Fermented Grape

how wine is produced

Debunking the world of wine

Discovering the world of wine

Demystifying the world of wine

wine fermentation

How wine is made

Harvesting grapes

The harvesting of wine grapes is one of the most important steps in the process of wine making. The time of the harvest is determined primarily by the ripeness of the grape as measured by sugar, acid and tannin levels but there is a huge degree of skill and intuition in choosing the perfect picking day.  Winemakers make their decision to pick based on the style of wine they wish to produce and the decision of when to pick is one of the key events that a vineyard owner will need to make. Picking too early or too late can dramatically impact a vintage with the harvest season between August and October in Europe and the United States and February and April in countries such as Australia, New Zealand and Chile. 

The weather is a major consideration with the wine maker juggling ripeness with the threat of heat, rain and hail which can damage the grapes and bring about disease. Rain during a harvest can dilute the grape juice and damage the flavour of the finished wine. 

 Vineyard owners use either the traditional picking by hand or mechanical harvesters and the latter has the advantage that night picking is easier which can ease subsequent handling. The question of using mechanical harvesting versus traditional hand picking is a source of contention in the wine industry. Mechanical harvesting of grapes has been one of the major changes in many vineyards in the last 50 years or so. First introduced commercially in the 1960's, it has been adopted in different wine regions for various economic, labour and wine making reasons.

A mechanical harvester works by beating the vine with rubber sticks to get the vine to drop its fruit onto a conveyor belt that brings the fruit to a holding bin. As technology improves mechanical harvesters have become more sophisticated in distinguishing grape clusters from leaves and other debris. But despite the improvement in equipment many harvesters still have difficulties in distinguishing between ripe, healthy grapes and unripe or rotted bunches which must then be sorted out manually at the winemaking facility. Another disadvantage is the potential of damaging the grape skins which can cause maceration and colouring of the juice that is particularly unwanted in the production of white and sparkling wine. The broken skins also bring the risk of oxidation and a loss of some of the aromatic qualities in the wine.

Despite the costs, some wineries prefer the use of human workers to hand-pick grapes. The main advantage is the skill of the worker to pick only healthy bunches and the gentler handling of the grapes. The production of some dessert wine like Bordeaux's Sauternes and  Tokaji Aszu from Hungary require that individual berries are picked from the bunches with noble rot which can only be done by hand. In very hilly areas with steep slopes it is generally not possible to run a mechanical harvester through the vineyard.

Throughout the history of wine, winemakers would use the sugar and acid levels of the grape as a guide in determining ripeness. Early winemakers tasted the grapes to gauge ripeness but nowadays winemakers usually use a refractometer to measure sugar levels and Brix or titration tests (using an indicator such as phenolphthalein) to determine the acidity within the grape.

Today winemakers place more of an emphasis on the "physiological" ripeness of the grape in terms of aspects such as the tannins. Currently, tasting is the only way to measure tannin ripeness, which takes experience and skill to do accurately. Viticulturists have not yet fully explained the complex processes that go into the ripening of tannins but most believe it begins with the polymerisation of small astringent tannins into larger molecules which are perceived by the taste buds as being softer.

From small picking baskets or trays, the wine pickers than pour them into a larger container which is then taken to the winery.

Into the winery

After the harvest, the grapes are taken into a winery and prepared for primary fermentation.

The key steps are: 

  • Sorting grapes for quality. Any rotten or raisined grapes, along with leaves and petioles, are removed. 
  • Destemming is the process of removing the grapes from the rachis (the main stem which holds the grapes). The decision to remove the stems is different for the production of red and white wines.  Most white wines are processed without destemming or crushing and are transferred from picking bins directly to the press. This is to avoid any extraction of tannin from either the skins or grape seeds. In some circumstances winemakers choose to crush white grapes for a short period of skin contact, usually for three to 24 hours to extract flavour and tannin from the skins. For red wine making, stems of the grapes are usually removed before fermentation since the stems have a relatively high tannin content and can give the wine certain aromas, particularly of green peppers.  Sometimes the winemaker may decide to leave the stems if the grapes themselves contain less tannin than desired, particularly if the stems have started to turn brown.
  •  Crushing is the process which squeezes the berries and breaks the skins to release the precious juice. Most larger wineries use a mechanical crusher/destemmer for these processes, but some small producers still use manual methods including the feet.In some cases, particularly with delicate grape varieties such such as Pinot noir, the grapes might be left uncrushed (called "whole berry") to keep fruity aromas through partial carbonic maceration. 

