Wine making

The winemaking process (or vinification) is the production of wine, starting with the selection of the fruit, its fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over millennia. Oenology is the science of wine and winemaking. The growing of grapes is viticulture.  There are many varieties of grapes for example, in Spain there are over 400 varieties planted throughout the country although 80% of the country´s wine production is made from only 20 grapes.

The factors that affect a wine style can be divided into two broad categories:

  • Nature: the natural factors defining the vine growing environment
  • Human: viticulture and winemaking decisions taken by the winemaker.

Let’s look at each category separately.


The main natural factors which affect wine are listed below.


Through photosynthesis, grape vines (the source of wine) like all other plants transform sunlight, water and carbon dioxide into glucose and oxygen. The latter is liberated into the atmosphere. Glucose, the other product of photosynthesis, is the plant’s food, together with nutrients found in the soil.


Grape vines need five things to thrive: heat, sunlight, carbon dioxide, water and nutrients. Out of the five, carbon dioxide is more or less always present in the same quantities everywhere.

However, it is the availability of the other four elements that shape the style and quality of wine.


The amount of heat the vines receive is one of the most defining factors influencing the wine. Some grape varieties prefer cold climates like riesling and pinot noir.  Others prefer much warmer areas while others are more versatile and can cope with a range of climates – with different results.


Generally, the greater the amount of sunlight, the more the vines will photosynthesize and accumulate glucose (sugar) in their grapes. Since sugar is converted into alcohol during fermentation, grapes containing more glucose produce wines with higher alcohol content and a fuller body.

There is a relationship between sunlight and heat however not always. For example high altitude wine growing areas are characterized by high amounts of sunlight but cooler temperatures.


Vines need water for two main reasons: to allow photosynthesis and to swell their berries. Ideally, vines need more water in the early stages of their development. Once the vegetative growth is complete, reducing water availability is beneficial for the grapes. Limiting water during the ripening phase will result in more concentrated and flavoursome berries from which to make wine.


Latitude is the main factor affecting sunlight and heat. Most vineyards in the world are planted between 30o and 50o latitude. During summer, days are longer with increasing distance from the Equator.


Higher altitudes mean lower temperatures. Altitude can be beneficial, especially in hot regions. They allow the growth of grapes that otherwise wouldn’t survive at such latitudes. Altitude normally ensures a higher temperature diurnal range.

Aspect and slope

The term aspect means the direction in which a slope faces. The factor is relevant for higher altitudes, not flat land of course. In the Northern Hemisphere, South-facing slopes get the most warmth (in the Southern Hemisphere, the North-facing aspect is the warmest one). At more extreme latitudes sunlight does not fall perpendicular to the ground; therefore a favourable aspect can be the difference in getting enough heat to make viticulture possible. Vineyards sitting on slopes have some advantages, especially in moderate and cold regions. If oriented with the right aspect these vineyards receive more sunlight compared to the ones on flat lands. The cold air tends to sink as low as possible therefore vineyards on a slope minimize the risk of spring frost. Also, the soil has better drainage.

Ocean currents

Ocean currents can have a localized warming or cooling effect. For example, the Gulf Stream (a warm ocean current) increases the temperature in North-Western Europe.


Fog is another cooling effect, decreasing local temperatures; it helps cool down scorching heat.


Soil composition affects vineyard temperature. In particular, stone and dark soils retain heat and increase the local temperature. This is why, for example, the Left Bank of Bordeaux (whose soil is rich in gravel) is warm enough to allow late ripening varieties (such as cabernet sauvignon) to thrive.


The draining capability of the soil has a huge impact on the mesoclimate of an area. It also has a great influence on the style of the resulting wine. In fact, soil nature is one of the defining elements of the so-called terroir of a wine.

Soils with bigger particles (stones, pebbles, sand) have greater draining capacity. Clay-based soils, whose particles are smallest, can become waterlogged in case of excessive rainfall. These soils require more energy to heat up and are defined as “cold”.

The interplay between soils, geology and viticulture is complex, and the exact implications of soil composition on wine is far from fully understood – hence there is no such thing as the “perfect” wine growing area. As a general rule, the best soils for winegrowing are those that retain enough water and release it to the vines when needed.

