WINE PRESSES: (a review for the lecture: Principles of Winemaking. Prof. Dr. Reinhard EDER, University of Life Science, BOKU, Vienna, Austria)

ABSTRACT

The aim of this paper is to present the various wine pressing techniques and systems which have been and are currently used in the wine production industry. Each instrument will be evaluated on both its advantages and disadvantages which will essentially demonstrate each respective design’s unique compatibility to specific forms of wine production. The wine making community’s continued pursuit of quality is the driving force behind innovation within its techniques and methods which is ultimately translated into the production of superior pressing techniques.

INTRODUCTION

The purpose of pressing is to recover the juice associated with the pulp and skin section of the grapes that are not readily released by natural draining. Presses can be classified into batch and continuous types. The composition of the press fractions has been long known to be different from that of the free-run fraction (Boulton, 1996). These differences can be positive in terms of the varietal character and flavour attributes and the precursors of certain aging components, but are often negative in terms of lower acidity and higher pH, excessive tannin and gum content (Boulton, 1996). The extent of these differences is determined by the condition of the fruit, the way in which pressure is applied, the nature of the screen and the movement of the skins. The batch presses are generally less prone to skin tearing (and subsequent phenol and tannin extraction) than the continuous presses (Boulton, 1996).Accumulation of suspended solids is concerning because it requires further clarification of the press fractions. There are similar concerns for higher tannin levels and high pH conditions (Boulton, 1996).These fractions generally require special attention such as fining and acidity adjustments that the free run fractions do not. The higher gum content would be expected to contribute to poorer settling and enhanced filter fouling in the latter stages of winemaking (Werrel, 1999). It was not until the introduction of the larger membrane presses in the mid 1970s that acceptable press fractions could be produced at rates comparable to those provided by the screw presses. Screw presses are now largely substituted by recent generations of membrane presses (Werrel, 1999). Just as the membrane press revolutionised the wine press industry and its capabilities in the 70s, with the continual passage of time, new and more refined presses will be produced and the industry will also continually grow and improve. The following section will consider the most basic and original presses; the basket press.

1. Batch Presses

Batch presses operate in a cycle in which the grapes are filled, pressurized, rotated and sometimes held at pressure then depressurized and emptied (Boulton, 1996). The filling time is determined by the capacity of the must

pump or conveyors and size of the press. The pressure is usually increased to a maximum pressure of 4 to 6 atm ( bars) in a stage over a period of between 1 and 2 hours (Boulton, 1996). Most batch presses (except basket presses) are rotated while the pressure is being applied so that a regular-shaped cake is developed. Although older and smaller models were often manually operated, most of these presses today have extensive programming capability and the pressures and holding times of the press can be pre-programmed (Werrel, 1999)

1.1 Basket Presses

The basket press is an upright, usually cylindrical, slatted or perforated “cage” which holds the grapes or must and a plate, just slightly smaller than the internal diameter of the cage. The plate is forced down while the juice or wine is expelled from the perforations or from between the slats of the cage usually at a right angle to the direction of the force. Basket presses range from simple presses, in which a wooden basket with vertical slats provides the restraining surface and a capstan is used to apply the pressure, to modern, automated and computerized presses made entirely out of stainless steel and that use hydraulically-actuated rams. The mechanical modifications of the basket press have been to mount the screen on its side and to provide a motor drive of an axial screw to move the pressing head (Boulton, 1996). Such presses are generally referred to as moving head presses. The moving head presses range from those with other one moving head to those in which a heas advances from each end (Boulton, 1996). Some have an internal threaded axle, while others have it external of the pressing cage (Boulton, 1996). There are even hydraulically displaced rams. Most of the moving head presses also have internal hoops connected by chains which are used to help in the breakup of the cake between successive pressings as the head is retracted.

Figure 1 This image displays a traditional basket press. Note the vertical slats from which the must is expelled and the manual lever which is used to press the grapes (Blake, 2001).

Advantages

o Basket presses are advantageous in that they generally press grapes softly

o Relative to other more complex presses the basket press is of less cost. This is illustrated by the fact that small wineries, with generally smaller budgets are more likely to buy basket presses than medium or large scale wineries.

Figure 2 Bar graph displaying the use of basket presses, tank/bladder press-load through doors and tank//bladder press-axial food across small, medium and large wineries.

Disadvantages

o The press’ open design means it has a relatively high degree of exposure to the surrounding air which increases the must’s risk of oxidation. Oxidation can be defined as the interaction between oxygen molecules and all the different substances they may contact, from metal to living tissue (Nye, 2001). Sensitive phenolics and aromatics in wine are negatively impacted by oxidation. SO2, NO2, CO2, or the strict exclusion of O2 are employed to combat this problem.

o The basket press is not hygienic, relative to other pressing techniques. Its open nature may make the must more susceptible to infection from the surrounding environment

o The basket and moving head presses are limited in that the juice channels in the cake are quickly closed off as pressure is applied. This results in a dry outer section of the cake with a wet core.

o Basket presses generally require more time relative to other presses to press a similar amount of grapes. This is because the cake in the basket press design has less surface area. In the market economy where time equals money, this may be a concern. However, this disadvantage was addressed by the creation of the moving head presses which had two cages as opposed to one.

o The use of the vertical basket press is now limited almost exclusively to home wine makers. Reasons for this include their small volumes difficulties in applying uniform pressure to all parts of the cake, the tendency to squirt juice at high pressures and the labour intensive operations of loading and unloading these presses (Boulton, 1996).