    It is possible to produce white wines from red grapes by the careful pressing of uncrushed fruit since this minimises contact between grape juice and skins. This process also increases the pH of the juice which may be needed if the grape juice is too acidic.

    In the case of rosé wines, the fruit is crushed and the dark skins are left in contact with the juice just long enough to extract the colour that the winemaker desires. The must is then pressed, and fermentation continues as if the winemaker was making a white wine.

Primary fermentation

Red wine is made from the must of red or black grapes, usually after destemming and produced by crushing the grapes into a suitable vessel. Must is the freshly pressed grape juice that contains the skins and pulp of the grape with the solid part of the must being called pomace. Fermentation occurs together with the grape skins, which give the wine its colour, a process called maceration.

White wine is made by fermenting juice which is made by pressing crushed grapes to extract a juice and the skins are removed. Sometimes white wine is made from red grapes by extracting their juice with minimal contact with the grape skins. Rosé wines are either made from red grapes where the juice is allowed to stay in contact with the dark skins long enough to pick up a pinkish colour or by blending red wine and white wine. White and rosé wines extract little of the tannins contained in the skins.

To start primary fermentation yeast may be added to the must or may occur naturally as a result of a yeast on the grapes or in the air. During this fermentation, which often takes between one and two weeks, the yeast converts most of the sugars in the grape juice into ethanol (alcohol) and carbon dioxide. The carbon dioxide is lost to the atmosphere and it is important that wine making facilities are well ventilated for reasons of safety.

After the primary fermentation of red grapes the free run wine is pumped off into tanks and the skins are pressed to extract the remaining juice and wine. The pressed wine is blended with the free run wine at the winemaker's discretion. The wine is kept at the correct temperature and the remaining sugars are converted into alcohol and carbon dioxide.

Yeast is normally already present on the grapes, often visible as a powdery appearance of the grapes. The primary, or alcoholic fermentation can be done with this natural yeast, but since this can give unpredictable results depending on the exact types of yeast that are present, cultured yeast is often added to the must. One of the main problems with the use of wild ferments is the failure for the fermentation to go to completion, that is some sugar remains unfermented. This can make the wine sweet when a dry wine is desired. Frequently wild ferments lead to the production of unpleasant acetic acid (vinegar) production as a by product.

During the primary fermentation, the yeast cells feed on the sugars in the must and multiply, producing carbon dioxide gas and alcohol. The temperature during the fermentation affects both the taste of the end product, as well as the speed of the fermentation. For red wines, the temperature is typically 22 to 25 °C, and for white wines 15 to 18 °C. For every gram of sugar that is converted, about half a gram of alcohol is produced, so to achieve a 12% alcohol concentration, the must should contain about 24% sugars. The sugar percentage of the must is calculated from the measured density, the must weight, with the help of a specialized type of hydrometer called a saccharometer. If the sugar content of the grapes is too low to obtain the desired alcohol percentage, sugar can be added (chaptalization). In commercial winemaking, chaptalization is subject to local regulations.

Alcohol of more than 12% can be achieved by using yeast that can withstand high alcohol. Some yeasts can produce 18% alcohol in the wine however extra sugar is added to produce a high alcohol content.


Pressing is the act of applying pressure to grapes or pomace in order to separate juice or wine from grapes and grape skins. Pressing is not always a necessary act in winemaking but if grapes are crushed there is a considerable amount of juice immediately released which is called free-run juice. 

