Winemaking, or vinification, is the process of wine production, from the selection of grapes to the bottling of finished wine. Wine production can be generally classified into two categories: still wine production and sparkling wine production. The science of wine and winemaking is known as oenology.

Process

After the harvest, the grapes are crushed and allowed to ferment. Red wine is made from the must (pulp) of red or black grapes that undergo fermentation together with the grape skins, while white wine is usually made by fermenting juice pressed from white grapes, but can also be made from must extracted from red grapes with minimal contact with the grapes' skins. Rosé wines are made from red grapes where the juice is allowed to stay in contact with the dark skins long enough to pick up a pinkish color, but little of the tannins contained in the skins.

During this primary fermentation, which often takes between one and two weeks, yeast converts most of the sugars in the grape juice into ethanol (alcohol). After the primary fermentation, the liquid is transferred to vessels for the secondary fermentation. Here, the remaining sugars are slowly converted into alcohol and the wine becomes clear. Some wine is then allowed to age in oak barrels before bottling, which add extra aromas to the wine, while others are bottled directly. The time from harvest to drinking can vary from a few months for Beaujolais nouveau wines to over twenty years for top wines. However, only about 10% of all red and 5% of white wine will taste better after 5 years, compared to after one year.Depending on the quality of grapes and the target wine style, some of these steps may be combined or omitted to achieve the particular goals of the winemaker. Many wines of comparable quality are produced using similar but distinctly different approaches to their production; quality is dictated by the attributes of the starting material and not necessarily the steps taken during vinification.

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 to the wine after the fermentation is done, a technique known as  
süssreserve.

The grapes

Of all factors affecting the quality of a wine, the quality of the grapes more than any other factor determines the quality of the wine. Their quality is not only affected by their variety, but also by the weather during the growing season, the soil, the time of harvest, and the way they are pruned. The combination of these effects is often referred to as their terroir. The most common genus of wine grape is Vitis vinifera, which includes nearly all varieties of European origin.

The grapes are usually harvested from the vineyard in the autumn (fall), in the northern hemisphere from early September until the beginning of November, or the middle of February until the beginning of March in the southern hemisphere.

Harvesting

Harvest is the picking of the grapes and in many ways the first step in wine production. Grapes are either harvested mechanically or by hand. The decision to harvest grapes is typically made by the winemaker and informed by the level of sugar (called °Brix), acid (TA or Titratable Acidity as expressed by tartaric acid equivalents) and pH of the grapes, as well as berry flavor, tannin development and overall disposition of the grapevine and weather forecasts. °Brix is a measure of the soluble solids in the grape juice and represents not only the sugars but also acids and to a lesser degree esters, terpenes and tannins. The level of sugar in the grapes is important not only because it will determine the final alcohol content of the wine, but also because it is an indirect index of grape maturity.

Mechanical harvesters are large tractors that straddle grapevine trellises and, using firm plastic or rubber rods, strike the fruiting zone of the grapevine to dislodge the grapes from the rachis. Mechanical harvestors have the advantage of being able to cover a large area of vineyard land in a relatively short period of time, and with a minimum investment of manpower per harvested ton. A disadvantage of mechanical harvesting is the indiscriminate inclusion of foreign non-grape material in the gondola by the action of the harvestor. Depending on the trellis system and grapevine canopy management, this foreign material can be moldy grapes, leaves, canes, metal debris, rocks and even small animals. Some winegrowers remove leaves and loose debris from the grapevine before mechanical harvesting to avoid such material being included in the harvested fruit. In the United States mechanical harvesting is seldom used for premium winemaking because of the indiscriminate picking and increased oxidation of the grape juice. In other countries (such as Australia and New Zealand), mechanical harvesting of premium winegrapes is more common because of general labor shortages.

Manual harvesting is the hand-picking of grape clusters from the grapevines. In the United States, grapes are traditionally picked into 30 pound boxes, and in many cases these boxes are consolidated into ½ ton bins or two-ton bins for transport to the winery. Manual harvesting has the advantage of using knowledgeable labor to not only pick the ripe clusters but also to leave behind the clusters that are not ripe or contain bunch rot or other defects. This can be an effective first line of defense to prevent inferior quality fruit from contaminating a lot or tank of wine.

Destemming
 
The corkscrew shaped feed auger sits on top of a mechanical crusher/destemmer. Grape clusters are then fed into the machine where they are systematically destemmed and transferred to the crusher.Destemming is the process of separating stems from the grapes. Depending on the winemaking procedure, this process may be undertaken before crushing with the purpose of preventing the development of crude tannins and unpalatable vegetal flavors in the resulting wine.

Crushing and primary fermentation

Crushing is the process of removing the grapes from the rachis and breaking the skins of the grapes. In smaller-scale wine making, the harvested grapes are sometimes crushed by trampling them bare-footed. However, in larger wineries, a mechanical crusher/destemmer is used. Because the stems of the grapes have a relatively high tannin content, they are usually removed beforehand; otherwise they can give a vegetal aroma to the wine (due to extraction of 2-methoxy-3-isopropylpyrazine which has an aroma reminiscent of green bell peppers.) However the winemaker can decide to leave them in if the grapes themselves contain less tannin than desired. If increased skin extraction is desired, a winemaker might choose to crush the grapes after destemming. In these cases the grapes pass between two rollers which squeeze the grapes enough to separate the skin and pulp, but not so much as to cause excessive shearing or tearing of the skin tissues. In some cases, notably with "delicate" red varietals such as Pinot noir or Syrah, all or part of the grapes might be left uncrushed (called "whole berry") to encourage the retention of fruity aromas through partial carbonic maceration.