Annual temperature range

This is the average difference in temperature between the hottest and the coldest months. It determines the length of the growing seasons and ultimately the amount of heat available throughout the year. This in turn affects which grape varieties are more suitable for a particular region.

In areas with a high annual temperature range, the seasons are well defined: the summers are hotter and the winters colder. As a result, the growing season for grapes is shorter. Usually regions further inland, far from large water masses, experience a high annual temperature range.

When the annual temperature range is small, the season differences are less defined, and the growing season is longer. This is beneficial for grapes that ripen late. The proximity to the sea or other large water masses generally favours a lower temperature range.

Diurnal range

Diurnal range is the difference in temperature between day and night. A higher diurnal range tends to be beneficial for grapes, especially in warm or hot climates. Cold nights allow the grape to rest and have a break from the heat of the day slowing down the loss of flavour compounds and acidity drop. This is why wines from higher altitudes are generally more elegant and aromatic, resulting in a fresher and higher acidity and a more defined character.


Temperature, sunlight and rainfall are the three factors that shape the overall climate of a region. Some regions experience more or less the same weather year in year out. In other areas, weather can differ greatly from one year to another. In these circumstances vintage is important.

Climate levels

In the wine world, people usually use three levels to refer to climate, with different degrees of granularity: macroclimate, mesoclimate and microclimate.

Macroclimate is the broad climate of an entire wine region. When talking about “climate” (without adding any prefix to the word), we generally refer to macroclimate.

Mesoclimate has a finer granularity, usually defining the conditions of a particular vineyard. Tens / hundreds of meters are the reference scale for a mesoclimate.

Microclimate has the finest granularity. It describes the conditions of a very small area, often in the order of one or few rows of vines.

Climates based on temperature

The average temperature during the growing season is used to classify wine regions into four climates: cool, moderate, warm and hot.

Styles vary greatly from one climate to another. Wines from colder climates typically tend to have a higher acidity and lighter body. Aroma-wise, these wines usually display a fresh fruit character; citrus, apple and pear fruit for whites, crunchy red fruit for reds.

On the other hand, wines from warmer regions often present a lower acidity, fuller body and higher alcohol content. The fruit aromas are much riper in warm regions: stone and tropical fruit for white wines, black and jammy fruit for reds.

The different climates and their effect on wine

The natural factors affecting wine are numerous and complexly intertwined. A number of variables exist to influence the environmental conditions of an area, which in turn affect the style and character of the wine produced.


The winemaking process is depicted below. There are many viticulture and winemaking decisions taken by the winemaker which will significantly influence the wine that is finally released.

The wine making process

1. Harvest

The exact moment for picking the grapes is the most sensitive decision wine growers have to make in the entire vineyard annual cycle. Timing is essential, and it does greatly influence the wine style. If a winemaker wants a riper, fruiter wine they will pick later rather than earlier. In a later harvest, grapes have higher sugar content, greater concentration of flavours and alcohol. An early harvest ensures there is less ripeness and alcohol in the wine but more acidity and variety in its aromatic profile. The key is balance. Therefore most winemakers will opt for a compromise between ripeness of fruit and acidity as their aim is to balance all the structural components in the juice.

Another important decision is the method of harvesting the grapes. The options available are manual or mechanised picking. The decision really depends on a number of factors. For example, the availability (and cost) of labour, the topography of the vineyard, weather conditions and the winemaker’s preferences.

Mechanized harvesting is less selective as the machinery shakes the vines and collects the fallen grapes, without differentiation. Its main advantage is speed so if timing is a constraint and the topography will allow its use, it will help to minimise oxidation or if bad weather is approaching.

Hand harvesting employs human labour to hand pick individual bunches of grapes. This process is time consuming, labour-intensive and usually more expensive (except where labour costs are lower). However, hand harvesting allows better grape selection as workers can be trained to reject less than optimal bunches. For this reason, hand picking is essential for certain styles of wine which require only grapes picked with the correct level of ripeness. In general, hand-picking causes less damage to vines.

2. Grape sorting and destemming

Once in the winery, the winegrower may choose for the grapes to be sorted. If they are sorted, human workers eliminate damaged or unsuitable grapes as well as the so-called “MOB” (Material Other than Grapes). MOB simply refers to the rubbish that can be unintentionally collected during the picking. Examples include: insects, small animals, leaves, garbage.

