The pressing of the grapes occurs at two different points in the process depending whether it is for red wine or white wine. As mentioned, white wine is produced with little to no maceration therefore the grapes are pressed as soon as they have been initially destemmed and crushed, and before fermentation begins. This ensures all the juice is released from the skins before they are discarded.
Contrastingly, for the making of red wine the grapes are not pressed until the alcoholic fermentation is complete. As the skins, seeds, and stems remain in the vessel during fermentation there is no rush to squeeze every last drop out before it occurs. Following the devatting, with the free run juices separated from the pomace, the remaining solids are transferred to a pressing machine where every possible drop of juice is squeezed out to create pressed juice (kept separate to free-run juice).
Pressed juice tends to have more volatile acidity and higher phenolics than free-run juice, depending on how much the skin is torn up in the process releasing more astringent elements. Therefore, it is only blended into wines in small portions at the end stage, or for the top tier wines there is no pressed juice and simply free-run. To reduce the amount of astringency in the resultant juice winemakers choose the level of pressure they apply to the pressing stage, balancing out how much breakage of skin there is. Each grape varietal has a different level of toughness that can be broken at varying pressures and produce more or less pressed juice.
Types of Presses
A press style that presses a certain amount of grapes before needing to be emptied and have the leftover solids removed before the next batch is added. They are considered the gentler approach to pressing, as the grapes are moved around less and skins less likely to tear. Unfortunately they are also more labour intensive, with regular changeovers of batches every time it fills up. Hence it is more favourable among traditionalists and those with smaller quantities of wine to produce.
The Batch Press process includes two main steps; applying pressure and rotating the vessel. The pressure applied is dependent on the winemakers’ wishes but is the catalyst to removing as much juice as possible, therefore it is done several times at different pressure levels to ensure only empty solids remain. Between each pressure change there is the rotating of the vessel, or manually breaking up the ‘cake’. The rotation promotes a more even pressing as turning over the solids that still remain can uncover alternative sections that were yet to be fully pressed. The resultant product also ends up being a more easily removed ‘cake’ of solids stuck together. Through this process, the liquid released is drained straight away and pumped into another container.
Batch Press 1 - Basket Press
The Basket Press was one of the very first mechanised presses, saving many people the effort of pressing by hand. Starting out as a simple wooden basket with a capstan (axle-rotating mechanism), it has now developed into a hydraulic press fully enclosed to prevent input of oxygen in the process. As a gentle style of press, it is favoured by those that have the time to dedicate to the slow process and keep a close eye on how evenly it presses each batch. There is no rushing this process as too much pressure in one go can snap the mechanism, slow and gentle is the benefits to this style.
Batch Press 2 - Moving Head Press
Pressing horizontally, rather than a single press coming down onto the solids there are two sides moving inwardly simultaneously to squash the solids between them. This helps to ensure an even amount of pressure is placed across the whole cake and compacts it to a greater extent while allowing the juices to flow out below it. Unfortunately, its efficiency to compact so quickly can occasionally cause juices to be trapped between layers of dry solids. Winemakers must be vigilant in rotating and breaking up the cake still in this press to ensure that it does not occur.
Batch Press 3 - Bladder Press
Often referred to as a pneumatic press, the bladder acts as a large balloon that is pumped with high amounts of air or water to press the grapes against perforated edges. Designed as a long cylindrical bladder inside a tank, the bladder applies even pressure to all sections of the cake while also cooling down the warm must if using cold water in the bladder. The remaining cake is in a donut shape and takes slightly longer to remove and clean than other presses but the resultant wine is more even and reliable.
Batch Press 4 - Membrane Press
A combination of a few presses, there is a large pressurised bladder that pushes the grapes horizontally (the bladder is not inside the vessel but pushes from one side to the next) into a mesh screen. The mesh screen allows the juices to drain instantly into a catchment without any of the solids getting through. Even though it still requires labour during cleaning, the actual process is quite seamless and gentle while being tightly enclosed to prevent any chance of oxidation. This press is often at the higher price range of equipment so not as often seen.
Using a helical screw or belt, the continuous presses feed grapes into the machine and presses the grapes, removing the solids as it continues through the machine. With a highly efficient process, this press is favoured by high volume producers to churn through large amounts of wine supply even if it breaks up the skins more than a Batch press.
Continuous Press 1 - Screw press
A large helical screw is used to transfer grapes across a perforated cylinder, this allows the juices to be released as the skins are continually pushed along through the machine. The tighter the screw gets the higher the pressure on the cake releasing more and more juice until it is dry. The harsh edges of the screw can break up a significant amount of the skin that brings with it more tannins and tiny solids that may fall through into the juice collected below.
Continuous Press 2 - Impulse press
Limiting the movement of the grapes to reduce the breakage, the Impulse press removes the screw from the press as grapes are fed in and then pulsed back and forth with pressure. This pushes the cake against the end of the press not dissimilar to a batch press, and sometimes just as slow. Minimal winemakers use this press as the time taken matches to the batch presses but the benefits are much lower.
Continuous Press 3 - Belt press
Using a series of pads filled with air sat upon a wire belt, the grapes are placed on the belt and fed through these pads that press down for an amount of time then moved to the next pad. The belt made of mesh allows the juices to seep through to the awaiting collection pans. Often used for whole-cluster pressing, it is not found across many wine regions due to the high levels of oxygen that can get into the machine.