1.2 Membrane Presses

Membrane presses function through the pressurization of gas in order to affect mechanical motion. The membrane is mounted on opposite sides of a cylindrical tank diametrically across the ends (Boulton, 1996). When evacuated, the membrane is drawn back against one-half of the circumference while the skins enter either through doors in the side walls or axially at one end. Drain screens are mounted along the length and the membrane is inflated like a balloon by an on-board air compressor. Membrane presses have gained widespread acceptance due to the preferred composition of their press fractions (Boulton, 1996).

Figure 3 This image displays an Australian pneumatic membrane wine press with a electronic control system

Advantages

o Membrane presses are beneficial in that they can be defined as a soft pressing device. In these presses the pressure is applied with a minimum of skin movement across the screen surface, and this leads to much less tearing and grinding of skins and seeds. As a result the tannin released and fine solids generated due to this action is greatly reduced and the press fractions are lower in both suspended solids and condensed phenols.

o In relation to, for example, the basket press, pneumatic presses generally allow for increased pressuring surface area.

o The design of the membrane reduces the grinding of the grape against the resistant surface when it is being pressed by the inflation of the membrane. The reduction of grinding is important as grinding can lead to the release of undesired phenolic compounds found in, for example, the pips.

o In relation to the basket press, the typical design of pneumatic presses decreases the exit time of the juice from the tank.

o Non-perforated pneumatic presses are generally easier to clean and have less risk of oxidation in relation to their perforated counterparts

o Non-perforated pneumatic presses are also beneficial in that mash standing can occur within them, given that they are sealed, and the juice would not be able to escape from the remaining cake.

o However, perforated pneumatic presses are beneficial in that free run juice is able to be expelled, and because of this, more grapes can be filled within the tank. Having the ability to increase the amount of grapes within the tank will inevitable decrease the quantity of time taken to press, which in itself is beneficial.

Disadvantages

o The most critical part of pneumatic press is the membrane, as it responsible for pressing the grapes. Pips, stones or other hard particles are liable to pop this membrane during pressing. Without it, the press loses its primary function, a consequence that may be entirely inopportune if this occurs right in the middle of the harvesting season. Therefore it is critical for the functioning of the press and the membranes susceptibility for damage given the presence of hard or sharp particles

1.3 Tank Press

The tank press is a pneumatic press, which received its name due to its often sealed ‘tank’ like structure. The "tank" is a closed cylinder, meaning only the ends are welded to the cylinder (Phillips, 2006). Tank presses may be either "open," in which the tank has perforated slots that allow the juice or wine to directly exit the press, or "closed," in which the juice is collected in slotted channels inside the press and only exit the press tank through a series of drains at one end. Tank presses may have either a bladder or a membrane inside which acts as the pressurizing device used to extract the press wine. After pressing the cake to a certain degree, there may be a brief interlude where pressing stops and the press cake is mixed. When pressing recommences it is usually at a slightly higher force then the previous stint. This cycle continues until the must is fully separated from the pomace, or separated as much as desired. After the desired amount of press juice

is extracted, the press cake is discharged. Tank presses may also function with the use of vacuum force. Instead of a membrane being inflated with the use of compressed air, the air is sucked away from the tank, a motion which presses the membrane against the grapes causing a release of the press juice.

Advantages

o Given its closed structure, mash standing can be performed within the tanko The design of the vacuum tank press allows for minimal exposure to oxygen, and

thus it is a very reductive process, which is desirable in order to combat the negative effects of oxidation

Disadvantages

o It can be more expensive than other presses o Given the open tank press is not completely isolated from contact with the

surrounding air, it may still be subject to oxidation, although perhaps to a lesser degree than other presses (basket press). Nevertheless, measures must be taken to reduce oxidation.

Figure 4 This figure displays a stainless steel closed tank press designed by the French company Diemme ( Diemme, 2003).

1.4 Pack Press

The pack press is based on the traditional ‘rustic’ press widely used in the cider industry. The press comprises a set of frames for containing the fruit (Ashurst, 2005). These are loaded, in stages, by placing a loose weave cloth over each rectangular frame and adding an appropriate quantity of the fruit mash over the frame by the operator who, then folds the cloth across to cover it over (Ashurst, 2005). Another frame is then placed on top and the process is repeated until several filled layers are formed. The stick is built up inside a rectangular tray, or bed, which is a collecting device and a platform raised by the a vertical hydraulic ram in bringing the top of the stack into contact with a fixed frame

(Ashurst, 2005). In the this way pressure is increased to expel the juice which runs down into a tray for collection.