Next, the grapes are destemmed. Destemming is the process that separates vine stems from the actual grapes. Destemming is an optional step usually done by machinery which crushes the grape (this step is unnecessary with machine harvesting).

Some winemakers allow whole bunches (grapes attached to their original stems) to ferment with destemmed berries. This technique, now popular with some premium pinot noir, increases the aromatic profile and tannin levels of the resulting wine (vine stems contain a lot of tannins). Whole bunches are not usually included in the fermentation process of grape varieties which are naturally high in tannins or where stems are bitter (for example cabernet sauvignon).

3. Crushing

This is another optional process (it is not carried for whole bunches or carbonic maceration), crushing breaks the skins of the berries, liberating a liquid often called “free run juice”. This liquid is usually of the highest quality and can be fermented, matured separately, or included in the blend later on.

While crushing is usually a mechanised process, it can also be done by human feet. Crushing should be as gentle as possible, to avoid damaging pips or stems. If these are crushed, they may release bitter flavours, damaging the overall wine profile. The key is balance; too little crushing will lead to an insufficient aroma extraction, while too much extraction will result in astringent and bitter aromas.

4. Pre-fermentation maceration

After the grapes are sorted, destemmed and crushed, they are ready to be fermented.  Before fermentation starts, the winemaker may choose to add an additional step which is to leave the skins in contact with the must (crushed fruit and skin). In red wines, the practice is called “pre-fermentation maceration” or “cold soaking”; in whites it is normally referred to as “skin contact”. If temperatures are 6-7°C or more, it may become necessary for the winemaker to cool the must to prevent fermentation kicking in too early.

Winemakers use pre-fermentation maceration to increase the complexity and enrich the aroma of a wine. The inclusion of the skins increases the transfer of colour and flavour compounds to the wine. However, it has no effect on tannin extraction.

It’s easy to discern the whites that have had some skin contact. The colour of these whites are normally more intense and the nose reveals savoury and natural notes. When skin contact is prolonged, the colour continues to build all the way into the first stages of fermentation. This is how orange wine is usually made.

5. Pressing

Pressing is probably the most crucial activity that occurs before fermentation. Pressing separates the liquid from the solid constituents (the pips, skin and fibres that make up the grape). Pressing is achieved by compression. The traditional vertical presses are filled from above and have a plate that progressively squeezes the must. More modern presses use other approaches, like inflatable rubber membranes or stainless steel cylinders.

Pressing is different from crushing. Crushing simply breaks grape berries, allowing the juice, pulp, and seeds to mingle with the skins and stems of the grapes. Pressing, on the other hand, is the process that separates the grape juice from the fibre and other solids that make up a berry. In both processes, winemakers try to avoid damaging the solid constituents of the must in order to avoid bitter and astringent flavours seeping into the wine. With pressing, such damage is avoided by the use of gentle and controllable pressure. In Champagne production, pressing is one of the most delicate phases – pressure is meticulously controlled and highly regulated under the appellations rules.

While squeezing the must, liquid is collected at each stage and passed through other winemaking steps. The liquids released at different stages of pressing are not the same. Each pressing stage is called a fraction. Winemakers usually separate the liquids produced during different fractions. Fractions can either be blended together or vinified separately in different ways (for example, one fraction may see oak while others may not).  Generally, the juices coming through at the start of pressing are lighter in tannins and have bright pure fruity flavours. The liquid produced towards the end of the pressing tends to be more tannic and astringent.

The point at which pressing occurs is one of the defining differences between white and red winemaking. For white wines, pressing takes place before fermentation begins; while for reds, it´s afterwards. White wines are fermented without the skin, therefore they don’t display red/black pigments and have a significantly lower amount of tannins. As for reds, fermentation occurs in a must which contains skin, pips and other solid parts. That’s why reds differ so much from whites in terms of colour and tannin content; colour and tannins are extracted from the skin during fermentation.

