Wine production
From Wikipedia, the free encyclopedia
Wine production, often referred to as vinification, is the process of converting grape juice into wine. The most common genus of wine grape is Vitis vinifera, which includes nearly all varieties of European origin. Wine production can be generally classified into two categories: Still wine production and sparkling wine production.
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[edit] Still Wine Production
Still wines are produced by undergoing a single alcoholic fermentation in tank or barrel such that the resulting wine is not effervescent. Most still wines are produced using a combination of the following fundamental steps: harvesting, crushing, pressing, fermentation, secondary (malolactic) fermentation, aging and bottling. 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.
[edit] Harvest
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.
[edit] Crushing
Crushing is the process of removing the grapes from the rachis and breaking the skins of the grapes. This task is typically accomplished with a single piece of equipment called a "crusher/destemmer" or sometimes just "crusher". Grapes are removed from the rachis to avoid the unwanted extraction of flavors or tannins which 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.) If increased skin extraction is desired, a winemaker might choose to crush the grapes after destemming. In these cases the grapes pass between to 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. 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.)
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.
[edit] 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.