Advantages

o The pack press is relatively cheap in comparison to other methods o It is a simple device, in that its repair does not always require advanced

devices

Disadvantages o They are ill suited for large producers, and are generally only used for

small scale producer (Ashurst, 2005)o It is a more difficult machine to clean, given its inclusion of multiple

weave cloth.

2. Continuous Presses

As opposed to batch presses, continuous presses can be in continual use and thus their production time and their production capacity is much higher, despite the fact that there is an inevitably price for mass production nature of continuous presses.

2.1 Continuous Tank Press

Tank presses may be designed for continuous pressing and is geared for large wine producers which specialize in mass production. The tank press allows the grapes to be easily and frequently deposited into one end of the tank, and the press cake should be removed in a similar timeframe. Grapes, as well as other foodstuff can be deposited into the first receiving chamber of the press, where it is pressed to a specific degree until it is moved to the second, and third and so on chambers to receive different degrees of pressing according to the operators desire. The must is collected in a receptacle which is either perforated, or the restraining surface is perforated, and then is relayed to a desired area. Tank presses may be powered through various means: ie: hydraulically or electrically.

Advantage o It is a closed system, which decreases the risk of oxidationo It is a highly productive and efficient systemo Its continuous nature reduces the need to over-pack the grapes, and reduces

cleaning time

Disadvantageo While this technique may seem beneficial in that it has the potential to

produce large quantities of must in a minimum time frame, there is a price to pay for this efficiency. The price, is that the wine which is produced using this

technique is generally of a lower quality than those produced using more refined, and time required techniques.

2.2 Screw Presses An alternative arrangement for developing pressure is to feed the skins into a cylinder in which a large helical screw is used to force the skins into a plug against a restriction at the far end. The restriction is usually a door which is held partially closed by hydraulic pressure. Liquid in the foodstuff is separated by gravity and feed pressure while solids is conveyed to the pressing dewatering zone. Screw presses have been generally replaced by membrane presses in modern wineries.

Figure 5 This figure displays a sectional view of a screw press

Advantages

o Again, the benefit of continuous presses is that its capacity for production is high, and the time needed for this production is relatively low.

Disadvantageso The movement across the cylindrical screen tears and grinds the pips and

outer skin tissue leading to press fractions which are considerably higher in mineral content, tannin and gums (Werrel, 1999).

o The press must that is expelled using a screw press are typically high in suspended solids content, typically above 4 volume to volume. This is an unacceptable level of suspended solids for white wine and further equipment will be needed to address this problem. Thus the quality of the wine is compromised by the press efficiency.

2.3 Belt Press

Belt presses have a continuous perforated belt that moves over several sets of rollers that apply the pressure to the grape skins. The juice extracted falls through the screen and is collected in one or more pans. These units have been used extensively for the production of whole cluster processing for sparkling wine production and can have very high capacities. They have not gained widespread acceptance for table wine production due to concerns of high solids in the juice, juice aeration and the difficulty of cleaning

Advantages

o Its capacity for production is high, and the time needed for this production is relatively low

o Disadvantages o Wine is at higher risk of oxidation due to more exposure than other types

of presses. o More difficult to clean because of the multiple and intricate surfaces of the

belt press.

2.4 Decanter

A decanter centrifuge separates solids from one or two liquid phases in one single continuous process. This is done using centrifugal forces that can be well beyond 3000 times greater than gravity. When subject to such forces, the denser solid particles are pressed outwards against the rotating bowl wall, while the less dense liquid phase forms a concentric inner layer. Different dam plates are used to vary the depth of the liquid - the so-called pond - as required. The sediment formed by the solid particles is continuously removed by the screw conveyor, which rotates at a different speed than the bowl. As a result, the solids are gradually "ploughed" out of the pond and up the conical "beach". The centrifugal force compresses the solids and expels the surplus liquid. The dried solids then discharge from the bowl. The clarified liquid phase or phases overflow the dam plates situated at the opposite end of the bowl.

Figure 6 This image displays a horizontal decanter centrifuge for solid particles (Diemme, 2003).

Advantages

o A decanter has a high production capacity, and as mentioned previously, this allows for minimum time investment for a large production value.

o Suitable for use in a variety of wine industries: big wineries, simple wines, mash heating, red wine, starch industry and waste eater treatment

Disadvantages

o Typical decanters require a substantial quantity of electrical power in order to function, causing higher production cost.

CONCLUSION

Wine presses are divided into two groups, batch presses and continual presses. The myriad of presses which belong to each group may be advantageous in their own merit and compatible to certain genres of wine production, but they also may be vastly incompatible for others. It is necessary for a wine producer to evaluate each potential press in relation to their goals and objectives and the genre of wine production they are involved in. For example, small, home made wine producers will smaller budgets may not have the financial capacity to invest in a state of the art membrane press, but would be well served with a version of the basket press. Similarly, large wine producers which are not as concerned with attaining a specific quality level, but rather are focused on quantity over quality will be more likely to choose a continuous press such as the tank press. By weighing each merit and disadvantage, one can make an informed choice about the press which would be most compatible to ones unique operation. With the continuation of innovation and discovery, these choices will not only increase, but they will become better, benefiting the wine industry and wine alike.

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