6. Clarification

Clarification techniques allow winemakers to get rid of solids (insoluble particles, sediment) from their wine producing a clearer wine. Some of these techniques are controversial. There are winemakers who swear the removal of these particles strip away the true character and flavours of a wine. Therefore the winemaker’s preferences determine the type and extent clarification techniques are used in the wine.

Clarification techniques can be roughly grouped into three categories: racking, fining and filtration.

Racking is the gentlest and least intrusive of the clarification method.  For this reason, it’s the only clarification technique used for certain types of wine. The liquid (containing solid particles) is left in a vessel for some time. Thanks to gravity, the insoluble matter gradually settles at the bottom of the container. The liquid is then pumped into a different vessel, leaving the solid particles behind at the bottom of the original container. Repeated racking, from one vessel to another, allows the winemaker to get rid of most of these particles.

Fining is the addition of certain substances (albumin, gelatine, bentonite, carbon) to the wine. These substances (called “fining agents”) are capable of bonding with the particles floating in the wine, forming bigger particles. The larger particles formed during the fining process are subsequently removed by filtration.

Filtration is the physical process of removing particles from the liquid. Wine is passed through either a sieve or a filter with holes or pores small enough to trap the solids while the liquid is collected separately.

Fining and filtration are regarded as by winemakers as an aggressive clarification method. As a result, some winemakers refuse to use these techniques. Wine made by such winemakers will usually bear the words “unfiltered” or something similar on the wine label. Wines that have not been fined or filtered may appear slightly hazier to the eye, but there is no detriment to the quality and aromatic profile of the wine.

Clarification techniques can be adopted at different stages of the winemaking process. Often white wines undergo some form of racking procedure before the start of fermentation. This is to avoid skin and other solid particles remaining in the must during fermentation. If such particles are not removed, it can result in the wine developing undesirable savoury aromas.

After fermentation, racking is used in white wines, to separate the liquid from the lees (dead yeast cells that remain after fermentation has completed).  However, some winemakers, favour contact with lees as it gives the wine more body and structure.

Winemakers also use clarification processes just before the wine is bottled. This is to ensure that the liquid is clear and without solid matter that may be off-putting to customer. At this stage, a more fine-grained filtration may also be used to ensure microbiological stability. In other words, yeasts and bacteria are filtered out from the wine to avoid it spoiling later when these bottles are on shop shelves.

7. Alcoholic fermentation

Alcoholic fermentation is the basis for the manufacturing of all alcoholic beverages. This complex biochemical process converts sugar into ethanol (alcohol) and carbon dioxide (CO2), thanks to the action of yeasts. The reaction produces other by-products, such as heat and chemical compounds that ultimately determine the flavours in wine.

Without the winemaker´s intervention, alcoholic fermentation would continue until all the sugar in the must is completely converted into alcohol – obtaining dry wine. However, there are a number of ways that a winemaker can stop alcoholic fermentation early. These include: adding spirit or sulphur dioxide to the must or filtering out the remaining yeasts. Some sugar is still left in the wine – also known as residual sugars. Some sweet wines (but not all) are produced by the winemakers purposely leaving some unfermented residual sugars.

Winemakers control alcoholic fermentation in two ways: yeast and temperature. Yeasts are microscopic fungi that, in the absence of oxygen, feed on sugar and produce alcohol. Yeasts are literally everywhere and the types are countless. From a winemaking standpoint, we are interested in a specific species of yeasts called Saccharomyces cerevisiae. These yeasts are alcohol and sulphur dioxide tolerant, perfect for wine fermentation.

When it comes to fermentation, winemakers have two choices. They can use the yeasts naturally present in the surrounding environment (in the winery and on the grape bloom) or commercially cultivated yeasts.

Wild (or spontaneous) fermentation involves ambient yeasts. These yeasts produce more aromatic wine with interesting flavours and greater complexity. However, winemakers have less control over wild yeasts. They are inconsistent and unpredictable which can be a problem in large-scale wine production.

The other option is to use commercially cultivated yeasts. These yeasts have a narrower aromatic profile but are easily controlled. They consistently produce predictable, attractive flavours.

The second way winemakers can influence fermentation is through temperature control. Alcoholic fermentation does not take place if the temperature is below 5°C or above 35°C. Lower temperatures during fermentation favour the development of fruity flavours. They also reduce the loss of more volatile flavours such as floral and other subtle notes.