Most red wines derive their color from grape skins (the exception being varieties or hybrids of non-vinifera vines which contain juice pigmented with the dark Malvidin 3,5-diglucoside anthocyanin) and therefore contact between the juice and skins is essential for color extraction. Red wines are produced by destemming and crushing the grapes into a tank and leaving the skins in contact with the juice throughout the fermentation (maceration). It is possible to produce white (colorless) wines from red grapes by the fastidious minimalization of contact between grape juice and skins (as in the making of Blanc de noirs sparkling wine, which is derived from Pinot noir, a red vinifera grape.)

A cap of grape skins forms on the surface of fermenting red wine.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 grapeseeds, as well as maintaining proper juice flow through a matrix of grape clusters rather than loose berries. In some circumstances winemakers choose to crush white grapes for a short period of skin contact, usually for three to 24 hours. This serves to extract flavor and tannin from the skins (the tannin being extracted to encourage protein precipitation without excessive Bentonite addition) as well as Potassium ions, which participate in bitartrate precipitation (cream of tartar) to increase the pH of the juice. This was a practice more common in the 1970's than today, though still practiced by some Sauvignon blanc and Chardonnay producers in California.

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

Yeast is normally already present on the grapes, often visible as a powdery appearance of the grapes. The 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.

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 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.

During or after the alcoholic fermentation, malolactic fermentation can also take place, during which specific strains of bacteria convert malic acid into the milder lactic acid. This fermentation is often initiated by inoculation with desired bacteria.

Pressing

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; if grapes are crushed there is a considerable amount of juice immediately liberated (called free-run juice) that can be used for vinification. Typically this free-run juice is of a higher quality than the press juice. However, most wineries do use presses in order to increase their production (gallons) per ton, as pressed juice can represent between 15%-30% of the total juice volume from the grape.

Presses act by positioning the grape skins or whole grape clusters between a rigid surface and a moveable surface and slowly decrease the volume between the two surfaces. Modern presses are able to follow a pressing program which dictates the duration and pressure at each press cycle, usually ramping from 0 Bar to 2.0 Bar. Sometimes winemakers choose pressures at which they wish to separate the streams of pressed juice, which is called making "press cuts." As the pressure increases on the grape skins so too increase the amount of tannin extracted into the juice, 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 pericarp, or skin, and seeds), pressed juice or wine tends to be lower in acidity with a higher pH than the free-run juice.

Before the advent of modern winemaking, most presses were basket presses made of wood and operated manually. Basket presses are composed of a cylinder of wooden slats on top of a fixed plate, with a moveable plate that can be forced downward (usually by a central ratcheting threaded screw.) The press operator would load the grapes or pomace into the wooden cylinder, place the top plate in place and begin to lower it until juice began to flow from the wooden slats. As the juice flow decreased to a minimum, the plate was ratcheted down again until a similar flowrate was achieved. This process would continue until the press operator determines that the quality of the pressed juice or wine is below standard, or all liquids have been pressed from the grape skins. Since the early 1990s, modern mechanical basket presses have seen a resurgence amongst higher-end producers seeking to replicate the gentle pressing of the historical basket presses. Because basket presses have relatively compact design, the press cake offers a longer relative pathway through which the juice must travel before leaving the press. It is believed by advocates of basket presses that this relatively long pathway through the grape or pomace cake serves as a filter to solids that would otherwise negatively impact the quality of the press juice.

With red wines, the must is pressed after the primary fermentation, which separates the skins and other solid matter from the liquid. With white, and rosé wine, there is no need to press. The wine is separated from the dead yeast (called its lees), and transferred to a new container for its secondary fermentation.

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.

Secondary fermentation and Bulk Aging
 
White oak barrels are often used for the bulk aging process.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 of wine, 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.

Amateur winemakers often use glass carboys, sometimes called demijohns with a capacity of 4.5 to 25 liters (approximately 1 to 6 gallons) to produce their wine. The vessel used for the process depends on both the amount of wine that is being produced, the grapes being used, and the goals of the winemaker.

Blending and Bottling

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.

A final dose of sulfite is added to help preseve 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.

Winemakers

A winemaker (AKA a vintner) is a person engaged in making wine. They are generally employed by wineries or wine companies, where their work includes:

-Cooperating with viticulturists
-Monitoring the maturity of grapes to ensure their quality and to determine the correct time for harvest
-Crushing and pressing grapes
-Monitoring the settling of juice and the fermentation of grape material
-Filtering the wine to remove remaining solids
-Testing the quality of wine by tasting
-Placing filtered wine in casks or tanks for storage and maturation
-Preparing plans for bottling wine once it has matured
-Making sure that quality is maintained when the wine is bottled

Today, these duties require an increasing amount of scientific knowledge, since laboratory tests are gradually supplementing or replacing traditional methods. Hence the vast majority of winemakers have, or are studying for, a Bachelor of Science degree (or similar) majoring in oenology. Winemakers can also be referred to as oenologists as they study oenology - the science of wine.                                                                                      

Source:Wikipedia

See also: www.d-i-wine.com, www.infozeek.net