spoilage microbes such as Brettanomyces.
Lees ageing may increase the time the wine is stored at the winery before release, and this can increase the cost of the final wine. Wines aged on lees (particularly gross lees) need to be monitored regularly and may be stirred, which incurs a labour cost. For example, acceptable to good, inexpensive Rueda wines are finished, packaged and released from the winery shortly after the end of fermentation. Those of higher quality may be aged in tank on their lees for a few months before bottling, and generally are sold at slightly higher prices.
If the wine is being matured in barrels, there is unlikely to be any significant additional cost to lees ageing on top of that for barrel maturation. Lees stirring may be costly in terms of labour if large numbers of barrels need to be opened and individually stirred.
Lees stirring is also known as batonnage.
Explain racking?
Racking is the process of transferring wine from one vessel to another with the aim of removing sediment from the wine. This sediment may be gross lees, fine lees or other solid material in the wine that has fallen to the bottom of the vessel over time (e.g. tiny fragments of grape skin or tartrate crystals). The wine is removed from the original vessel through a valve near the bottom of the vessel (above the top of the sediment) and pumped or poured into the top of a new, clean vessel.
Racking can be an oxidative process. Some makers of red wine will increase the oxygen exposure by deliberately splashing the wine. It is also feasible to protect the wine from oxygen during this time, which may be preferential for aromatic or fruity wines, by using pressure from inert gas to push the wine out through a hose into the new vessel, which would have been flushed with an inert gas.
Racking may be carried out once or several times during the maturation process, depending on how long the wine is stored, whether it is stored with or without lees and whether the winemaker wants to increase oxygen exposure by racking.
Explain blending?
In winemaking, the term ‘blending’ usually refers to the mixing together of two or more batches of wine, as opposed to the blending of two different substances (e.g. mixing Süssreserve into the wine to sweeten it). Blending can take place at any time during the winemaking process, but is most often carried out just prior to finishing and packaging. Blending may involve combining wines:
- from different grape varieties
- from different locations (from different vineyards, different regions or even different countries)
- from different grape growers or businesses that sell grapes, must or wine
- from different vintages
- that have been treated differently in the winery (e.g. wine made from free run juice and wine made from press juice, or wine matured in oak with wine that has been stored in stainless steel or concrete)
- that have been treated equally in the winery but are in different vessels for logistical reasons (e.g. unless a wine is made in very small quantities, wine fermented or matured in barrels will need to be blended together to make up the required volume).
What can and cannot be blended and in what proportions often depends on local legislation. For example, in the EU, for a wine to be labelled with a Protected Designation of Origin (PDO), 100 per cent of the grapes must come from the defined geographical area. Equally, Brunello di Montalcino DOCG must be made from 100 per cent Sangiovese, whereas in Chianti Classico DOCG, Sangiovese can be blended with other specified grape varieties.
What are the key reasons for blending wines?
Balance
– Blending may help to increase or moderate the levels of certain characteristics of the wine to produce a wine that is better balanced, and in this way enhance quality. For example, a batch of wine from a warmer vineyard or that has been picked late may be blended with a batch of wine from a cooler vineyard or that has been picked early to fine tune acidity levels. The use of different grape varieties is a common way to enhance the balance of the wine, and there are many well-known blending partners. For example, Merlot provides body and ripe, plummy fruit to a blend with Cabernet Sauvignon, which, when not fully ripe, can be too astringently tannic on its own.
Consistency
– Significant variation among bottles of a single product in a single vintage is usually seen as a fault. Therefore, blending can be necessary to ensure a certain volume of a consistent product. There are some wine styles where consistency is needed across different years. This includes most styles of Sherry and non-vintage sparkling wine, but also many inexpensive wines where the consumer may not expect nor accept significant vintage variation.
Style
– Blending is also often fundamental in reaching a desired style. Even where vintage variation may be accepted and promoted, winemakers often make their wines to a certain ‘house style’ or want to create wines of certain quality levels within their range (e.g. a premium or super-premium wine for bottle ageing and a lower-priced wine (e.g. mid-priced) for earlier consumption). Equally, some rosés are made by blending red and white wine, and this method allows very precise control over colour and flavour profile (how much contribution the red wine makes).
Complexity
– The blending of two or more parcels of wine may lead to a greater range of flavours, and in this way enhance the complexity and therefore quality of the final wine.
Minimise faults
– Blending may also be carried out to help reduce the presence of a wine fault. For example, if wine in one barrel is showing significant volatile acidity (see Faults), that wine may be sterile-filtered to remove acetic acid bacteria and then blended into a larger volume of un-faulty wine to lower the concentration and sensory perception of acetic acid.
Volume
– In areas with small vineyard holdings, winemakers are likely to need to blend the wines from different vineyards to produce viable volumes of certain wines. Similarly, in poor vintages or in cases where the producer has minimal vineyard holdings, grapes, must or wine may need to be bought in to satisfy the needs of their customers.
Price
– Many wines, especially those that are inexpensive or mid-priced, will be made to be sold profitably at a certain price point. In this case, blending different parcels of wine can help to create a certain style and quality. For example, Chardonnay is sometimes blended with grape varieties such as Trebbiano and Semillon. The latter two grapes are generally cheaper to buy than Chardonnay, keeping the price point of the wine low. As one of the most recognised grape varieties internationally, the inclusion of Chardonnay in the blend helps the wine to sell.
Of course, there are also reasons why a winemaker might choose not to blend certain grape varieties or parcels of wine. For example, a winemaker may choose to produce wine that comes from a single vineyard to maintain the character of the fruit and/or quality of wine coming from that particular vineyard. The term ‘single vineyard’ on the label can also make the wine seem more rare and distinctive from a marketing point of view. Additionally, some grape varieties have pronounced and distinctive aromas (such as Sauvignon Blanc and Riesling) and blending with other grape varieties may dilute the character of these grape varieties.
The blending process generally starts with blending trials using measuring cylinders and small volumes of wines to distinguish the proportion of each wine in the favoured blend before conducting on a much larger scale. This is a process that requires a high level of skill and experience. The blending process can be extremely challenging, especially when blending young wines that are destined for long ageing; the winemaker needs to estimate how the blend will develop in the future. Logistics and business also play a part; money is lost if volumes of wine are not used or sold on to other producers.
Blending is best carried out before stabilisation in case any instabilities arise from the blend; for example, tartrate stability is dependent on pH level, and this can be altered by blending wines with different pH levels.