Higher temperatures during fermentation are necessary for the extraction of colour and tannins.

This is why white wines ferment at a significantly lower temperature when compared to red wines. In white wine fermentation temperatures range from 12 – 22°C. While temperatures in the fermentation of red wines range from 20 – 32°C.

Throughout the fermentation process, temperature is constantly monitored. The ease in which temperature can be controlled is one of the major advantages of modern winemaking. It is also one of the main reasons for the huge leap in wine quality over the last 20 years.

Alcoholic fermentation of the must can take place in different types of vessels: oak, concrete or stainless steel vats. However oak is never used for red wine fermentation and this is for a practical reason. Oak barrels are sealed, while in red wine fermentation the vessels need to be open.

During fermentation of red wine, the must contains pulp, skin and other solid compounds (collectively known as cap). The cap, if left alone, would just float on top of the fermenting liquid. This would lead to a poor extraction of colour and tannins. The different cap management techniques all aim to improve the contact between the cap and juice. Since these cap management techniques require the cap to be stirred, they make oak barrels (that are closed) impractical vessels for the purpose.

In addition, temperature control with oak vessels is inefficient due to the lack of embedded instruments. Therefore oak is used mainly for the fermentation of white wine, usually when the winemaker wants it to have toasty flavours, more body and some degree of oxidation.

Concrete and stainless steel vats are inert vessels. They don’t impart any aromas. Consequently, they favour a more reductive (as opposed to oxidised) style of winemaking and they allow easier temperature control.

8. Post-fermentation maceration

This is an optional step that only takes place in red winemaking. After fermentation has completed, a winemaker can decide to leave the fermented juice in contact with the skins for a variable amount of time (anything from 3 to 100 days).

Post-fermentation maceration (or extended maceration as it’s also referred to) primarily serves to enhance flavour and tannin extraction. For some grape varieties, winemakers have found that an extended period of maceration encourages a suppler and round tannic structure. This happens because over time the tannins undergo a polymerisation process. Their molecules bind, becoming bigger and less bitter.

The duration of the maceration usually depends on the grape variety and, above all, on the style the winemaker wants to obtain.

9. Rack-off lees

Only white wines undergo this optional step. When fermentation ends, the wine normally appears cloudy. This cloudiness is caused by dead yeast cells (called lees) and grape skin fragments which are still in the liquid. To remove these particles, some sort of clarification technique is used. The extent to which the wine is clarified at this stage is a winemaker decision.

Some winemakers may decide to leave some fine lees in contact with the liquid for a set time. This practice is called lees contact. If left alone, these small particles will settle at the bottom of the vessel forming hydrogen sulphide which would spoil the overall aroma of the wine. To avoid lees settling at the bottom of the vessel, winemakers usually use a technique called Bâtonnage or lees stirring. With the help of a long baton, the lees at the bottom of the barrel are periodically moved so that they fully mix with the wine.

Lees contact changes the body of the wine. It gives a richer texture and a denser mouthfeel. If lees contact is prolonged (typically more than 9 months), it also imparts flavours of its own. Bread dough, pastry, biscuits and a delicate toast-like undertone are typical. These flavours may sound familiar to Champagne lovers. That is because after the second fermentation in bottle, Champagne wine stays on their lees for several months.

10. Drain

This step is only relevant for red wine production. For white wines draining and pressing happen before fermentation. For red wines, they take place after fermentation.

Liquid wine is drained from the vat which allows unpressed juice (also referred to as “free run wine” or “vin de goutte”) to separate from the cap (the mass of pulp, skin and seeds). The remaining cap usually retains about 20% of the wine.

11. Pressing

This step is only relevant for red wine production. After draining, more juice can be obtained by pressing the cap as indicated previously. The fractions obtained in this way are usually richer in colour and tannins. See also section 5 Pressing.

The remaining cap, after draining, is pressed to squeeze out the remaining press wine (or “vin de presse”). The press wine tends to be dark, harsh and unpalatable. It is usually richer in colour and tannins and is then mixed with unpressed juice. After pressing out the liquid, the remaining cap (skins, pips and stalks – or “dry marc”) remains. It is often reused in the vineyard as fertilizer – in some regions it is distilled to make a clear alcohol, often flavoured with other fruits.