Modern presses dictate the duration and pressure at each press cycle, usually ramping from 0 Bar to 2.0 Bar. Sometimes winemakers choose pressures which separate the streams of pressed juice, called making "press cuts." As the pressure increases the amount of tannin extracted from the skins into the juice increases, often rendering the pressed juice excessively tannic or harsh. Because of the location of grape juice constituents in the berry (water and acid are found primarily in the mesocarp or pulp, whereas tannins are found primarily in the exocarp, or skin, and seeds), pressed juice or wine tends to be lower in acidity with a higher pH than the free-run juice.

With red wines, the must is pressed after primary fermentation, which separates the skins and other solid matter from the liquid. With white wine, the liquid is separated from the must before fermentation . With rosé, the skins may be kept in contact for a shorter period to give colour to the wine, in that case the must may be pressed as well. After a period in which the wine stands or ages, the wine is separated from the dead yeast and any solids that remained (called lees), and transferred to a new container where any additional fermentation may take place.

Pigeage is a French winemaking term for the traditional stomping of grapes in open fermentation tanks. To make certain types of wine, grapes are put through a crusher and then poured into open fermentation tanks. Once fermentation begins, the grape skins are pushed to the surface by carbon dioxide gases released in the fermentation process. This layer of skins and other solids is known as the cap. As the skins are the source of the tannins, the cap needs to be mixed through the liquid each day, or "punched," which traditionally is done by stomping through the vat.

Cold stabilisation is a process used in winemaking to reduce tartrate crystals (generally potassium bitartrate) in wine. These tartrate crystals look like grains of clear sand, and are also known as "wine crystals" or "wine diamonds". They are formed by the combination of tartaric acid and potassium, and may appear to be sediment in the wine, though they are not. During the cold stabilising process after fermentation, the temperature of the wine is dropped to close to freezing for 1–2 weeks. This will cause the crystals to separate from the wine and stick to the sides of the holding vessel. When the wine is drained from the vessels, the tartrates are left behind. They may also form in wine bottles that have been stored under very cold conditions.

Secondary fermentation

The next process in the making of red wine is secondary fermentation. This is a bacterial fermentation which converts malic acid to lactic acid and is therefore called Malolactic fermentation . This process decreases the acid in the wine and softens the taste of the wine as malic acid is generally unpleasant.

This is carried out either as an intentional procedure in which specially cultivated strains of such bacteria are introduced into the maturing wine, or it can happen by chance if uncultivated lactic acid bacteria are present.

Malolactic fermentation usually results in an increase on the pH of the wine. This should be monitored and not allowed to rise above a pH of 3.55 for whites or a pH of 3.80 for reds. 

The use of lactic acid bacteria is the reason why some chardonnays can taste "buttery" due to the production of diacetyl by the bacteria. All red wines go through complete malolactic fermentation, both to lessen the acid of the wine and to remove the possibility that malolactic fermentation will occur in the bottle. White wines vary in the use of malolactic fermentation during their making with lighter aromatic wines such as Riesling generally not undergoing the process.  The fuller white wines such as barrel fermented chardonnay, are more commonly put through malolactic fermentation. Sometimes a partial fermentation, for example, somewhere less than 50% might be employed.

Variations on the above procedure exist. With sparkling wines such as Champagne, an additional fermentation takes place inside the bottle, trapping carbon dioxide and creating the characteristic bubbles. Sweet wines are made by ensuring that some residual sugar remains after fermentation is completed. This can be done by harvesting late (late harvest wine), freezing the grapes to concentrate the sugar (ice wine), or adding a substance to kill the remaining yeast before fermentation is completed; for example, high proof brandy is added when making port wine. In other cases the winemaker may choose to hold back some of the sweet grape juice and add it later after the fermentation is done, a technique known as süssreserve.

During the secondary fermentation and aging process, which takes three to six months, the fermentation continues very slowly. The wine is kept under an airlock to protect the wine from oxidation. Proteins from the grape are broken down and the remaining yeast cells and other fine particles from the grapes are allowed to settle. Potassium bitartrate will also precipitate, a process which can be enhanced by cold stabilization to prevent the appearance of (harmless) tartrate crystals after bottling. The result of these processes is that the originally cloudy wine becomes clear. The wine can be racked during this process to remove the lees.

