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Harvesting and Juicing the Wine Grapes

by Staff Writer - B. Shaughnessy | February 08, 2012

The essential component of wine is quality grape juice. It is therefore paramount that the juices of wine grapes are extracted in an efficient way while not sacrificing the quality of the grape. Depending on the type of wine produced as well as the conditions of the grape itself, vintners have a multitude of options to choose for grape juice extraction.

The technical name for the substance produced from grape crushing is must. With the production of must, several factors must be taken into account before production can begin. First, the grapes must be harvested and potentially be de-stalking. From this point grapes are crushed and in the case of white wine drained. Continuing with white wine production, the drained skins are further pressed and the juices are collected from this process. In red wine production the must is pressed after the juice has been fermented.

The first critical decision that must be made is the grape harvest date. Grapes must reach a stage where the balance between sugars and acids are optimal. The grapes must also be fully ripened and contain matured polyphenols with browned pips. This is important to prevent unripe tastes from being introduced into the resulting juice. First indications of grape maturity are seen during the veraison stage of grape growth. In the veraison stage of grape growth, the grapes begin to show signs of ripening with a change in skin color. Dark black grape varieties begin to show color in their skins while white grape varieties begin to show a degree of translucence in their skins. Metabolic changes to the grapes are also seen within the veraison stage of grape growth. Specifically within the veraison stage, sugars begin to accumulate while the presence of acids begins to decrease. The final decision to harvest a crop is determined by scientific methodology and ultimately a taste test by the vintner.

After a grape crop has been determined to be optimal for harvest, the next question that must be answered is which method of harvesting will be used. The two methods of grape harvesting available are either manual picking with field workers or the use of mechanized harvesting machines. Both processes have their advantages and disadvantages. Also, one very important step in the process of harvesting is the application of an anti-oxidation compound such as potassium metabisulfite before harvesting commences.

The method of harvesting is determined by the required quality level of the grape as well as the available financial resources of the winery. The method of picking grapes by hand is used when the resulting grapes must be of the highest quality. Hand picking grapes is ideal for the production of sweet wines where the selection of noble rotted berries is essential. Hand picking may also be employed when vineyards are inaccessible to the mechanized grape picking machines. The downside to hand picking are the high costs and time required.

Mechanical grape harvesting is a method where a machine is responsible for picking the grapes from the vine. Mechanical harvesters are dedicated tractor-like devices that use strong vibrations to shake the grapes free of the vine. The benefits in using a mechanical grape picker include decreased costs and decreased harvesting time. Many vintners have come to prefer the use of mechanical pickers to hand picking. The main reason for this is the decreased picking time. With the reduced time required for picking, vintners can zero in on the absolute ideal time for a grape crop to be harvested. Mechanical pickers are not without their problems. The nature of the vibrations used to shake grapes free of the vines can be violent and damaging. However the process of engineering mechanical harvesters is continually moving forward. Mechanical harvesters built today utilize better technologies that will only continue to improve over time.

One of the first decisions to be made after harvesting is the fate of the grape stalk. Grape stalks are a good source of tannins and can be included within the must if additional tannic properties are desired. However if these qualities are not desired than the harvested grapes must be destalked. This is a fairly simple process accomplished by a mechanical device. The grapes are fed into a cylinder with perforated holes found on the cylinder surface. These holes are roughly the diameter of the grapes. Arrays of fingers are mounted to a spindle running down the center of the cylinder. When loaded with grapes, the fingers rotate and encourage the grapes to fall off the stalks and through the holes perforated in the cylinder. The action of the fingers resembles a kitchen mixer moving at a slow speed. After the destalking process is complete, the grapes are advanced to the next stage of must production while the stalks can be reutilized for the production of a vegetable based spirit.

The next phase of must production involves the crushing of the grapes. Traditionally the process of crushing was performed with human feet stomping the grapes. This process has all but been eliminated with modern winemaking. Today specialized machines have been developed to control the crushing process, as several key criteria must be maintained. During the crushing process it is imperative that the skins of the grape are not torn or mishandled. Improper handling of the grape skins can damage the extraction of aromas. Stalks must be handled as gently as possible and not ripped open as this would lead to a rich influx of tannins in the resulting must. The main objective of crushing is the extraction of free-run juice and the proper handling of the solid parts of the grape. Grape crushers are designed with this criteria in mind and can perform the action of crushing effectively and efficiently.

After the crushing of the grapes, different procedures are utilized for the type of wine being used. In red wine production, the resulting must is fermented and then grape solids are pressed. In white wine production, the grape solids are pressed first, before fermentation.

Pressing involves extracting additional juices as well as polyphenols and aromas from the grape skins. In the case of white wine production, the skins are drained from the juice as pressing occurs before fermentation. The mechanics of pressing are generally all the same for different grape types. Pressing occurs when a special machine applies pressure to the mass of grape skins to extract the remaining juices. Historically the technology used for pressing grapes was a simple basket with a vertical screw designed to apply downward pressure. This press design was simple and remained unchanged for thousands of years. Unfortunately the basket press design has several drawbacks in its operation. In a basket press operation, juices are extracted very slowly and oxidation is a significant problem. Therefore, modern press designs have been designed to overcome the limitations of the simple basket press design. Press designs include the horizontal screw press that automates the loading and unloading of grape skins, the pneumatic press that features an efficient low squeeze pressure juice extraction technique, tank presses which can perform the actions of pressing in an oxygen free environment and finally continuous screw presses for large operations requiring non-stop grape pressing in a high volume environment.

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