12. Malolactic fermentation

Malolactic fermentation (MLF) is a chemical reaction that transforms malic acid into lactic acid. The former is tart (think a crunchy green apple), the latter soft and buttery. When a wine is subjected to MLF, its perceived acidity is lower and broader.

MLF is carried out by a strain of bacteria called Oenococcus Oeni. MLF usually occurs after alcoholic fermentation has finished (but it can take place at other times). It spans from two weeks to three months in duration. Since there are bacteria in the environment capable of transforming malic acid into lactic acid, MLF will naturally occur unless it is artificially inhibited. However, it is common for winemakers to inoculate controlled strains of Oenococcus Oeni to have more predictable and consistent results.

MLF is carried out for virtually all reds and for some whites. In white winemaking, whether to apply MLF is an important decision. If a winemaker wants to impart a lower and broader acidity to their wine, and add buttery or creamy flavours, then MLF is used. Another reason why winemakers may want their wines to go through MLF is stabilisation. When a wine doesn’t undergo MLF, the reaction may take place anyway later in the bottle. This would turn the wine cloudy, giving it an unpleasant sour aroma. Additionally, wines subjected to MLF are also more resistant to microbiological spoilage.

There are circumstances where MLF should be avoided for instance, when it’s important to preserve a high acidity in the wine (such as sweet wines). Another example is when the winemaking focus is to retain the original primary flavours of the grape. This last reason explains why most aromatic grape varieties, such as sauvignon blanc and riesling, do not normally undergo MLF.

13 & 15. Blending

Blending is a far broader concept than one imagines. Winemakers can blend:

  • different grape varieties – the most common idea of blending
  • fruit coming from different vineyards – this routinely happens with most producers that buy grapes from growers
  • wines that are vinified in different ways – for example, blending a percentage of wine from a stainless steel vat with a portion of wine matured in an oak barrel
  • liquids coming from different pressing fractions – such as blending free-run juice with pressed wine
  • wine from different vintages – blending different vintage wine.

The possibilities are endless. Blending can happen at any stage of the vinification process. However, in most cases blending takes place straight after fermentation or during the maturation process. There are four main reasons why winemakers blend their wines: balance, complexity, consistency and style.

Balance. Blending allows winemakers to even out undesirable characteristics to achieve balance in their final product, hence enhancing its overall quality. There are many ways that a winemaker can achieve balance. For example, free run juice can be blended with pressed wine to give more tannin to a red wine. To prevent oak flavours dominating the final wine, a winemaker might choose a portion of the wine to be matured in oak then blend this with another portion that sees no wood at all.

Complexity. Strictly connected to balance is complexity. The concoction of different grape varieties (or different vinification processes) augments the aromatic complexity of a wine as different varieties confer different flavours and nuances.

Consistency. Consumers normally expect to taste the same product regardless of variations in bottles, production batches or vintages. Consistency is important. Even when maintaining the exact same vinification process, the resulting wine may differ from batch to batch. That is due to slight differences in the raw material (grapes), variation in vats or oak barrels, etc. Blending everything together in large vats is usually just what winemakers need to smooth out inconsistencies.

Style. Blending can also be carried out to achieve a particular style. Some winemakers’ top goal is to produce wines with a clearly recognisable house style, regardless of the variations that might occur from batch to batch, or from vintage to vintage.

Blending may occur after malolactic fermentation and/or after maturation.

14. Maturation

The term “maturation” refers to the period of time after fermentation but before the actual consumption of the wine. Not every wine benefits from long maturation. In fact, 90% of the wines are best consumed in the first one or two years after the vintage indicated on the label.

Another erroneous generalisation is that all premium wines can (and should) be aged. If the main objective of the winemaker is to retain and enhance the aromatic primary flavours of the grapes – then odds are the wine wouldn’t benefit from extended maturation (even if it’s a high quality wine). This is because with age, a wine tends to lose its fresh and fruity character and instead develops different types of flavours.