The secondary fermentation usually takes place in either large stainless steel vessels with a volume of several cubic meters, or oak barrels, depending on the goals of the winemakers. Unoaked wine is fermented in a barrel made of stainless steel or other material having no influence in the final taste of the wine. Depending on the desired taste, it could be fermented mainly in stainless steel to be briefly put in oak, or have the complete fermentation done in stainless steel. Oak could be added as chips used with a non-wooden barrel instead of a fully wooden barrel. This process is mainly used in cheaper wine.

Barrel aging of wine and maturation

Red wine is often transferred to oak barrels to mature for a period of weeks or months after primary fermentation with the intention to increase complexity and concentration of flavour & aroma to the wine and soften harsh tannins. After this maturation the wine must then be settled or clarified and adjustments made prior to filtration and bottling. 

The most common barrels are the Bordeaux barriques style which hold 225 litres (59 US gallons) followed by the Burgundy style barrel which hold 228 litres (60 US gallons) and is called a pièce. Some producers use the hogshead 300-litre (79 US gallons) barrel. 

Because the word 'barrique' is so famous and associated with the wines of Bordeaux, the term 'barrique aged' is often incorrectly used to describe any wine that has been aged in small barrels to distinguish it from wines aged in much larger oak casks. Barrique only refers to the size of the barrel and does not mean that it is new or used oak; high, medium or low toast or French, American or any other country.

Since an oak barrel is porous it allows evaporation and oxygenation to occur in wine but typically not at levels that would cause it to be spoiled. A  Bordeaux barriques barrel once filled can lose around 20-25 litres of mostly alcohol and water in a year through evaporation and the winemaker has to top up the barrel to overcome this loss. Since small amounts of oxygen are allowed to pass through the barrel this helps reduce the intensity of tannins.

Phenols within the wood of the barrel  interact to produce specific flavours associated with vanilla. The degree of "toast" on the barrel also gives the finished wine certain flavours. White wines matured in oak can have flavour characteristics including caramel, cream, smoke, spice and vanilla. Mocha and toffee flavours can be added to red wine by maturing in oak.

The length of time a wine spends in the barrel is dependent on the grape variety and style of wine. The majority of oak flavour is added to the wine in the first few months the wine is in contact with oak but longer maturing can increase the impact of certain phenolic compounds. Generally speaking most wines are aged for 1-2 years but there are exceptions.

French oak typically comes from the forests of Allier,Limousin, Nevers, Tronçais and Vosges each having their own characteristics. American oak is generally grown in Missouri, Minnesota and Wisconsin where many wine barrels are from. 

The tighter grain of French oak allows for a more gradual integration of flavours in the wine and American oak is generally more intensely flavoured than French oak with more sweet and vanilla notes. Winemakers choose American oak typically for big reds, base wines for "assemblage", or for warm climate Chardonnays. Besides being derived from different species, a major difference between American and French oak comes from the preparation of the wood. The tighter grain and less watertight nature of French oak means coopers need to split the wood along the grain. The wood is then aged or "seasoned" for 24 to 36 months in the open air, in a so-called wood-yard. Even though American coopers may use a kiln-dry method to season the wood, almost all others will season American oak in exactly the same way as French. Open air seasoning has the advantage of leaching undesirable chemical components and bitter tannins, mellowing the oak in a way that kiln-dry methods are incapable of replicating.

New barrels give more flavours than do previously used barrels. Over time many of the oak properties get "leached" out of the barrel with layers of natural deposits left from the wine building up on the wood to where after 3 to 5 vintages there may be little or no oak flavours being added to the wine. In addition, oxygen transport through the oak and into the wine, which is required for maturation, becomes severely limited after 3–5 years. 

