Pressing-Clarification of White Grape Must
Wine Grape Presses
Presses can be divided into two major categories: batch presses and continuous presses. Batch pressing is performed not in a single step, but with several cycles of pressing interrupted by crumblings of the press contents to redistribute them and open new paths for juice drainage. Examples of batch presses include basket, moving head, and membrane presses. The second category of presses is the continuous press, which is based on an infinite Archimedes screw that presses the pomace slowly to an outlet whose size is controlled. This enables the operator to set the exerted pressure on the pomace as desired.
The basket press, otherwise known as the vertical screw press, has remained virtually unchanged since its inception by the monks of the Middle Ages (Figure 4.2). Although originally made using wood slats most basket presses use stainless steel. The mode of operation is simple, merely increasing the pressure on the mass of skins by the lid of the press, causing the cells to rupture and releasing their contents. The pressing can be done by a rotating action on a vertical, central metal capstan, or by means of hydraulic pressure on a disk on top of the skins.
Advantages and Disadvantages
The quality of juice extracted by vertical hydraulic presses is indisputable, since the pressure is exerted without grinding the grapes. The juice has a low concentration of suspended solids due to the filtration resulting from the cake thickness. Because of the nature of the pressing, the grape solids remain reasonably static, resulting in a wine that may be less bitter than would be obtained from a membrane or moving head press.
The free-run can vary from 450 to 500 liters (120-130gal) per ton, depending on the cultivar and preceding winemaking procedures. Further juice or wine recovery will depend on the pressure and duration of pressing and can be an additional 200 to 250 liters (50-65gal). The total press cycle can last 1.5 to 24 hours. The larger the diameter of the press, the more quickly the juice will run away from the bed of grape skins.
Reds versus White Varieties
Wineries that specialize in delicate and finicky varietals such as Pinot Noir or Viognier tend to prefer basket presses. Conversely, the hard-to-press varietals such as Riesling, Muscat are frequently pressed in membrane presses.
There are several variations on the basic theme, including deep or shallow baskets, the latter still being extensively used in the Champagne region of northern France. The larger the diameter of the press, the more quickly the juice will drain from the bed of grape skins. Basket presses offer removable and forkliftable baskets and press pans. This allows the winery to own two or more complete basket assemblies.
Moving Head Press
An important adaptation of the vertical basket press is the moving head press or commonly referred to as the horizontal plate press or horizontal screw press (Figure 4.3). A moving head press consists of a slotted cylinder containing a system of plates, hoops and chains. Grapes are loaded into the press through a hatch in the upper, raised portion of the press, which is then closed, and the press rotates, tumbling the grapes.
Advantages and Disadvantages
An apparent advantage of hydraulically operated horizontal moving head presses is that the movement of the plate is independent of the rotation of the cage. Apart from facilitating cake crumbling, cage rotation can be desirable during filling as it allows for improved immediate drainage of juice from crushed grapes and for greater quantities of crushed grapes to be loaded into the press. The primary drawback of moving head presses is the progressive reduction in drainage surface during pressing.
The membrane press is another commonly used batch press today (Figure 4.4). Introduced in the 1950's, its basic design principle is that a rubber bladder is placed inside an enclosed horizontal stainless-steel cylindrical tank with drainage ducts. To load the press, the membrane is drawn back using a vacuum to expose the entire cylindrical volume. Presses are either loaded through one or two sliding doors on the side of the cylinder or through an axial feed on a swivel mount on the end of the cylinder. Either way, loading is periodically paused to close the press and rotate it to evenly distribute the loaded grapes around the tank.
The specific operation of a membrane press can significantly influence juice yield, quality, and throughput. The pressure in the sac is built up gradually in cycles, where each cycle is composed of the following: pressing the must gradually while rotating the drum, holding the pressure at its maximum value and releasing the pressure while reversing the direction of rotation (to break the pomace cake), then pressing gradually again while changing the direction. The most recent membrane presses have fully programmable systems, permitting the operator to carry out several dozen sequences within a pressing cycle. Press programs can broadly be categorized as either standard or sequential programs.
Some manufacturers have adopted quite different membrane and screen arrangements. By far the most popular design is the side-mounted press configuration with opposite drainage ducts (Figure 4.5). Another alternative membrane/screen arrangement is to have the membrane mounted centrally with a central star axle. The membrane inflates, filling the empty cavity with air and gently pushing the berries through strainers located along all the sides of the drum.
Continuous Screw Press
The continuous screw press was developed for large wineries producing large volumes of wine of commercial quality. They avoid the time and labor costs associated with cyclical filling and emptying. Crushed grapes are pumped into the press via a hopper at one end of the press. A continuous screw press consists of a screw inside of a hollow pipe (referred to as an Archimedes' screw), which is situated in an enclosed drum.
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