A wine benefits from long ageing when two conditions are met. Firstly, the wine needs to have structure. That is, a high level of tannins, acidity and/or alcohol. Secondly, the flavours in the wine need to be concentrated, so that they can evolve in an interesting way over time (typically, primary fresh fruit aromas fade, giving way to secondary and tertiary flavours). Even though this aspect is often overlooked, certain styles of white wines can also age beautifully. Since white wines don’t have tannins to act as preservatives, acidity is paramount.

Maturation can be either anaerobic or aerobic. This is an important distinction as it hugely impacts the chemical reactions happening in the wine over time.

Anaerobic maturation means that the wine ages without being in contact with oxygen. This happens if the maturation vessel is airtight (implying it does not allow air to come in contact with the liquid). Stainless steel and concrete vats are inert and airtight. Typically wine is stored for a limited time in those vessels – before being bottled.

The main container for anaerobic maturation is actually the glass bottle. Producers can bottle and store their wines in their storage areas for years before releasing them (most Rioja producers do this for Reserva and Gran Reserva wines). Alternatively, wine lovers can purchase bottles and store them for years (hopefully under the right conditions) before uncorking their matured wines.

Irrespective of the particular vessel used, anaerobic maturation has two important characteristics: lack of oxygen and no flavour imparted by the vessel. The extent to which these characteristics exist have an enormous influence on the way the wine develops over time. In anaerobic maturation, the wine will not feature secondary flavours (such as roasted nuts, cigar box, clove, etc.). Instead, the wine will maintain a purer bouquet where fruit dominates. Fresh fruit aromas will give way to cooked and preserved fruit. In red wine, think red (or black) fruit jam, prunes and dates. In white wine: dried apple skin, dried apricot and marmalade. Apart from this fruit development, other aromas emerge during anaerobic maturation these include: petrol, kerosene, toast, hay, mushrooms and honey for whites; while in reds flavours such as leather, earth, mushroom, tobacco, savoury and meat will emerge.

Aerobic maturation takes place in the presence of oxygen. Contact with oxygen (unsurprisingly) triggers oxidation – a chemical reaction which changes the structure and the aroma of the wine. Oxidative wines have a richer texture, greater complexity and a characteristic aromatic profile. These wines are less fruity but acquire savoury, toasty and earthy notes.

Oxidised vs Oxidative

In wine jargon, oxidised and oxidative denote different things – even though each has one thing in common – oxygen. An oxidised wine is a faulty wine: the liquid has been unintentionally and harmfully exposed to oxygen. Oxidised wines lose colour, fruity flavours and taste vinegary. While in an oxidative wine, the winemaker has deliberately decided to give the wine controlled exposure to oxygen. It’s a stylistic decision that affects the wine texture and flavours.

In aerobic maturation, wine is typically aged in some form of wood container. Oak is the preferred choice, both for historical and practical reasons. The use of wood to age wine for an extended period of time leads to a second major effect on the wine profile. Oak (like all wood) is not an inert vessel. It imparts its own specific flavours to the liquid contained within. With oak, things get a bit more complicated, so please bear with me.

The impact of oak maturation on wine depends on five main factors:

  • Size of the wood vessel: smaller barrels impart more flavours to the wine, as proportionally more liquid is in contact with the wood
  • Amount of time the wine is in contact with wood: as you can imagine, the more time the wine spends in contact with oak, the more the oak flavours are transferred to the wine
  • The extent of previous barrel usage: new barrels impart more flavours than older barrels.
  • The nature of the wood: generally two types of oak are used to build wine barrels: European and American oak. European oak tends to impart more savoury and toasty flavours (roasted wood, cigar box), while American oak gives more sweet aromas (vanilla, coconut).
  • The level of “toastiness” in the barrel: the way barrels are made matters. To create a barrel, wood staves are heated over an open fire, bent and held together with iron rings. During this process, the wood can be “toasted” to different degrees. A barrel that is highly toasted will confer more mocha and butterscotch nuances.

The combination of the oxidation and wood container described above affects the flavour of wine in a recognisable way. A wine that is matured in oak assumes a complex array of flavours: toast, toffee, butterscotch, cloves, nutmeg, charred wood, chocolate, coffee, cinnamon, vanilla.

The use of oak in maturation depends on several factors: grape variety (some varietals don’t cope well with wood), intensity of flavours (more concentrated wines can withstand oak without losing their original character) and – above all – the winemaker’s vision. Most of the times the winemaker is seeking balance.

