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 preprogrammed (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 tank
o 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 oxidation
o It is a highly productive and efficient system
o Its continuous nature reduces the need to over-pack the grapes, and reduces
cleaning time
Disadvantage
o 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.
Disadvantages
o 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.
BIBLIOGRAPHY
Ashurst, P. R. Chemistry and Technology of Soft Drinks and Fruit Juices. Oxford, UK:
Blackwell Pub., 2005. Print.
Blake, M. "Frogie's World." 2001. Web. 11 Sept. 2010.
Boulton, Roger B. Principles and Practices of Winemaking. New York: Chapman &
Hall, 1996. Print.
Diemme. "Press Tank." Catalogue. 2003. Web. 13 Sept. 2010.
Nye, Bill. "What Is Oxidation?" WiseGEEK: Clear Answers for Common Questions.
2001. Web. 11 Sept. 2010. <http://www.wisegeek.com/what-is-oxidation.htm>.
Phillips, Curtis. "Product Review: Choosing the Best Tank Press For Your Winery."
Wine Business. 15 Feb. 2006. Web. 13 Sept. 2010.
<http://www.winebusiness.com/wbm/?go=getArticle&dataId=42352>.
Vivwine. "Google Image Result for Http://www.vintekaus.com.au/vt1700a.gif." Google.
Australian Wine Cooperation, 1998. Web. 12 Sept. 2010.
<http://www.google.ca/imgres?imgurl=http://www.vintekaus.com.au/vt1700a.gif&imgre
furl=http://
Werrel, Phill. "New World Winemakin." Wine Making Technology : The Wine Yeast and
Fermentation Specialists. 1999. Web. 13 Sept. 2010.
<http://www.newworldwinemaker.com/>.