Laboratory testing

Whether the wine is aging in tanks or barrels, tests are run periodically in a laboratory to check the wine's quality. Common tests include °Brix, pH, titratable acidity, residual sugar, free or available sulfur, total sulfur, volatile acidity and percent alcohol. Additional tests include those for the crystallization of cream of tartar (potassium hydrogen tartrate) and in the case of white wines the precipitation of heat unstable protein.

These tests may be performed throughout the making of the wine and just before bottling. If the results of laboratory testing are worse than desired a winemaker can take action, for example the addition of more sulfur  dioxide and sensory testing may also lead to remedies such as the addition of a protein to soften the taste of the wine.

°Brix is one measure of the soluble solids in the grape juice and represents not only the sugars but also includes many other soluble substances such as salts, acids and tannins, sometimes called Total Soluble Solids (TSS). Brix is usually measured with a refractometer while the other methods use a hydrometer. 

Because sugar is the dominant compound in grape juice, these units are effectively a measure of sugar level. The level of sugar in the grapes determines the final alcohol content of the wine as well as indirect index of grape maturity. Brix (Bx for short) is measured in grams per hundred grams of solution, so 20 Bx means that 100 grams of juice contains 20gm of dissolved compounds. There are other common measures of sugar content of grapes, Specific gravity, Oechsle (Germany) and Beaume (France). In the French Baumé (Be° or Bé° for short) one Be° corresponds approximately to one percent alcohol. One Be° is equal to 1.8 Brix, that is 1.8 grams of sugar per one hundred grams. Therefore, to achieve one percent alcohol the winemaker adds 1.8 grams per 100 ml or 18 grams per litre, a practice known as chaptalization.

The Volatile Acidity test confirms if there is any steam distillable acids in the wine, mainly acetic acid (the dominant component of vinegar), but lactic, butyric, propionic, and formic acid can also be found.  The amount of volatile acidity found in sound grapes is negligible, because it is a by-product of microbial metabolism. Because acetic acid bacteria require oxygen to grow, eliminating any air in wine containers or the use of sulfur dioxide and inoculation with a low-V.A. producing strain of Saccharomyces may deter acetic acid producing yeast. In addition, removing mouldy grapes also prevents possible problems associated with acetic acid bacteria.  A relatively new method for removal of volatile acidity from a wine is reverse osmosis. Blending may also help so a wine with high V.A. can be filtered (to remove the microbe responsible) and blended with a low V.A. wine, so that the acetic acid level is below the required level.

Sulfur dioxide can be readily measured with relatively simple laboratory equipment. There are several methods available; a typical test involves acidification of a sample with phosphoric acid, distillation of the liberated SO2, and capture by hydrogen peroxide solution.

Blending, fining and finishing

Different batches of wine can be mixed before bottling in order to achieve the desired taste. The winemaker can correct perceived inadequacies by mixing wines from different grapes and batches that were produced under different conditions. These adjustments can be as simple as adjusting acid or tannin levels, to as complex as blending different varieties or vintages to achieve a consistent taste.

Fining agents are used during winemaking to remove tannins, reduce astringency and remove microscopic particles that could cloud the wines. The winemakers decide on which fining agents are used and these may vary from product to product and even batch to batch (usually depending on the grapes of that particular year).

Gelatin has been used in winemaking for centuries and is recognized as a traditional method for wine fining, or clarifying. It is also the most commonly used agent to reduce the tannin content. Generally no gelatin remains in the wine because it reacts with the wine components, as it clarifies, and forms a sediment which is removed by filtration prior to bottling.

Besides gelatin, other fining agents for wine are often derived from animal products, such as micronized potassium casseinate (casein is milk protein), egg whites, egg albumin, bone char, bull's blood, isinglass (Sturgeon bladder), PVPP (a synthetic compound), lysozyme, and skim milk powder. Some aromatized wines contain honey or egg-yolk extract.

Non-animal-based filtering agents are also often used, such as bentonite (a volcanic clay-based filter), diatomaceous earth, cellulose pads, paper filters and membrane filters (thin films of plastic polymer material having uniformly sized holes).