Maturation is one of the stages where winemakers’ stylistic choices most clearly impact the wine’s character. There are some winemakers that avoid oak like a bad disease, claiming it covers the authentic flavours of wine. Others heavily use new oak and extensive maturation in bottle to accentuate secondary and tertiary flavours over fruity ones.

16. Clarification and stabilization

Clarification techniques may be applied to the wine after maturation as well if deemed appropriate by the winemaker to clear and stable it.. See also section 6 Clarification. Clarification tends to stabilize wine, since it removes some of the same particles that promote instability. The gradual oxidation that occurs during barrel aging also has a naturally stabilizing effect.

Stabilization. As a complex chemical mixture dependent on the activity of microorganisms, wine can be unstable and reactive to changes in its environment. Once bottled, a wine may be exposed to extremes of temperature and humidity, as well as violent movement during transportation and storage. These may cause cloudiness, sedimentation and/or the formation of tartrate crystals; more seriously, they may also cause spoilage or the production of carbonic gas. In these cases the winemaker may decide to utilize stabilization to make the wine evolve in a predictable manner.

There may be temperature instability which can be prevented by undergoing “cold stabilization”, in which it is cooled to near its freezing point to provoke crystallization before bottling. Microbiological instability is also possible if the wine has been sterilized by filtration making it possible for it to contain live yeast cells and bacteria. If both alcoholic and malolactic fermentation have run to completion, and neither excessive oxygen nor Brettanomyces yeast are present, this ought to cause no problems; modern hygiene has largely eliminated spoilage by bacteria such as acetobacter, which turns wine into vinegar. If there is residual sugar, however, it may undergo secondary fermentation, creating dissolved carbon dioxide as a by-product. When the wine is opened, it will be spritzy or “sparkling”. In a wine intended to be still this is regarded as a serious fault; it can even cause the bottle to explode. Similarly, a wine that has not been put through complete malolactic fermentation may undergo it in bottle, reducing its acidity, generating carbon dioxide, and adding a diacetyl butterscotch aroma. Brettanomyces yeasts add 4-ethylphenol, 4-ethylguaiacol and isovaleric acid horse-sweat aromas. These phenomena may be prevented by sterile filtration, by the addition of relatively large quantities of sulfur dioxide and sometimes sorbic acid, by mixing in alcoholic spirit to give a fortified wine of sufficient strength to kill all yeast and bacteria, or by pasteurization.

17. Packaging

Before being released on the market, it is crucial that the wine is packed into a suitable container. The vast majority of wine is stored in glass bottles because they are cheap to produce, sturdy, air-tight and most importantly don’t taint the wine. Because of this, to date glass represents the best material for long wine maturation. Of course, glass bottles do also have their disadvantages. They are heavy, rigid and add a significant amount to the cost of transportation.

However, glass is not the only choice for wine storage. Plastic bottles, “bag-in-box” packs, ceramic and aluminium cans are also viable alternatives.

Bottles need a form of closure to keep it sealed until the wine is ready to be consumed. Different types of closures vary the amount of air (oxygen) that can pass into the bottle and come in contact with the liquid. A small and controlled entry of oxygen allows the development of tertiary flavours. Other closures, on the other hand, are completely airtight. In those cases, no oxidation will occur. The goal of airtight closures is to preserve the fruit aromas of the wine.

There are different types of closures. Cork, the traditional closure for wines, is still the most widely used. Cork is a porous material which allows a small quantity of oxygen to come into contact with the wine contained in the bottle. Other closures include: synthetic (plastic) cork, screw caps and vino-seal (or glass stoppers).

18. Release

After the wine is bottled and labelled, it is technically ready to be sold into the market. In practice, some producers may decide (either due to stylistic reasons or regulation constraints) to keep the bottles for a while (allowing them to mature) before actually releasing them to the public.

The vast majority of wines are intended for early consumption, as previously mentioned. For the small minority of wine that can be aged, drinkers can decide how long to keep the wine before actually drinking it. There is no standard ideal maturation period for a wine. It really depends upon the grapes, wine style, producer and, above all, personal preference.



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