The most common preservative used in winemaking is sulfur dioxide, normally added in one of the following forms: liquid sulfur dioxide, sodium or potassium metabisulphite. Another useful preservative is potassium sorbate.

Sulfur dioxide has two primary actions, firstly it is an anti microbial agent and secondly an anti oxidant. In the making of white wine it can be added prior to fermentation and immediately after alcoholic fermentation is complete. If added after alcoholic ferment it will have the effect of preventing or stopping malolactic fermentation, bacterial spoilage and help protect against the damaging effects of oxygen.

Additions of up to 100 mg per litre (of sulfur dioxide) can be added, but the available or free sulfur dioxide should be measured by the aspiration method and adjusted to 30 mg per liter. Available sulfur dioxide should be maintained at this level until bottling. For rose wines smaller additions should be made and the available level should be no more than 30 mg per liter.

In the making of red wine, sulfur dioxide may be used at high levels (100 mg per liter) prior to ferment to assist in colour stabilisation. Otherwise, it is used at the end of malolactic fermentation and performs the same functions as in white wine. However, small additions (say 20 mg per liter) should be used to avoid bleaching red pigments and the maintenance level should be about 20 mg per liter. Furthermore, small additions (say 20 mg per liter) may be made to red wine after alcoholic ferment and before malolactic ferment to overcome minor oxidation and prevent the growth of acetic acid bacteria.

Without the use of sulfur dioxide, wines can readily suffer bacterial spoilage no matter how hygienic the winemaking practice.


Potassium sorbate is effective for the control of fungal growth, including yeast, especially for sweet wines in bottle. However, one potential hazard is the metabolism of sorbate to geraniol which is a potent and unpleasant by-product. The production of geraniol occurs only if sorbic acid is present during malolactic fermentation. To avoid this, either the wine must be sterile bottled or contain enough sulfur dioxide to inhibit the growth of bacteria. Sterile bottling includes the use of filtration.

Filtration in winemaking is used to accomplish two objectives, clarification and microbial stabilization. In clarification, large particles that affect the visual appearance of the wine are removed. In microbial stabilization, organisms that affect the stability of the wine are removed therefore reducing the likelihood of re-fermentation or spoilage.

The process of clarification is concerned with the removal of particles; those larger than 5–10 micrometers for coarse polishing, particles larger than 1–4 micrometers for clarifying or polishing. Microbial stabilization requires a filtration of at least 0.65 micrometers. However, filtration at this level may lighten a wine's color and body. Microbial stabilization does not imply sterility. It simply means that a significant amount of yeast and bacteria have been removed.

Clarification of the wine can take place naturally by putting the wine into refrigeration at 35°F (2°C). The wine takes about a month to settle and it is clear. No chemicals are needed.

Manipulating wine

Because of a difficult season or climatic changes some wine makers are forced to employ controversial techniques to change the composition of the finished wine.

These include:

  • Spinning cone - This is a treatment which reduces alcohol levels using a series of spinning filters in a vacuum. The technique is particularly controversial and almost never openly used by wine producers.
  • Reverse osmosis - Used to concentrate a wine or remove certain compounds it is a high efficiency filtration technique which can separate water and  alcohol from flavour/aroma producing compounds. 
  • Micro-oxidation - A well used technique where oxygen is piped into the fermentation tanks to soften tannins.
  • Flash Détente - Must is heated rapidly then cooled quickly to make unripe, vegetal grapes palatable when a harvest is particularly poor. 
  • Addition of acid -Some wine makers in warmer climates add Tartaric Acid where alcohol and fruit dominate.
  • Chaptalization -The addition of sugar to grape must in cool climate areas to boost fermentation. 

Bottling and labelling

A final dose of sulfite is added to help preserve the wine and prevent unwanted fermentation in the bottle. The wine bottles then are traditionally sealed with a cork, although alternative wine closures such as synthetic corks and screwcaps, which are less subject to cork taint, are becoming increasingly popular.